Web Services Business Process Execution Language Version 2.0
Working Draft 01, 01208
September DecemberMay 20042005
Document identifier:
wsbpel-specification-draft-01
Location:
http://www.oasis-open.org/apps/org/workgroup/wsbpel/
Editors:
Assaf Arkin <arkin@intalio.com>
Sid Askary <saskary@nuperus.com>
Ben Bloch <ben_b54@hotmail.com>
Francisco Curbera,
IBM <curbera@us.ibm.com>
Yaron Goland, BEA <ygoland@bea.com>
Neelakantan Kartha,
Sterling Commerce <N_Kartha@stercomm.com>
Canyang Kevin Liu,
SAP <kevin.liu@sap.com>
Satish Thatte,
Microsoft <satisht@microsoft.com>
Prasad Yendluri,
webMethods <pyendluri@webmethods.com>
Alex Yiu,
Oracle <alex.yiu@oracle.com>
Contributors:
{FirstName} {Last Name}, {Organization}
Editor’s Notes – KevinL – this section should be consolidated with Appendix H
This document defines a notation for
specifying business process behavior based on Web Services. This notation is
called Web Services Business Process
Execution Language for Web Services (abbreviated to BPEL4WS
WS-BPEL in the rest of this document).
Processes in BPEL4WSWS-BPEL
export and import functionality by using Web Service interfaces exclusively.
Business processes can be described
in two ways. Executable business processes model actual behavior of a participant
in a business interaction. Business protocols, in contrast, use process
descriptions that specify the mutually visible message exchange behavior of
each of the parties involved in the protocol, without revealing their internal
behavior. The process descriptions for business protocols are called abstract
processes. BPEL4WSWS-BPEL
is meant to be used to model the behavior of both executable and abstract
processes.
BPEL4WSWS-BPEL
provides a language for the formal specification of business processes and business
interaction protocols. By doing so, it extends the Web Services interaction
model and enables it to support business transactions. BPEL4WSWS-BPEL
defines an interoperable integration model that should facilitate the expansion
of automated process integration in both the intra-corporate and the
business-to-business spaces.
This is a draft version of the WS-BPEL TC specification, updated from the origninal BPEL4WS V1.1 specification dated May 5, 2003 that was submitted to the WS BPEL TC. See: http://www.oasis-open.org/apps/org/workgroup/wsbpel/download.php/2046/BPEL%20V1-1%20May%205%202003%20Final.pdf
If you are on the <wsbpel@lists.oasis-open.org> list for committee members, send comments there. If you are not on that list, subscribe to the <wsbpel-comment@lists.oasis-open.org> list and send comments there. To subscribe, send an email message to <mailto:wsbpel-comment-request@lists.oasis-open.org> with the word "subscribe"as the body of the message.
For information on whether any patents have been disclosed that may be essential to implementing this specification, and any offers of patent licensing terms, please refer to the Intellectual Property Rights section of the WS-BPEL TC web page http://www.oasis-open.org/committees/tc_home.php?wg_abbrev=wsbpel
Copyright © 2004 OASIS Open, Inc. All Rights Reserved.
Table of Contents
1. Introduction
3. Relationship with Other Specifications
4. What Changed from BPEL4WS 1.0This Section Has Been Deleted
4.1. Core Concepts Clarification
4.2. Terminology Changes
4.3. Feature Changes
5. Core Concepts and Usage Patterns
6. Defining a Business Process
6.1. Initial Example
6.2. The Structure of a Business Process
6.4. The Lifecycle of a Business Process
7. Partner Link Types, Partner Links, and Endpoint References
7.1. Partner Link Types
7.2. Partner Links
7.3. Business Partners This
Section Has Been Deleted
7.4. Endpoint References
8.1. Motivation
8.2. Defining Properties
9.1. Expressions
9.2. Variables
9.3. Assignment
10. Correlation
10.1. Message Correlation
10.2. Defining and Using Correlation Sets
11. Basic Activities
11.1. Standard Attributes for Each Activity
11.2. Standard Elements for Each Activity
11.3. Invoking Web Service Operations
11.4. Providing Web Service Operations
11.5. Updating Variable Contents
11.6. Signaling Faults
11.7. Waiting
11.8. Doing Nothing
12.1. Sequence
12.2. Switch
12.3. While
12.4. Pick
12.5. Flow
13. Scopes
13.1. Data Handling and Partner Links
13.2. Error Handling in Business Processes
13.3. Compensation Handlers
13.4. Fault Handlers
13.5. Event Handlers
13.6. IsolatedSerializable
Scopes
14. Extensions for Executable Processes
14.1. Expressions
14.2. Variables
14.3. Assignment
14.4. Correlation
14.5. Web Service Operations
14.6. Terminating a Service Instance
14.7. Compensation
14.8. Event Handlers
15. Extensions for Business Protocols
15.1. Variables
15.2. Assignment
16. Examples
16.1. Shipping Service
16.2. Loan Approval
16.3. Multiple Start Activities
Appendixes
C. XSD Schemas
D. Notices
E. Intellectual Property Rights
G. References
H. Committee Members (Non-Normative)
1. Introduction
The goal of the Web Services effort is to achieve universal interoperability between applications by using Web standards. Web Services use a loosely coupled integration model to allow flexible integration of heterogeneous systems in a variety of domains including business-to-consumer, business-to-business and enterprise application integration. The following basic specifications originally defined the Web Services space: SOAP, Web Services Description Language (WSDL), and Universal Description, Discovery, and Integration (UDDI). SOAP defines an XML messaging protocol for basic service interoperability. WSDL introduces a common grammar for describing services. UDDI provides the infrastructure required to publish and discover services in a systematic way. Together, these specifications allow applications to find each other and interact following a loosely coupled, platformindependent model.
Systems integration requires more than the ability to conduct simple interactions by using standard protocols. The full potential of Web Services as an integration platform will be achieved only when applications and business processes are able to integrate their complex interactions by using a standard process integration model. The interaction model that is directly supported by WSDL is essentially a stateless model of synchronous or uncorrelated asynchronous interactions. Models for business interactions typically assume sequences of peer-to-peer message exchanges, both synchronous and asynchronous, within stateful, long-running interactions involving two or more parties. To define such business interactions, a formal description of the message exchange protocols used by business processes in their interactions is needed. The definition of such business protocols involves precisely specifying the mutually visible message exchange behavior of each of the parties involved in the protocol, without revealing their internal implementation. There are two good reasons to separate the public aspects of business process behavior from internal or private aspects. One is that businesses obviously do not want to reveal all their internal decision making and data management to their business partners. The other is that, even where this is not the case, separating public from private process provides the freedom to change private aspects of the process implementation without affecting the public business protocol.
Business protocols must clearly be described in a platform-independent manner and must capture all behavioral aspects that have cross-enterprise business significance. Each participant can then understand and plan for conformance to the business protocol without engaging in the process of human agreement that adds so much to the difficulty of establishing cross-enterprise automated business processes today.
What are the concepts required to describe business protocols? And what is the relationship of these concepts to those required to describe executable processes? To answer these questions, consider the following::
· Business protocols invariably include data-dependent behavior. For example, a supply-chain protocol depends on data such as the number of line items in an order, the total value of an order, or a deliver-by deadline. Defining business intent in these cases requires the use of conditional and time-out constructs.
· The ability to specify exceptional conditions and their consequences, including recovery sequences, is at least as important for business protocols as the ability to define the behavior in the "all goes well" case.
· Long-running interactions include multiple, often nested units of work, each with its own data requirements. Business protocols frequently require cross-partner coordination of the outcome (success or failure) of units of work at various levels of granularity.
If we wish to provide precise predictable descriptions of service behavior for crossenterprise business protocols, we need a rich process description notation with many features reminiscent of an executable language. The key distinction between public message exchange protocols and executable internal processes is that internal processes handle data in rich private ways that need not be described in public protocols.
In thinking about the data handling aspects of business protocols it is instructive to consider the analogy with network communication protocols. Network protocols define the shape and content of the protocol envelopes that flow on the wire, and the protocol behavior they describe is driven solely by the data in these envelopes. In other words, there is a clear physical separation between protocol-relevant data and "payload" data. The separation is far less clear cut in business protocols because the protocol-relevant data tends to be embedded in other application data.
BPEL4WSWS-BPEL
uses a notion of message properties , which are a
type of variable property, to identify
protocol-relevant data embedded in messages. Properties can be viewed as
"transparent" data relevant to public aspects as opposed to the
"opaque" data that internal/private functions use. Transparent data
affects the public business protocol in a direct way, whereas opaque data is
significant primarily to back-end systems and affects the business protocol
only by creating nondeterminism because the way it affects decisions is opaque.
We take it as a principle that any data that is used to affect the behavior of
a business protocol must be transparent and hence viewed as a property.
The implicit effect of opaque data manifests itself through
nondeterminism in the behavior of services involved in business protocols.
Consider the example of a purchasing protocol. The seller has a service that
receives a purchase order and responds with either acceptance or rejection
based on a number of criteria, including availability of the goods and the
credit of the buyer. Obviously, the decision processes are opaque, but the fact
of the decision must be reflected as behavior alternatives in the external
business protocol. In other words, the protocol requires something like a
switch activity in the behavior of the seller's service but the selection of
the branch taken is nondeterministic. Such nondeterminism can be modeled by
allowing the assignment of a nondeterministic or opaque value to a message
property, typically from an enumerated set of possibilities. The property can
then be used in defining conditional behavior that captures behavioral
alternatives without revealing actual decision processes. BPEL4WSWS-BPEL
explicitly allows the use of nondeterministic data values to make it possible
to capture the essence of public behavior while hiding private aspects.
The basic concepts of BPEL4WSWS-BPEL
can be applied in one of two ways. A BPEL4WSWS-BPEL
process can define a business protocol role, using the notion of abstract
process. For example, in a supply-chain protocol, the buyer and the seller are
two distinct roles, each with its own abstract process. Their relationship is
typically modeled as a partner link. Abstract processes use all the concepts of
BPEL4WSWS-BPEL
but approach data handling in a way that reflects the level of abstraction
required to describe public aspects of the business protocol. Specifically,
abstract processes handle only protocol-relevant data. BPEL4WSWS-BPEL
provides a way to identify protocol-relevant data as message properties. In
addition, abstract processes use nondeterministic data values to hide private
aspects of behavior.
It is also possible to use BPEL4WSWS-BPEL
to define an executable business process. The logic and state of the process
determine the nature and sequence of the Web Service interactions conducted at
each business partner, and thus the interaction protocols. While a BPEL4WSWS-BPEL
process definition is not required to be complete from a private implementation
point of view, the language effectively defines a portable execution format for
business processes that rely exclusively on Web Service resources and XML data.
Moreover, such processes execute and interact with their partners in a
consistent way regardless of the supporting platform or programming model used
by the implementation of the hosting environment.
Even where private implementation aspects use platform-dependent
functionality, which is likely in many if not most realistic cases, the
continuity of the basic conceptual model between abstract and executable
processes in BPEL4WSWS-BPEL
makes it possible to export and import the public aspects embodied in business
protocols as process or role templates while maintaining the intent and
structure of the protocols. This is arguably the most attractive prospect for
the use of BPEL4WSWS-BPEL
from the viewpoint of unlocking the potential of Web Services because it allows
the development of tools and other technologies that greatly increase the level
of automation and thereby lower the cost in establishing cross-enterprise
automated business processes.
In summary, we believe that the two usage patterns of business
protocol description and executable business process description require a
common core of process description concepts. In this specification we clearly
separate the core concepts from the extensions required specifically for the
two usage patterns. The BPEL4WSWS-BPEL
specification is focused on defining the common core, and adds only the
essential extensions required for each usage pattern.
BPEL4WSWS-BPEL
defines a model and a grammar for describing the behavior of a business process
based on interactions between the process and its partners. The interaction
with each partner occurs through Web Service interfaces, and the structure of
the relationship at the interface level is encapsulated in what we call a
partner link. The BPEL4WSWS-BPEL
process defines how multiple service interactions with these partners are
coordinated to achieve a business goal, as well as the state and the logic
necessary for this coordination. BPEL4WSWS-BPEL
also introduces systematic mechanisms for dealing with business exceptions and
processing faults. Finally, BPEL4WSWS-BPEL
introduces a mechanism to define how individual or composite activities within
a process are to be compensated in cases where exceptions occur or a partner
requests reversal.
BPEL4WSWS-BPEL
is layered on top of several XML specifications: WSDL 1.1, XML Schema 1.0, and
XPath1.0. WSDL messages and XML Schema type definitions provide the data model
used by BPEL4WSWS-BPEL
processes. XPath provides support for data manipulation. All external resources
and partners are represented as WSDL services. BPEL4WSWS-BPEL
provides extensibility to accommodate future versions of these standards,
specifically the XPath and related standards used in XML computation.
2. Notational Conventions
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC 2119].
Namespace URIs of the general form "some-URI" represent some application-dependent or context-dependent URI as defined in [RFC 2396].
This specification uses an informal syntax to describe the XML grammar of the XML fragments that follow:
· The syntax appears as an XML instance, but the values indicate the data types instead of values.
· Grammar in bold has not been introduced earlier in the document, or is of particular interest in an example.
· <-- description --> is a placeholder for elements from some "other" namespace (like ##other in XSD).
· Characters are appended to elements, attributes, and as follows: "?" (0 or 1), "*" (0 or more), "+" (1 or more). The characters "[" and "]" are used to indicate that contained items are to be treated as a group with respect to the "?", "*", or "+" characters.
· Elements and attributes separated by "|" and grouped by "(" and ")" are meant to be syntactic alternatives.
· The XML namespace prefixes (defined below) are used to indicate the namespace of the element being defined.
· Examples starting with <?xml contain enough information to conform to this specification; other examples are fragments and require additional information to be specified in order to conform.
· XSD schemas and WSDL definitions are provided as a formal definition of grammars [XML Schema Part 1] and [WSDL 1.1].
This specification uses a number of namespace prefixes throughout; their associated URIs are listed below. Note that the choice of any namespace prefix is arbitrary and not semantically significant.
· xsi - "http://www.w3.org/2001/XMLSchema-instance"
· xsd - "http://www.w3.org/2001/XMLSchema"
· wsdl - "http://schemas.xmlsoap.org/wsdl/"
· plnk – http://schemas.xmlsoap.org/ws/2004/03/partner-link/”
· bpws – “http://schemas.xmlsoap.org/ws/2004/03/business-process/”
3. Relationship with Other Specifications
BPEL4WSWS-BPEL
depends on the following XML-based specifications: WSDL 1.1, XML Schema 1.0 and
XPath 1.0.
Among these, WSDL has the most influence on the BPEL4WSWS-BPEL
language. The BPEL4WSWS-BPEL
process model is layered on top of the service model defined by WSDL 1.1. At
the core of the BPEL4WSWS-BPEL
process model is the notion of peer-to-peer interaction between services
described in WSDL; both the process and its partners are modeled as WSDL services.
A business process defines how to coordinate the interactions between a process
instance and its partners. In this sense, a BPEL4WSWS-BPEL
process definition provides and/or uses one or more WSDL services, and provides
the description of the behavior and interactions of a process instance relative
to its partners and resources through Web Service interfaces. That is, BPEL4WSWS-BPEL
defines the message exchange protocols followed by the business process of a
specific role in the interaction.
The definition of a BPEL4WSWS-BPEL
business process also follows the WSDL model of separation between the abstract
message contents used by the business process and deployment information
(messages and portType versus binding and address information). In particular,
a BPEL4WSWS-BPEL
process represents all partners and interactions with these partners in terms
of abstract WSDL interfaces (portTypes and operations); no references are made
to the actual services used by a process instance.
However, the abstract part of WSDL does not define the
constraints imposed on the communication patterns supported by the concrete
bindings. Therefore a BPEL4WSWS-BPEL
process may define behavior relative to a partner service that is not supported
by all possible bindings, and it may happen that some bindings are invalid for
a BPEL4WSWS-BPEL
process definition.
While WS-BPEL attempts to provide as much compatibility with WSDL 1.1 as possible there are two areas where such compatibility has proven impossible. One area, discussed later in this document, is in fault naming. The other area is in support for overloaded operation names in WSDL portTypes. A BPEL processor MUST reject any WSDL portType definition that includes overloaded operation names. This restriction was deemed appropriate as overloaded operations are rare, they are actually banned in the WS-I Basic Profile and supporting them was felt to introduce more complexity than benefit.
A BPEL4WSWS-BPEL
process is a reusable definition that can be deployed in different ways and in
different scenarios, while maintaining a uniform application-level behavior
across all of them. Note that the description of the deployment of a BPEL4WSWS-BPEL
process is out of scope for this specification.
Introduction of service reference container
(“bpws:service-ref”) is meant to avoid inventing a private BPEL4WSWS-BPEL
mechanism for web service endpoint references and to provide pluggability of
different versions of service referencing or endpoint addressing schemes being
used within a BPEL program without having explicit dependency to a particular
version of specification.
With respect to [WS-I Basic Profile] (Basic Profile 1.10)
all BPEL implementations SHOULD be configurable such that they can
participate in Basic Profile 1.10 compliant
interactions. A BPEL implementation MAY allow the Basic Profile 1.10
configuration to be disabled, even for scenarios encompassed by the Basic
Profile 1.10.
4. What
Changed from BPEL4WS 1.0This Section Has Been
Deleted
The BPEL4WS 1.1 specification is an enhancement of
the BPEL4WS 1.0 specification [15]. The 1.1 version has five new authors who
brought a fresh viewpoint and deep industry experience. Their contributions are
reflected in a number of enhancements in this version.
The 1.1 version incorporates numerous corrections
and clarifications based on the feedback received on the 1.0 version. In
addition, the 1.1 version differs from the 1.0 version in the following
substantive ways.
4.1. Core
Concepts Clarification
We believe that the two usage patterns of business
protocol description and executable business process description require a
common core of process description concepts. In the 1.1 version of the
specification we clearly separate the core concepts from the extensions
required specifically for the two usage patterns. The main body of the
specification defines the core concepts. The Extensions for Executable Processes
and the Extensions for Business Protocols are defined in separate sections at
the end of the specification. The separation of core concepts from extensions
allows features required for specific usage patterns to be defined in a
composable manner. It is conceivable that further extensions will be developed
over time as the usage of the specification matures.
4.2. Terminology
Changes
The following terminology changes have occurred
·Service
Links are now called Partner Links
·Service
Link Types are now called Partner Link Types
·Service
References are now called Endpoint References
·Containers
are now called Variables
The formal syntax has also been changed to reflect
these terminology changes, including the replacement of the current partner
element with a partnerLink element to reflect the fact that such a
link is a conversational interface rather than reflective of a business
relationship. A partner element reflective of a business relationship
is added as described in the next section.
4.3. Feature Changes
The following changes have been made:
·The
terminate activity is now strictly limited to executable processes.
·Partner
Link Type Roles are now limited to a single WSDL portType.
·A
new partner element is added to allow grouping of Partner Links based
on expected business enterprise relationships.
·Endpoint
references are now manifested as a service reference container
(“bpws:service-ref”).
·Message
Properties are now limited to only be simple types.
·Web
service interactions in abstract processes are now permitted to omit references
to variables for inbound and outbound message data.
·Opaque
assignment in abstract processes may now target Boolean variables, and
variables of simple but unbounded types. In the latter case the semantics
requires creation of a unique value similar to a GUID.
·The
syntax for defining variables has been changed to use three mutually exclusive
attributes messagetype, type and element. The first points to a WSDL message
type definition. The second points to an XML Schema simple type. The third
points to an XML Schema global element definition. This allows one to define
variables using something other than WSDL message types. Only variables that
are defined using messagetypes can be used as input or output targets in
messaging operations.
·The
ability to provide an in-line WSDL message type has been removed, since the
vast majority of the uses of this feature will be replaced by the usage of XML
Schema simple types and global elements.
·Correlation
sets have now been added to the uniqueness requirement so that it is not legal
to have two web service interactions outstanding if they have the same partner,
port type, operation and correlation set(s).
·In
case of activity termination, the activities wait, reply and invoke
are added to receive as being instantly terminated rather than being
allowed to finish.
·The
variable provided as the value of the faultVariable attribute in a catch
handler to hold fault data is now scoped to the fault handler itself
rather than being inherited from the associated scope.
·Variables
and correlation sets can now be associated with local scopes rather than with
the process as a whole. This permits easier management of visibility and
lifetime for variables and repeated initiation of local correlation sets to allow
multiple correlated conversations during, e.g., iterative behavior.
·Event
handlers can now be associated with scopes, to permit a process or scope to be
prepared to receive external events and requests concurrently with the main
activity of the process or scope. This is especially helpful for events and
requests that cannot be “scheduled” relative to the main activity,
but may occur at unpredictable times.
·The
Future Directions section has been dropped since this version forms the
starting point for a formal standards process, which will define those
directions.
5. Core Concepts and Usage Patterns
As noted in the introduction, we believe that the two usage
patterns of business protocol description and executable business process
description require a common core of process description concepts. In this
specification we clearly separate the core concepts from the extensions
required specifically for the two usage patterns. The BPEL4WSWS-BPEL
specification is focused on defining the common core, and adds only the
essential extensions required for each usage pattern. These extensions are
described in separate sections (Extensions for Executable Processes
and Extensions for Business Protocols).
In a number of cases, the behavior of a process in a certain combination of circumstances is undefined, e.g., when a variable is used before being initialized. In the definition of the core concepts we simply note that the semantics in such cases is not defined.
BPEL4WSWS-BPEL
takes it as a general principle that compliant implementations MAY choose to
perform static analysis to detect and reject process definitions that may have
undefined semantics. Such analysis is necessarily pessimistic and therefore
might in some cases prevent the use of processes that would not, in fact,
create situations with undefined semantics, either in specific uses or in any
use.
In the executable usage pattern for BPEL4WSWS-BPEL,
situations of undefined semantics always result in standard faults in the BPEL4WSWS-BPEL
namespace. These cases will be described as part of the Extensions
for Executable Processes in the specification. However, it is
important to note that BPEL4WSWS-BPEL
uses two exactly one standard
internal faults for its core control semantics,
namely, bpws:forcedTermination and bpws:joinFailure.
These This are
is the only two standard
faults that plays a role
in the core concepts of BPEL4WSWS-BPEL.
Of course, the occurrence of faults specified in WSDL portType definitions
during web service invocation is accounted for in the core concepts as well.
6. Defining a Business Process
6.1. Initial Example
Before describing the structure of business processes in
detail, this section presents a simple example of a BPEL4WSWS-BPEL
process for handling a purchase order. The aim is to introduce the most basic
structures and some of the fundamental concepts of the language.
The operation of the process is very simple, and is
represented in the following figure. Dotted lines represent sequencing. Free
grouping of sequences represents concurrent sequences. Solid arrows represent
control links used for synchronization across concurrent activities. Note that
this is not meant to be a definitive graphical notation for BPEL4WSWS-BPEL
processes. It is used here informally as an aid to understanding.
On receiving the purchase order from a customer, the process initiates three tasks concurrently: calculating the final price for the order, selecting a shipper, and scheduling the production and shipment for the order. While some of the processing can proceed concurrently, there are control and data dependencies between the three tasks. In particular, the shipping price is required to finalize the price calculation, and the shipping date is required for the complete fulfillment schedule. When the three tasks are completed, invoice processing can proceed and the invoice is sent to the customer.
The WSDL portType offered by the service to its customers
(purchaseOrderPT) is shown in the following WSDL document. Other WSDL
definitions required by the business process are included in the same WSDL
document for simplicity; in particular, the portTypes for the Web Services
providing price calculation, shipping selection and scheduling, and production
scheduling functions are also defined there. Observe that there are no bindings
or service elements in the WSDL document. A BPEL4WSWS-BPEL
process is defined "in the abstract" by referencing only the portTypes
of the services involved in the process, and not their possible deployments.
Defining business processes in this way allows the reuse of business process
definitions over multiple deployments of compatible services.
The partner link types included at the bottom of the WSDL
document represent the interaction between the purchase order service and each
of the parties with which it interacts (see Partner Link Types, Partner
Links, and Endpoint References). Partner link types can be used to
represent dependencies between services, regardless of whether a BPEL4WSWS-BPEL
business process is defined for one or more of those services. Each partner
link type defines up to two "role" names, and lists the portTypes
that each role must support for the interaction to be carried out successfully.
In this example, two partner link types, "purchasingLT" and
"schedulingLT", list a single role because, in the corresponding
service interactions, one of the parties provides all the invoked operations: The
"purchasingLT" partner link represents the connection between the
process and the requesting customer, where only the purchase order service
needs to offers a service operation ("sendPurchaseOrder"); the
"schedulingLT" partner link represents the interaction between the
purchase order service and the scheduling service, in which only operations of
the latter are invoked. The two other partner link types,
"invoicingLT" and "shippingLT", define two roles because
both the user of the invoice calculation and the user of the shipping service
(the invoice or the shipping schedule) must provide callback operations to
enable asynchronous notifications to be asynchronously sent
("invoiceCallbackPT" and "shippingCallbackPT" portTypes).
1234567890123456789012345678901234567890123456789012345678901234567890
1 2 3 4 5 6
<definitions targetNamespace="http://manufacturing.org/wsdl/purchase"
xmlns:sns="http://manufacturing.org/xsd/purchase"
xmlns:pos="http://manufacturing.org/wsdl/purchase"
xmlns="http://schemas.xmlsoap.org/wsdl/">
<types>
<xsd:schema > <xsd:import namespace="http://manufacturing.org/xsd/purchase"
schemalocation="http://manufacturing.org/xsd/purchase.xsd"/></xsd:schema>
</types>
<message name="POMessage">
<part name="customerInfo" type="sns:customerInfo"/>
<part name="purchaseOrder" type="sns:purchaseOrder"/>
</message>
<message name="InvMessage">
<part name="IVC" type="sns:Invoice"/>
</message>
<message name="orderFaultType">
<part name="problemInfo" element=”sns:OrderFault"/>
</message>
<message name="shippingRequestMessage">
<part name="customerInfo" element="sns:customerInfo"/>
</message>
<message name="shippingInfoMessage">
<part name="shippingInfo" element="sns:shippingInfo"/>
</message>
<message name="scheduleMessage">
<part name="schedule" element="sns:scheduleInfo"/>
</message>
<!-- portTypes supported by the purchase order process -->
<portType name="purchaseOrderPT">
<operation name="sendPurchaseOrder">
<input message="pos:POMessage"/>
<output message="pos:InvMessage"/>
<fault name="cannotCompleteOrder"
message="pos:orderFaultType"/>
</operation>
</portType>
<portType name="invoiceCallbackPT">
<operation name="sendInvoice">
<input message="pos:InvMessage"/>
</operation>
</portType>
<portType name="shippingCallbackPT">
<operation name="sendSchedule">
<input message="pos:scheduleMessage"/>
</operation>
</portType>
<!-- portType supported by the invoice services -->
<portType name="computePricePT">
<operation name="initiatePriceCalculation">
<input message="pos:POMessage"/>
</operation>
<operation name="sendShippingPrice">
<input message="pos:shippingInfoMessage"/>
</operation>
</portType>
<!-- portType supported by the shipping service -->
<portType name="shippingPT">
<operation name="requestShipping">
<input message="pos:shippingRequestMessage"/>
<output message="pos:shippingInfoMessage"/>
<fault name="cannotCompleteOrder"
message="pos:orderFaultType"/>
</operation>
</portType>
<!-- portType supported by the production scheduling process -->
<portType name="schedulingPT">
<operation name="requestProductionScheduling">
<input message="pos:POMessage"/>
</operation>
<operation name="sendShipingSchedule">
<input message="pos:scheduleMessage"/>
</operation>
</portType>
<plnk:partnerLinkType name="purchasingLT">
<plnk:role name="purchaseService">
<plnk:portTypename="pos:purchaseOrderPT"/>
</plnk:role>
</plnk:partnerLinkType>
<plnk:partnerLinkType name="invoicingLT">
<plnk:role name="invoiceService">
<plnk:portTypename="pos:computePricePT"/>
</plnk:role>
<plnk:role name="invoiceRequester">
<plnk:
portType name="pos:invoiceCallbackPT"/>
</plnk:role>
</plnk:partnerLinkType>
<plnk:partnerLinkType name="shippingLT">
<plnk:role name="shippingService">
<plnk: portType name="pos:shippingPT"/>
</plnk:role>
<plnk:role name="shippingRequester">
<plnk: portType name="pos:shippingCallbackPT"/>
</plnk:role>
</plnk:partnerLinkType>
<plnk:partnerLinkType name="schedulingLT">
<plnk:role name="schedulingService">
<plnk:portType name="pos:schedulingPT"/>
</plnk:role>
</plnk:partnerLinkType>
</definitions>
The business process for the order service is defined next. There are four major sections in this process definition:
·
The <variables> section defines the data variables
used by the process, providing their definitions in terms of WSDL message
types, XML Schema simple types (simple or
complex), or XML Schema elements. Variables allow processes to
maintain state data and process history based on messages exchanged.
· The <partnerLinks> section defines the different parties that interact with the business process in the course of processing the order. The four partnerLinks shown here correspond to the sender of the order (customer), as well as the providers of price (invoicingProvider), shipment (shippingProvider), and manufacturing scheduling services (schedulingProvider). Each partner link is characterized by a partner link type and a role name. This information identifies the functionality that must be provided by the business process and by the partner service for the relationship to succeed, that is, the portTypes that the purchase order process and the partner need to implement.
·
The <faultHandlers> section contains fault
handlers defining the activities that must be performed in response to faults
resulting from the invocation of the assessment and approval services. In BPEL4WSWS-BPEL,
all faults, whether internal or resulting from a service invocation, are
identified by a qualified name. In particular, each WSDL fault is identified in
BPEL4WSWS-BPEL
by a qualified name formed by the target namespace of the WSDL document in
which the relevant portType and fault are defined, and the ncname of the fault.
It is important to note, however, that because WSDL 1.1 does not require that
fault names be unique within the namespace where the operation is defined, all
faults sharing a common name and defined in the same namespace are
indistinguishable. In spite of this serious WSDL limitation, BPEL4WSWS-BPEL
provides a uniform naming model for faults, in the expectation that future
versions of WSDL will provide a better fault-naming model.
· The rest of the process definition contains the description of the normal behavior for handling a purchase request. The major elements of this description are explained in the section following the process definition.
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<process name="purchaseOrderProcess"
targetNamespace="http://acme.com/ws-bp/purchase"
xmlns="http://schemas.xmlsoap.org/ws/2004/03/business-process/"
xmlns:lns="http://manufacturing.org/wsdl/purchase">
<documentation xml:lang="EN">A simple example of a BPEL4WSWS-BPEL process for handling a purchase order.</documentation>
<partnerLinks>
<partnerLink name="purchasing"
partnerLinkType="lns:purchasingLT"
myRole="purchaseService"/>
<partnerLink name="invoicing"
partnerLinkType="lns:invoicingLT"
myRole="invoiceRequester"
partnerRole="invoiceService"/>
<partnerLink name="shipping"
partnerLinkType="lns:shippingLT"
myRole="shippingRequester"
partnerRole="shippingService"/>
<partnerLink name="scheduling"
partnerLinkType="lns:schedulingLT"
partnerRole="schedulingService"/>
</partnerLinks>
<variables>
<variable name="PO" messageType="lns:POMessage"/>
<variable name="Invoice" messageType="lns:InvMessage"/>
<variable name="POFault" messageType="lns:orderFaultType"/>
<variable name="shippingRequest" messageType="lns:shippingRequestMessage"/>
<variable name="shippingInfo" messageType="lns:shippingInfoMessage"/>
<variable name="shippingSchedule" messageType="lns:scheduleMessage"/>
</variables>
<faultHandlers>
<catch faultName="lns:cannotCompleteOrder" faultVariable="POFault" faultMessageType="lns:orderFaultType">
<reply partnerLink="purchasing"
portType="lns:purchaseOrderPT"
operation="sendPurchaseOrder"
variable="POFault"
faultName="cannotCompleteOrder"/>
</catch>
</faultHandlers>
<sequence>
<receive partnerLink="purchasing"
portType="lns:purchaseOrderPT"
operation="sendPurchaseOrder"
variable="PO">
</receive>
<flow>
<documentation> A parallel flow to handle shipping, invoicing and scheduling </documentation>
<links>
<link name="ship-to-invoice"/>
<link name="ship-to-scheduling"/>
</links>
<sequence>
<assign>
<copy>
<from variable="PO" part="customerInfo"/>
<to variable="shippingRequest"
part="customerInfo"/>
</copy>
</assign>
<invoke partnerLink="shipping"
portType="lns:shippingPT"
operation="requestShipping"
inputVariable="shippingRequest"
outputVariable="shippingInfo">
<sources> <source linkName="ship-to-invoice"/>
</sources>
</invoke>
<receive partnerLink="shipping"
portType="lns:shippingCallbackPT"
operation="sendSchedule"
variable="shippingSchedule">
<sources> <source linkName="ship-to-scheduling"/>
</sources>
</receive>
</sequence>
<sequence>
<invoke partnerLink="invoicing"
portType="lns:computePricePT"
operation="initiatePriceCalculation"
inputVariable="PO">
</invoke>
<invoke partnerLink="invoicing"
portType="lns:computePricePT"
operation="sendShippingPrice"
inputVariable="shippingInfo">
<targets> <target linkName="ship-to-invoice"/>
</targets>
</invoke>
<receive partnerLink="invoicing"
portType="lns:invoiceCallbackPT"
operation="sendInvoice"
variable="Invoice"/>
</sequence>
<sequence>
<invoke partnerLink="scheduling"
portType="lns:schedulingPT"
operation="requestProductionScheduling"
inputVariable="PO">
</invoke>
<invoke partnerLink="scheduling"
portType="lns:schedulingPT"
operation="sendShippingSchedule"
inputVariable="shippingSchedule">
<targets> <target linkName="ship-to-scheduling"/>
</targets>
</invoke>
</sequence>
</flow>
<reply partnerLink="purchasing"
portType="lns:purchaseOrderPT"
operation="sendPurchaseOrder"
variable="Invoice"/>
</sequence>
</process>
6.2. The Structure of a Business Process
This section provides a quick summary of the BPEL4WSWS-BPEL
syntax. It provides only a brief overview; the details of each language
construct are described in the rest of this document.
The basic structure of the language is:
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<process name="ncname" targetNamespace="uri"
queryLanguage="anyURI"?
expressionLanguage="anyURI"?
suppressJoinFailure="yes|no"?
abstractProcess="yes|no"?
xmlns="http://schemas.xmlsoap.org/ws/2004/03/business-process/">
<import namespace="uri" location="uri" importType="uri"/>*
<partnerLinks>?
<!-- Note: At least one role must be specified. -->
<partnerLink name="ncname" partnerLinkType="qname"
myRole="ncname"? partnerRole="ncname"?>+
</partnerLink>
</partnerLinks>
<partners>?<variables>?
<partner name="ncname">+
<partnerLink name="ncname"/>+
</partner>
</partners>
<variable name="ncname" messageType="qname"?
type="qname"? element="qname"?/>+
</variables>
<correlationSets>?
<correlationSet name="ncname" properties="qname-list"/>+
</correlationSets>
<faultHandlers>?
<!-- Note: There must be at least one fault handler or default. -->
<catch faultName="qname"? faultVariable="ncname"?
faultMessageType="qname"? faultElement="qname"?>*
activity
</catch>
<catchAll>?
activity
</catchAll>
</faultHandlers>
<eventHandlers>?
<!-- Note: There must be at least one onEvent or onAlarm handler. -->
<onEvent partnerLink="ncname" portType="qname"?
operation="ncname" messageType="qname" variable="ncname"
messageExchange="ncname"? >*
<correlations>?
<correlation set="ncname" initiate="yes|rendezvous|no"?/>+
</correlations>
activity
</onEvent>
<onAlarm>* ( <for expressionLanguage="anyURI"?>duration-expr</for> | <until expressionLanguage="anyURI"?>deadline-expr</until> )? <repeatEvery expressionLanguage="anyURI"?>duration-expr</repeatEvery>? activity
</onAlarm>
</eventHandlers>
activity
</process>
The top-level attributes are as follows:
·
queryLanguage. This attribute specifies the default
XML query language used for selection of nodes in assignment, property
definition, and other uses. The default value for this
attribute is: "urn:oasis:names:tc:wsbpel:2.0:sublang:xpath1.0", which
represents the usage of XPath 1.0 within WS-BPEL 2.0. The URI of the
corresponding XPath 1.0 specification is: The
default value for this attribute is XPath 1.0, represented by the URI of the
XPath 1.0 specification: http://www.w3.org/TR/1999/REC-xpath-19991116.
·
expressionLanguage. This attribute specifies the
expression language used in the process. The default value for this
attribute is: "urn:oasis:names:tc:wsbpel:2.0:sublang:xpath1.0", which
represents the usage of XPath 1.0 within WS-BPEL 2.0. The URI of the
corresponding XPath 1.0 specification is: The
default for this attribute is XPath 1.0, represented by the URI of the XPath
1.0 specification: http://www.w3.org/TR/1999/REC-xpath-19991116.
· suppressJoinFailure. This attribute determines whether the joinFailure fault will be suppressed for all activities in the process. The effect of the attribute at the process level can be overridden by an activity using a different value for the attribute. The default for this attribute is "no" at the process level. When this attribute is not specified for an activity, it inherits its value from its closest enclosing activity or from the process if no enclosing activity specifies this attribute.
· abstractProcess. This attribute specifies whether the process being defined is abstract (rather than executable). The default for this attribute is "no".
The value of the queryLanguage and expressionLanguage attributes on the
<process> element are global defaults and can be overridden on
specific activities like <assign> using the mechanisms defined later in
this specification. In addition the queryLanguage attribute is also
available for use in defining BPEL property aliases in WSDL. BPEL processors
MUST:
· statically determine which languages are referenced by queryLanguage or expressionLanguage attributes either in the BPEL process definition itself or in any BPEL property definitions in associated WSDLs and
· if any referenced language is unsupported by the BPEL processor then the processor MUST NOT process the submitted BPEL process definition.
Note that: <documentation> construct may be added to
virtually all BPEL4WSWS-BPEL
constructs as the formal way to annotate processes definition with human
documentation. Examples of <documentation> construct can be found in
previous section. Detailed description of <documention> is provided in
next section, as it is a part of “Language Extensibility”.
The token "activity" can be any of the following:
· <receive>
· <reply>
· <invoke>
· <assign>
· <throw>
·
<terminateexit>
· <wait>
· <empty>
· <sequence>
· <switch>
· <while>
· <pick>
· <flow>
· <scope>
· <compensate>
· <rethrow>
· <validate>
The syntax of each of these elements, except <terminateexit>,
<compensate> and <rethrow>, is considered in the following
paragraphs.
·
Although <terminateexit>
is permitted as an interpretation of the token activity, it is only available
in executable processes and as such is defined in the section on Extensions for Executable Processes.
· <compensate> activity can be used ONLY within a fault handler or a compensation handler (i.e. <catch>, <catchAll> and <compensationHandler> elements).
· <rethrow> activity can be used ONLY within a fault handler (i.e. <catch> and <catchAll> elements).
The <receive> construct allows the business process to do a blocking wait for a matching message to arrive. The portType attribute on the <receive> activity is optional. If the portType attribute is included for readability, the value of the portType attribute MUST match the portType value implied by the combination of the specified partnerLink and the role implicitly specified by the activity (See also partnerLink description in the next section). The optional messageExchange attribute is used to associate a <reply> activity with a <receive> activity. (For details of messageExchange, please see “Providing Web Service Operations” section).
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<receive partnerLink="ncname" portType="qname"? operation="ncname"
variable="ncname"? createInstance="yes|no"?
messageExchange="ncname"?
standard-attributes>
standard-elements
<correlations>?
<correlation set="ncname" initiate="yes|rendezvous|no"?/>+
</correlations> <fromPart part="ncname" toVariable="ncname"/>*
</receive>
The <reply> construct allows the business process to send a message in reply to a message that was received through a <receive>. The combination of a <receive> and a <reply> forms a request-response operation on the WSDL portType for the process. The portType attribute on the <reply> activity is optional. If the portType attribute is included for readability, the value of the portType attribute MUST match the portType value implied by the combination of the specified partnerLink and the role implicitly specified by the activity (See also partnerLink description in the next section). The optional messageExchange attribute is used to associate a <reply> activity with an inbound message activity, such as, <receive>, <onMessage> and <onEvent>. (For details of messageExchange, please see “Providing Web Service Operations” section).
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<reply partnerLink="ncname" portType="qname"? operation="ncname"
variable="ncname"? faultName="qname"?
messageExchange="ncname"?
standard-attributes>
standard-elements
<correlations>?
<correlation set="ncname" initiate="yes|rendezvous|no"?/>+
</correlations> <toPart part="ncname" fromVariable="ncname"/>*
</reply>
The <invoke> construct allows the business process to invoke a one-way or requestresponse operation on a portType offered by a partner. The portType attribute on the <invoke> activity is optional. If the portType attribute is included for readability, the value of the portType attribute MUST match the portType value implied by the combination of the specified partnerLink and the role implicitly specified by the activity (See also partnerLink description in the next section).
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<invoke partnerLink="ncname" portType="qname"? operation="ncname"
inputVariable="ncname"? outputVariable="ncname"?
standard-attributes>
standard-elements
<correlations>?
<correlation set="ncname" initiate="yes|rendezvous|no"?
pattern="in|out|out-in"/>+
</correlations>
<catch faultName="qname"faultName=“qname”? faultVariable="ncname"?
faultMessageType="qname"?>*
activity
</catch>
<catchAll>?
activity
</catchAll>
<compensationHandler>?
activity
</compensationHandler> <toPart part="ncname" fromVariable="ncname"/>* <fromPart part="ncname" toVariable="ncname"/>*
</invoke>
The <assign> construct can be used to update
the values of variables with new data. An <assign> construct can contain
any number of elementary assignments. The syntax of the assignment activity is:
The <assign> construct can be used to update
the values of variables with new data. An <assign> construct can contain
any number of elementary assignments, including <copy> assign elements or
data update operations defined as extension under other namespaces. The syntax
of the assignment activity is:
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<assign validate="yes|no"? standard-attributes>
standard-elements
(<copy> from-spec to-spec </copy> | <extensibleAssign> ...assign-element-of-other-namespace... </extensibleAssign>) +<copy>+
from-spec
to-spec
</copy>
</assign>
The <validate> construct can be used to validate the values of variables against their associated XML and WSDL data definition. The construct has a variables attribute, which points to the variables being validated. The syntax of the validate activity is:
<validate variables="ncnames" />
The <throw> construct generates a fault from inside the business process.
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<throw faultName="qname" faultVariable="ncname"? standard-attributes>
standard-elements
</throw>
The <wait> construct allows you to wait for a given time period or until a certain time has passed. Exactly one of the expiration criteria must be specified.
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<wait standard-attributes>
standard-elements ( <for expressionLanguage="anyURI"?>duration-expr</for> | <until expressionLanguage="anyURI"?>deadline-expr</until> )
</wait>
The <empty> construct allows you to insert a "no-op" instruction into a business process. This is useful for synchronization of concurrent activities, for instance.
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<empty standard-attributes>
standard-elements
</empty>
The <sequence> construct allows you to define a collection of activities to be performed sequentially in lexical order.
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<sequence standard-attributes>
standard-elements
activity+
</sequence>
The <switch> construct allows you to select exactly one branch of activity from a set of choices.
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<switch standard-attributes>
standard-elements
<case>+ <condition expressionLanguage="anyURI"?> ... bool-expr ... </condition>
activity
</case>
<otherwise>?
activity
</otherwise>
</switch>
The <while> construct allows you to indicate that an activity is to be repeated until a certain success criteria has been met.
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<while standard-attributes> standard-elements <condition expressionLanguage="anyURI"?> ... bool-expr ... </condition>
activity
</while>
The <pick> construct allows you to block and wait for a suitable message to arrive or for a time-out alarm to go off. When one of these triggers occurs, the associated activity is performed and the pick completes.
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<pick createInstance="yes|no"? standard-attributes>
standard-elements
<onMessage partnerLink="ncname" portType="qname"?
operation="ncname" variable="ncname"?
messageExchange="ncname"? >+
<correlations>?
<correlation set="ncname" initiate="yes|rendezvous|no"?/>+
</correlations> <fromPart part="ncname" toVariable="ncname"/>*
activity
</onMessage>
<onAlarm>* ( <for expressionLanguage="anyURI"?>duration-expr</for> | <until expressionLanguage="anyURI"?>deadline-expr</until> )? <repeatEvery expressionLanguage="anyURI"?>duration-expr</repeatEvery>?
activity
</onAlarm>
</pick>
The portType attribute on the <onMessage> activity is optional. If the portType attribute is included for readability, the value of the portType attribute MUST match the portType value implied by the combination of the specified partnerLink and the role implicitly specified by the activity (See also partnerLink description in the next section). The optional messageExchange attribute is used to associate a <reply> activity with a <onMessage> activity. (For details of messageExchange, please see “Providing Web Service Operations” section).
The <flow> construct allows you to specify one or more activities to be performed concurrently. Links can be used within concurrent activities to define arbitrary control structures.
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<flow standard-attributes>
standard-elements
<links>?
<link name="ncname">+
</links>
activity+
</flow>
The <scope> construct allows you to define a nested activity with its own associated variables, fault handlers, and compensation handler.
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<scope variableAccessSerializableisolated="yes|no" standard-attributes>
standard-elements <partnerLinks>? ... see above under <process> for syntax ...
</partnerLinks>
<variables>?
... see above under <process> for syntax ...
</variables>
<correlationSets>?
... see above under <process> for syntax ...
</correlationSets>
<faultHandlers>?
... see above under <process> for syntax ...
</faultHandlers>
<compensationHandler>?
... see above under <process> for syntax ...
</compensationHandler>
<terminationHandler>?
...
</terminationHandler>
<eventHandlers>?
...
</eventHandlers>
activity
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The <compensate> construct is used to invoke compensation on an inner scope that has already completed normally. This construct can be invoked only from within a fault handler or another compensation handler.
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<compensate scope="ncname"? standard-attributes>
standard-elements
</compensate>
Note that the "standard-attributes" referred to above are:
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name="ncname"? suppressJoinFailure="yes|no"?
where the default values are as follows:
· name: No default value (that is, the default is unnamed)
· suppressJoinFailure: When this attribute is not specified for an activity, it inherits its value from its closest enclosing activity or from the process if no enclosing activity specifies this attribute.
and that the "standard-elements" referred to above are:
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<targets>? <joinCondition expressionLanguage="anyURI"?>? ... bool-expr ... </joinCondition>
<target linkName="ncname" />+ <joinCondition expressionLanguage="anyURI"?>? ... bool-expr ... </joinCondition> </target></targets><sources>?
<source linkName="ncname">+ <transitionCondition expressionLanguage="anyURI"?>? ... bool-expr ... </transitionCondition> </source></sources>
6.3. Language Extensibility
BPEL4WSWS-BPEL
contains constructs that are generally sufficient for expressing abstract and
executable business processes. In some cases, however, it might be necessary to
“extend” the BPEL4WSWS-BPEL
language with additional constructs from other XML namespaces.
BPEL4WSWS-BPEL
supports extensibility by allowing namespace-qualified attributes to appear on
any BPEL4WSWS-BPEL
element and by allowing elements from other namespaces to appear within BPEL4WSWS-BPEL
defined elements. This is allowed in the XML Schema specifications for BPEL4WSWS-BPEL.
Extensions MUST NOT change the semantics of any element or
attribute from the BPEL4WSWS-BPEL
namespace.
The <documentation> construct is designed to be an
integral part of Language Extensibility. The content of <documentation>
are for human-consumption. Example types for those content are: plain text,
HTML and XHTML. Tool-implementation specific information (e.g. the graphical
layout details) should be added through elements and attributes of other
namespaces, using the general BPEL4WSWS-BPEL
Language Extensibility mechanism.
6.4. Document Linking
A BPEL4WSWS-BPEL
process definition relies on XML Schema and WSDL 1.1 for the definition of
datatypes and service interfaces. Process definitions also rely on other
constructs such as partner link types, message variable properties and property aliases
(defined later in this specification) which are defined within WSDL 1.1
documents using the WSDL 1.1 language extensibility feature.
The <import> element is used within a BPEL4WSWS-BPEL
process to explicitly indicate a dependency on external XML Schema or WSDL
definitions. Any number of <import> elements may appear as initial
children of the <process> element, before any other child element. Each
<import> element contains three mandatory attributes
· namespace. The namespace attribute specifies the URI namespace of the imported definitions.
· location. The location attribute contains a URI indicating the location of a document that contains relevant definitions in the namespace specified. The document located at the URI MUST contain definitions belonging to the same namespace as indicated by the namespace attribute.
· importType. The importType attribute identifies the type of document being imported by providing the URI of the encoding language. The value MUST be set to "http://www.w3.org/2001/XMLSchema" when importing XML Schema 1.0 documents, and to "http://schemas.xmlsoap.org/wsdl/" when importing WSDL 1.1 documents.
The presence of an <import> element should be interpreted as a hint to
the BPEL4WSWS-BPEL
processor. In particular, processors are not required to retrieve the imported
document from the location specified on the <import> element.
6.5 The Lifecycle of a Business Process
As noted in the introduction, the interaction model that is
directly supported by WSDL is essentially a stateless client-server model of
synchronous or uncorrelated asynchronous interactions. BPEL4WSWS-BPEL,
builds on WSDL by assuming that all external interactions of the business process
occur through Web Service operations. However, BPEL4WSWS-BPEL
business processes represent stateful long-running interactions in which each
interaction has a beginning, defined behavior during its lifetime, and an end.
For example, in a supply chain, a seller's business process might offer a
service that begins an interaction by accepting a purchase order through an
input message, and then returns an acknowledgement to the buyer if the order
can be fulfilled. It might later send further messages to the buyer, such as
shipping notices and invoices. The seller's business process remembers the
state of each such purchase order interaction separately from other similar
interactions. This is necessary because a buyer might be carrying on many
simultaneous purchase processes with the same seller. In short, a BPEL4WSWS-BPEL
business process definition can be thought of as a template for creating
business process instances.
The creation of a process instance in BPEL4WSWS-BPEL
is always implicit; activities that receive messages (that is, receive activities
and pick activities) can be annotated to indicate that the occurrence of that
activity causes a new instance of the business process to be created. This is
done by setting the createInstance attribute of such an activity to "yes". When a message is
received by such an activity, an instance of the business process is created if
it does not already exist (see Providing
Web Service Operations and Pick).
To be instantiated, each business process must contain at
least one such "start activity." That is, a receive/pick activity annotated with a createInstance="yes" attribute. See section 11.4 for more details on start
activities.This must be an initial activity in the sense that
there is no basic activity that logically precedes it in the behavior of the
process.
If more than one start activity is enabled concurrently, then all such activities must use at least one correlation set and must use the same correlation sets (see Correlation and the Multiple Start Activities example).
If a process contains exactly one start activity then the use of
correlation sets is unconstrained. If exactly one start activity is expected to
instantiate the process, the use of correlation sets is unconstrained. This
includes a pick with multiple onMessage branches; each such branch can use different correlation sets or no
correlation sets.
A business process instance is terminated in one of the following ways:
· When the activity that defines the behavior of the process as a whole completes. In this case the termination is normal.
· When a fault reaches the process scope, and is either handled or not handled. In this case the termination is considered abnormal even if the fault is handled and the fault handler does not rethrow any fault. A compensation handler is never installed for a scope that terminates abnormally.
·
When a process instance is explicitly terminated by
a an terminateexit activity (see Terminating
the Service Instance). In this case the termination
is abnormal.
The structure of the main processing section is defined by the outer <sequence> element, which states that the three activities contained inside are performed in order. The customer request is received (<receive> element), then processed (inside a <flow> section that enables concurrent behavior), and a reply message with the final approval status of the request is sent back to the customer (<reply>). Note that the <receive> and <reply> elements are matched respectively to the <input> and <output> messages of the "sendPurchaseOrder" operation invoked by the customer, while the activities performed by the process between these elements represent the actions taken in response to the customer request, from the time the request is received to the time the response is sent back (reply).
A receive activity for an inbound request/response operation is said to be open if that activity has been performed and no corresponding reply activity has been performed. If the process instance reaches the end of its behavior, and one or more receive activities remain open, then the status of the instance becomes undefined. This condition indicates a modeling error that was not detected by static analysis.
The example makes the implicit assumption that the customer request can be processed in a reasonable amount of time, justifying the requirement that the invoker wait for a synchronous response (because this service is offered as a request-response operation). When that assumption does not hold, the interaction with the customer is better modeled as a pair of asynchronous message exchanges. In that case, the "sendPurchaseOrder" operation is a one-way operation and the asynchronous response is sent by invoking a second one-way operation on a customer "callback" interface. In addition to changing the signature of "sendPurchaseOrder" and defining a new portType to represent the customer callback interface, two modifications need to be made in the preceding example to support an asynchronous response to the customer. First, the partner link type "purchasingLT" that represents the process-customer connection needs to include a second role ("customer") listing the customer callback portType. Second, the <reply> activity in the process needs to be replaced by an <invoke> on the customer callback operation.
The processing taking place inside the <flow> element consists of three <sequence> blocks running concurrently. The synchronization dependencies between activities in the three concurrent sequences are expressed by using "links" to connect them. The links are defined inside the flow and are used to connect a source activity to a target activity. (Note that each activity declares itself as the source or target of a link by using the nested <source> and <target> elements.) In the absence of links, the activities nested directly inside a flow proceed concurrently. In the example, however, the presence of two links introduces control dependencies between the activities performed inside each sequence. For example, while the price calculation can be started immediately after the request is received, shipping price can only be added to the invoice after the shipper information has been obtained; this dependency is represented by the link (named "ship-to-invoice") that connects the first call on the shipping provider ("requestShipping") with sending shipping information to the price calculation service ("sendShippingPrice"). Likewise, shipping scheduling information can only be sent to the manufacturing scheduling service after it has been received from the shipper service; thus the need for the second link ("ship-to-scheduling").
Observe that information is passed between the different activities in an implicit way through the sharing of globally visible data variables. In this example, the control dependencies represented by links are related to corresponding data dependencies, in one case on the availability of the shipper rates and in another on the availability of a shipping schedule. The information is passed from the activity that generates it to the activity that uses it by means of two global data variables ("shippingInfo" and "shippingSchedule").
Certain operations can return faults, as defined in their WSDL definitions. For simplicity, it is assumed here that the two operations return the same fault ("cannotCompleteOrder"). When a fault occurs, normal processing is terminated and control is transferred to the corresponding fault handler, as defined in the <faultHandlers> section. In this example the handler uses a <reply> element to return a fault to the customer (note the "faultName" attribute in the <reply> element).
Finally, it is important to observe how an assignment activity is used to transfer information between data variables. The simple assignments shown in this example transfer a message part from a source variable to a message part in a target variable, but more complex forms of assignments are also possible.
7. Partner Link Types, Partner Links, and Endpoint References
A very important, if not the most important, use case for BPEL4WSWS-BPEL
will be in describing cross-enterprise business interactions in which the
business processes of each enterprise interact through Web Service interfaces
with the processes of other enterprises. An important requirement for realistic
modeling of business processing in this environment is the ability to model the
required relationship with a partner process. WSDL already describes the
functionality of a service provided by a partner, at both the abstract and
concrete levels. The relationship of a business process to a partner is
typically peer-to-peer, requiring a two-way dependency at the service level. In
other words, a partner represents both a consumer of a service provided by the
business process and a provider of a service to the business process. This is
especially the case when the interactions are based on asynchronous messaging
rather than on remote procedure calls. The notion of Partner links is used to
directly model peer-to-peer conversational partner relationships. Partner links
define the shape of a relationship with a partner by defining the message and
port types used in the interactions in both directions. However, the actual
partner service may be dynamically determined within the process. BPEL4WSWS-BPEL
uses a notion of endpoint reference, manifested as a service reference
container (“bpws:service-ref”), to represent the dynamic data
required to describe a partner service endpoint.
It is important to emphasize that the notions of partner link
and endpoint reference used here are preliminary. The specification for these
concepts as they relate to Web Services is still evolving, and we expect
normative definitions for them to emerge in future. The BPEL4WSWS-BPEL
specification will be updated to conform to the expected future standards.
7.1. Partner Link Types
A partner link type characterizes the conversational relationship between two services by defining the "roles" played by each of the services in the conversation and specifying the portType provided by each service to receive messages within the context of the conversation. The following example illustrates the basic syntax of a partner link type declaration:
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<partnerLinkType name="BuyerSellerLink"
xmlns="http://schemas.xmlsoap.org/ws/2004/03/partner-link/">
<role name="Buyer">
< portType name="buy:BuyerPortType"/>
</role>
<role name="Seller">
< portType name="sell:SellerPortType"/>
</role>
</partnerLinkType>
Each role specifies exactly one WSDL portType.
In the common case, portTypes of the two roles originate from separate namespaces. However, in some cases, both roles of a partner link type can be defined in terms of portTypes from the same namespace. The latter situation occurs for partner link types that define "callback" relationships between services.
The partner link type definition can be a separate artifact independent of either service's WSDL document. Alternatively, the partner link type definition can be placed within the WSDL document defining the portTypes from which the different roles are defined.
The extensibility mechanism of WSDL 1.1 is used to define partnerLinkType as a new definition type to be placed as an immediate child element of a <wsdl:definitions> element in all cases. This allows reuse of the WSDL target namespace specification and, more importantly, its import mechanism to import portTypes. For cases where a partnerLinkType declaration is linking the portTypes of two different services, the partnerLinkType declaration can be placed in a separate WSDL document (with its own targetNamespace).
The syntax for defining a partnerLinkType is:
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<definitions name="ncname" targetNamespace="uri"
xmlns="http://schemas.xmlsoap.org/wsdl/">
...
<plnk:partnerLinkType name="ncname">
<plnk:role name="ncname" >
<plnk:portType name="qname"/>
</plnk:role>
<plnk:role name="ncname">?
<plnk: portType name="qname"/>?
</plnk:role>
</plnk:partnerLinkType>
...
</definitions>
This defines a partner link type in the namespace indicated by the value of the "targetNamespace" attribute of the WSDL document element. The portTypes identified within roles are referenced by using QNames as for all top-level WSDL definitions.
Note that in some cases it can be meaningful to define a partner link type containing exactly one role instead of two. That defines a partner linking scenario where one service expresses a willingness to link with any other service, without placing any requirements on the other service.
Examples of partnerLinkType declarations are found in various business process examples in this specification.
7.2. Partner Links
The services with which a business process interacts are
modeled as partner links in BPEL4WSWS-BPEL.
Each partner link is characterized by a partnerLinkType. More than one partner
link can be characterized by the same partnerLinkType. For example, a certain
procurement process might use more than one vendor for its transactions, but
might use the same partnerLinkType for all vendors.
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<partnerLinks>
<partnerLink name="ncname" partnerLinkType="qname"
myRole="ncname"? partnerRole="ncname"?>+
</partnerLink>
</partnerLinks>
Each partnerLink is named, and this name is used for all service interactions via that partnerLink. This is critical, for example, in correlating responses to different partnerLinks for simultaneous requests of the same kind (see Invoking Web Service Operations and Providing Web Service Operations).
The role of the business process itself is indicated by the attribute myRole and the role of the partner is indicated by the attribute partnerRole. In the degenerate case where a partnerLinkType has only one role, one of these attributes is omitted as appropriate.
Note that the partnerLink declarations specify the shape of
the relationships that the BPEL4WSWS-BPEL
process will employ in its behavior. Before operations on a partner's service
can be invoked via a partnerLink, the binding and communication data for the
partner service must be available. The relevant information about a partner
service can be set as part of business process deployment. This is outside the
scope of BPEL4WSWS-BPEL.
However, it is also possible to select and assign actual partner services
dynamically, and BPEL4WSWS-BPEL
provides the mechanisms to do so via assignment of endpoint references. In
fact, because the partners are likely to be stateful, the service endpoint
information needs to be extended with instance-specific information. BPEL4WSWS-BPEL
allows the endpoint references implicitly present in partnerLinks to be both
extracted and assigned dynamically, and also to be set more than once. See Assignment for the mechanisms used for dynamic
assignment of endpoint references to partner services.
A partnerLink can be declared within a process or scope element. The name of a partnerLink should be unique within its own scope. Access to partnerLink follows common lexical scoping rules, similar to the rules for variables. A partnerLink resolves to the nearest enclosing scope, regardless of the type of the partnerLink. If a local partnerLink declared in an enclosing scope, the local partnerLink will be used in local assignments and message sending/receiving activities. The lifecyle of a partnerLink is same as the lifecycle of the scope declaring the partnerLink. The initial binding information of a partnerLink can be set as a part of business process deployment, regardless whether it is declared on the process or scope element level.
7.3. Business Partners
While a partner link represents a conversational
relationship between two partner processes, relationships with a business
partner in general require more than a single conversational relationship to be
established. To represent the capabilities required from a business partner,
BPEL4WSWS-BPEL uses the partner element.
A partner is defined as a subset of the partner links of the process, as shown
in the example below.
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<partner name="SellerShipper"
xmlns="http://schemas.xmlsoap.org/ws/2004/03/partner-link/">
<partnerLink name="Seller"/>
<partnerLink name="Shipper"/>
</partner>Partner definitions are optional and need not cover
all the partner links defined in the process. From the process perspective a
partner definition introduces a constraint on the functionality that a business
partner is required to provide. In the example above, the partner definition
states that the same business partner (“SellerShipper”) is required
to provide the services associated with the the roles of seller and shipper.
Partner definitions MUST NOT overlap, that is, a partner link MUST NOT appear
in more than one partner definition.
The syntax for partner definitions is given below:
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<partners>
<partner name="ncname">+
<partnerLink name="ncname"/>+
</partner>
</partners>7.4. Endpoint References
WSDL makes an important distinction between portTypes and ports. PortTypes define abstract functionality by using abstract messages. Ports provide actual access information, including communication service endpoints and (by using extension elements) other deployment related information such as public keys for encryption. Bindings provide the glue between the two. While the user of a service must be statically dependent on the abstract interface defined by portTypes, some of the information contained in port definitions can typically be discovered and used dynamically.
The fundamental use of endpoint references is to serve as the
mechanism for dynamic communication of port-specific data for services. An
endpoint reference makes it possible in BPEL4WSWS-BPEL
to dynamically select a provider for a particular type of service and to invoke
their operations. BPEL4WSWS-BPEL
provides a general mechanism for correlating messages to stateful instances of
a service, and therefore endpoint references that carry instance-neutral port
information are often sufficient. However, in general it is necessary to carry
additional instance-identification tokens in the endpoint reference itself.
Every partner role in a partnerLink in a BPEL4WSWS-BPEL
process instance is assigned a unique endpoint reference in the course of the
deployment of the process or dynamically by an activity within the process.
Endpoint references associated with parterRole and myRole of
partnerLinks are manifested as service reference containers
(“bpws:service-ref”). This container is used as envelope to wrap
around the actual endpoint reference value, when a BPEL4WSWS-BPEL
process interacts the endpoint reference of a partnerLink. It provides
pluggability of different versions of service endpoint referencing schemes
being used within a BPEL program. The design pattern here is similar to those
of expression language, also known as open-content models.
The “bpws:service-ref” has a required
attribute called “reference-scheme” to denote the namespace URI of
the service endpoint reference scheme which is being used. The
“bpws:service-ref” has an optional attribute called
“reference-scheme” to to denote the URI of the reference
interpretation scheme of service endpoint, which is the child element of
bpws:service-ref.
Most likely, the URI of reference scheme and the namespace URI of the child element of bpws:service-ref are not necessarily the same. Typically, this optional attribute is used ONLY when the child element of the “bpws:service-ref” is ambiguous by itself. The optional attribute supplies further information to disambiguate the usage of the content. One example would be: different treatments of wsdl:service element, if wsdl:service is used as the endpoint reference.
If that attribute is not specified, use the namespace URI of the content element within the wrapper to determine the reference scheme of service endpoint.
If the attribute is specified, use the URI as the reference scheme of service endpoint and treat the content element within the wrapper accordingly. When the "reference-scheme" attribute is specified, the URI value is most likely different from the namespace URI of the content element for EPR.
When the BPEL container fails to interpret the combination of the "reference-scheme" attribute and the content element OR just the content element alone, a standard fault "bpws:unsupportedReference" must be thrown.
When BPEL4WSWS-BPEL
users interact and manipulate endpoint references of partnerLinks, The
“bpws:service-ref” element are NOT exposed to BPEL4WSWS-BPEL
users in all cases. For example, when people try to do an assignment from the
endpoint reference of myRole of partnerLink-A to that of partnerRole of
partner-B. On the contrary, if people try to do assignment from a messageType
or element based variable through expression or from a service-ref literal-form
of from-spec, the “bpws:service-ref” is visible to BPEL4WSWS-BPEL
users.
8. VariableMessage Properties
8.1. Motivation
8.1.1 Motivation for Message Properties
The data in a message consists conceptually of two parts: application data and protocol relevant data, where the protocols can be business protocols or infrastructure protocols providing higher quality of service. An example of business protocol data is the correlation tokens that are used in correlation sets (see Correlation). Examples of infrastructure protocols are security, transaction, and reliable messaging protocols. The business protocol data is usually found embedded in the application-visible message parts, whereas the infrastructure protocols almost always add implicit extra parts to the message types to represent protocol headers that are separate from application data. Such implicit parts are often called message context because they relate to security context, transaction context, and other similar middleware context of the interaction. Business processes might need to gain access to and manipulate both kinds of protocol-relevant data. The notion of message properties is defined as a general way of naming and representing distinguished data elements within a message, whether in application-visible data or in message context. For a full accounting of the service description aspects of infrastructure protocols, it is necessary to define notions of service policies, endpoint properties, and message context. This work is outside the scope of
BPEL4WSWS-BPEL.
Message properties are defined here in a sufficiently general way to cover
message context consisting of implicit parts, but the use in this specification
focuses on properties embedded in application-visible data that is used in the
definition of business protocols and abstract business processes.
8.1.2 Motivation for Variable Properties
Message properties are an instance of a more generic mechanism, variable properties. All variables in BPEL can have properties defined on them. Properties are useful on non-message variables as a way to isolate the BPEL process’s logic from the details of a particular variable’s definition. Using properties a BPEL process can isolate its variable initialization logic in one place and then set and get properties on that variable in order to manipulate it. If the variable’s definition is later changed the bulk of the BPEL process definition that manipulates that variable can remain unchanged.
8.2. Defining Properties
A property definition creates a globally unique name and
associates it with an XML Schema simple type..
The intent is not to create a new type. The intent is to create a name that has
greater significance than the type itself. For example, a sequence number can
be an integer, but the integer type does not convey this significance, whereas
a globally named sequence-number property does. Properties can occur anywhere
in a message, including in the message contextvariable.
A typical use for a property in BPEL4WSWS-BPEL
is to name a token for correlation of service instances with messages. For
example, a social security number might be used to identify an individual
taxpayer in a long-running multiparty business process regarding a tax matter.
A social security number can appear in many different message types, but in the
context of a tax-related process it has a specific significance as a taxpayer
ID. Therefore a global name is given to this use of the type by defining a
property, as in the following example:
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<definitions name="properties"
targetNamespace="http://example.com/properties.wsdl"
xmlns:tns="http://example.com/properties.wsdl"
xmlns:txtyp="http://example.com/taxTypes.xsd"
xmlns="http://schemas.xmlsoap.org/wsdl/">
<! -- import schema taxTypes.xsd -- > <!-- define a correlation property -->
<bpws:property name="taxpayerNumber"
type="txtyp:SSN"/>
...
</wsdl:definitions>
TIn correlation, the
property name must have global significance to be of any use. Properties such
as price, risk, response latency, and so on, which are used in conditional
behavior in a business process, have similar global and public significance. It
is likely that they will be mapped to multiple messages, and therefore they
need to be globally named as in the case of correlation properties. Such
properties are essential, especially in abstract processes.
Even in the general case of properties on XML typed WS-BPEL variables the property name should maintain its generic nature. The name is intended to identify a certain kind of value, often with an implied semantic. Any variable the property is available on is therefore expected to provide a value that meets not just the syntax of the property definition but also its semantics.
The WSDL extensibility mechanism is used to define properties so that the target namespace and other useful aspects of WSDL are available.
The BPEL4WSWS-BPEL
standard namespace, "http//schemas.xmlsoap.org/ws/2004/03/business-process/", is used for property definitions. The syntax for a property
definition is a new kind of WSDL definition as follows:
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<wsdl:definitions name="ncname">
<bpws:property name="ncname" type="qname"/>
...
</wsdl:definitions>
Properties used in business protocols are typically embedded in application-visible message data. The notion of aliasing is introduced to map a global property to a field in a specific message part. The property name becomes an alias for the message part and location, and can be used as such in Expressions and Assignment in abstract business processes. As an example, consider the following WSDL message definition:
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<definitions name="messages"
targetNamespace="http://example.com/taxMessages.wsdl"
xmlns:txtyp="http://example.com/taxTypes.xsd"
xmlns="http://schemas.xmlsoap.org/wsdl/">
<!-- define a WSDL application message --> <message name="taxpayerInfo"> <part name="identification" element="txtyp:socialsecnumber"/> </message> ...</definitions>
The definition of a global property and its location in a particular field of the message are shown in the next WSDL fragment:
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<definitions name="properties"
targetNamespace="http://example.com/properties.wsdl"
xmlns:tns="http://example.com/properties.wsdl"
xmlns:txtyp="http://example.com/taxTypes.xsd"
xmlns:txmsg="http://example.com/taxMessages.wsdl"
xmlns="http://schemas.xmlsoap.org/wsdl/">
<!-- define a correlation property -->
<bpws:property name="taxpayerNumber" type="txtyp:SSN"/>
...
<bpws:propertyAlias propertyName="tns:taxpayerNumber"
messageType="txmsg:taxpayerInfo" part="identification">
<query> /txtyp:socialsecnumber </query>
</bpws:propertyAlias>
</definitions>
The bpws:propertyAlias defines a globally named property tns:taxpayerNumber as an alias for a location in the identification part of the message type txmsg:taxpayerInfo.
The syntax for a propertyAlias definition is:
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<definitions name="ncname"
...
>
<bpws:propertyAlias propertyName="qname"
messageType="qname" part="ncname"> <query queryLanguage="anyURI"?>? ... queryString ... </query> </bpws:propertyAlias>
...
</wsdl:definitions>
The interpretation of the message and part attributes, as well as the <query> element is the same as in the corresponding from-spec in copy assignments (see Assignment).
8.3 Defining Property Aliases
Properties used in business protocols are typically embedded in application-visible variables. The notion of aliasing is introduced to map a global property to a field in a specific message part or variable value. The property name becomes an alias for the message part and/or location, and can be used as such in Expressions and Assignment in abstract business processes. As an example, consider the following WSDL message definition:
<definitions name="messages"
targetNamespace="http://example.com/taxMessages.wsdl"
xmlns:txtyp="http://example.com/taxTypes.xsd"
xmlns="http://schemas.xmlsoap.org/wsdl/">
<!-- define a WSDL application message -->
<message
name="taxpayerInfoMsg">
<part name="identification"
element="txtyp:taxPayerInfoElem"/>
</message>
...
</definitions>
The definition of a global property and its location in a particular field of the message are shown in the next WSDL fragment:
<definitions name="properties"
targetNamespace="http://example.com/properties.wsdl"
xmlns:tns="http://example.com/properties.wsdl"
xmlns:txtyp="http://example.com/taxTypes.xsd"
xmlns:txmsg="http://example.com/taxMessages.wsdl"
xmlns="http://schemas.xmlsoap.org/wsdl/">
<!-- define a correlation property
-->
<bpws:property
name="taxpayerNumber" type="txtyp:SSN"/>
...
<bpws:propertyAlias
propertyName="tns:taxpayerNumber"
messageType="txmsg:taxpayerInfoMsg"
part="identification">
<query>
/txtyp:taxPayerInfoElem/txtyp:socialsecnumber
</query>
</bpws:propertyAlias>
<bpws:propertyAlias propertyName="tns:taxpayerNumber"
element="txtyp:taxPayerInfoElem">
<query>
/txtyp:taxPayerInfoElem/txtyp:socialsecnumber
</query>
</bpws:propertyAlias>
</definitions>
The first bpws:propertyAlias defines a globally named property tns:taxpayerNumber as an alias for a location in the identification part of the message type txmsg:taxpayerInfo.
The second bpws:propertyAlias provides a second definition for the same globally named property tns:taxpayerNumber but this time as an alias for a location inside of the element txtyp:taxPayerInfoElem.
The presence of both aliases means that it is possible to retrieve the social security number from both a variable holding a message of messageType txmsg:taxpayerInfo as well as an element defined using txtyp:taxPayerInfoElem.
The syntax for a propertyAlias definition is:
<definitions name="ncname"
... >
<bpws:propertyAlias propertyName="qname"
messageType="qname"? part="ncname"?
type="qname"? element="qname"?>
<query queryLanguage="anyURI"?>?
... queryString ...
</query>
</bpws:propertyAlias>
...
</wsdl:definitions>
A property alias element MUST use one of the three following combinations of attributes:
- messageType and part,
- type or
- element.
When a propertyAlias is used with the messageType/part combination then the property MUST be available on all BPEL variables where the qname value used in the messageType attribute on the declaration of both the variable and the propertyAlias are identical. The part attribute and query element is applied against the BPEL messageType variable to either set or get the property variable in the same way that the part attribute and query element are used in the first from and to specs in copy assignments.
If a propertyAlias is used with a type attribute then the property MUST be available on all BPEL variables where the qname value used in the type attribute on the declaration of both the variable and the propertyAlias are identical. The query is applied against the BPEL variable to either set or get the property variable in the same way that the query is used in the first from and to specs in copy assignments when applied against BPEL variables defined using a type.
If a propertyAlias is used with an element attribute then the property MUST be available on all BPEL variables where the qname value used in the element attribute on the declaration of both the variable and the propertyAlias are identical. The query is applied against the BPEL variable to either set or get the property variable in the same way that the query is used in the first from and to specs in copy assignments when applied against BPEL variables defined using an element definition.
A BPEL process MUST NOT be accepted for processing if it defines two or more propertyAlias’s for the same property name and BPEL variable type. For example, it is not legal to define two propertyAlias’s for the property taxpayernumber and the messageType txmsg:taxpayerInfoMsg. The same logic would prohibit having two propertyAliases on the same element qname and property name value or two propertyAliases on the same type qname and property name value.
9. Data Handling
Business processes model stateful interactions. The state
involved consists of messages received and sent as well as other relevant data
such as time-out values. The maintenance of the state of a business process
requires the use of state variables, which are called variables in BPEL4WSWS-BPEL.
Furthermore, the data from the state needs to be extracted and combined in
interesting ways to control the behavior of the process, which requires data
expressions. Finally, state update requires a notion of assignment. BPEL4WSWS-BPEL
provides these features for XML data types and WSDL message types. The XML
family of standards in these areas is still evolving, and using the process-level
attributes for query and expression languages provides for the incorporation of
future standards.
The extensions required for abstract and executable processes
are concentrated in the datahandling feature set. Executable processes are
permitted to use the full power of data selection and assignment but are not
permitted to use nondeterministic values. Abstract processes are restricted to
limited manipulation of values contained in variablemessage properties but are permitted
to use nondeterministic values to reflect the consequences of hidden private
behavior. Detailed differences are specified in the following sections.
9.1. Expressions
BPEL4WSWS-BPEL
uses several types of expressions. The kinds of expressions used are as follows
(relevant usage contexts are listed in parentheses):
· Boolean-valued expressions (transition conditions, join conditions, while condition, and switch cases)
· Deadline-valued expressions ("until" attribute of onAlarm and wait)
· Duration-valued expressions ("for" attribute of onAlarm and wait, repeatEvery attribute of onAlarm)
· General expressions (assignment)
BPEL4WSWS-BPEL
provides an extensible mechanism for the language used in these expressions.
The language is specified by the expressionLanguage attribute of the process element. In
addition, language constructs that require or allow conditional expressions
(such as “switch”, “while” and others) provide the
ability to override the default expression language for individual expressions.
Compliant implementations of the current version of BPEL4WSWS-BPEL
MUST support the use of XPath 1.0 as the expression language. XPath 1.0 is
indicated by the default value of the expressionLanguage
attribute, which is:
urn:oasis:names:tc:wsbpel:2.0:sublang:xpath1.0
which represents the usage of XPath 1.0 within WS-BPEL 2.0. The URI of the
corresponding XPath 1.0 specification is:
http://www.w3.org/TR/1999/REC-xpath-19991116
Given an expression language, it must be possible to query data from variables, to extract property values, and to query the status of links from within expressions. This specification defines those functions for XPath 1.0 only, and it is expected that other expressionlanguage bindings will provide equivalent functionality. The rest of this section is specific to XPath 1.0.
BPEL4WSWS-BPEL
introduces several extension functions to XPath's built-in functions to enable
XPath 1.0 expressions to access information from the process. The extensions
are defined in the standard BPEL4WSWS-BPEL
namespace http://schemas.xmlsoap.org/ws/2004/03/businessprocess/.
The prefix "bpws:" is associated with this namespace.
Any qualified names used within XPath expressions are resolved
by using namespace declarations currently in scope in the BPEL4WSWS-BPEL
document at the location of the expression.
The arguments to all XPath functions defined in this specification MUST be given as quoted strings. The previous requirement MUST be statically enforced. It is therefore illegal to pass into a BPEL XPath function any XPath variables, the output of XPath functions, a XPath location path or any other value that is not a quoted string. This means, for example, that getVariableProperty("varA","propB") meets the previous requirement while getVariableProperty( $varA, string(getVariableProperty("varB","propB") ) does not. Note that the previous requirement institutes a restriction which does not exist in the XPath standard.
The following functions are defined by this specification:
bpws:getVariableProperty ('variableName', 'propertyName')
This function extracts global property values from variables.
The first argument names the source variable for the data and the second is the
qualified name (QName) of the global property to select from that variable (see Message Propertiessee Variable Properties). If the referenced property is not defined or if
there does not exist a propertyAlias to associate the property with the
referenced variable then the semantics of the process is undefined. If the given property does not appear in any of the
parts of the variable's message type, then the semantics of the process is
undefined. The return value of this function is calculated by
applying the appropriate propertyAlias for the requested property to the
current value of the submitted variable. If the application of the XPATH in the
propertyAlias to the variable value results in a response that contains
anything other than exactly one information item and/or a collection of
Character information items then a bpws:selectionFailure fault MUST be thrownthe
semantics of the process is undefined.The
return value of this function is a node set containing the single node
representing the property. If the given property definition selects a node set
of a size other than one, then the semantics of the process is undefined.
bpws:getLinkStatus ('linkName')
This function returns a Boolean indicating the status of the link (see Link Semantics). If the status of the link is positive the value is true, and if the status is negative the value is false. This function MUST NOT be used anywhere except in a join condition. The linkName argument MUST refer to the name of an incoming link for the activity associated with the join condition. These restrictions MUST be statically enforced.
These BPEL4WSWS-BPEL-defined
extension functions are available for use within all XPath 1.0 expressions.
The syntax of XPath 1.0 expressions for BPEL4WSWS-BPEL
is considered in the following paragraphs.
9.1.1. Boolean Expressions
These are expressions that conform to the XPath 1.0 Expr production where the evaluation results in Boolean values.
9.1.2. Deadline-Valued Expressions
These are expressions that conform to the XPath 1.0 Expr production where the evaluation results in values that are of the XML Schema types dateTime or date. Note that XPath 1.0 is not XML Schema aware. As such, none of the built-in functions of XPath 1.0 are capable of producing or manipulating dateTime or date values. However, it is possible to write a constant (literal) that conforms to XML Schema definitions and use that as a deadline value or to extract a field from a variable (part) of one of these types and use that as a deadline value. XPath 1.0 will treat that literal as a string literal, but the result can be interpreted as a lexical representation of a dateTime or date value.
9.1.3. Duration-Valued Expressions
These are expressions that conform to the XPath 1.0 Expr production where the evaluation results in values that are of the XML Schema type duration. The preceding discussion about XPath 1.0's XML Schema unawareness applies here as well.
9.1.4. General Expressions
These are expressions that conform to the XPath 1.0 Expr production where the evaluation results in any XPath value type (string, number, or Boolean).
Expressions with operators are restricted as
follows:
·All
numeric values including arbitrary constants are permitted with the equality or
relational operators (<, <=, =, !=, >=, >").
·Values
of integral (short, int, long, unsignedShort, and so on) type including
constants are permitted in numeric expressions, provided that only integer
arithmetic is performed. In practice, this means that division is disallowed.
It is difficult to enforce this restriction in XPath 1.0 because XPath 1.0
lacks integral support for types. The restriction should be taken as a
statement of intent that will be enforced in the future when expression
languages with more refined type systems become available.
·Only
equality operators (=, !=) are permitted when used with values of string type
including constants.
These restrictions reflect XPath 1.0 syntax and
semantics. Future alternative standards in this space are expected to provide
stronger type systems and therefore support more nuanced constraints. The
restrictions are motivated by the fact that XPath general expressions are meant
to be used to perform business protocol-related computation such as retry
loops, line-item counts, and so on, that must be transparent in the process
definition. They are not meant to provide arbitrary computation. This is the
motivation for the constraint that numerical expressions deal only with integer
computation, and for disallowing arbitrary string manipulation through
expressions.
9.2. Variables
Business processes specify stateful interactions involving the exchange of messages between partners. The state of a business process includes the messages that are exchanged as well as intermediate data used in business logic and in composing messages sent to partners.
Variables provide the means for holding messages that constitute the state of a business process. The messages held are often those that have been received from partners or are to be sent to partners. Variables can also hold data that are needed for holding state related to the process and never exchanged with partners.
The type of each variable may be a WSDL message type, an XML Schema type (simple or complex)an XML Schema simple type or an XML
Schema element. The syntax of the variables declaration is:
<variables>
<variable name="ncname" messageType="qname"?
type=”qname”? element=”qname”?/>+
</variables>
The name of a variable should be unique within its own scope. Variable access follows common lexical scoping rules. A variable resolves to the nearest enclosing scope, regardless of the type of the variable. If a local variable has the same name as a variable defined in an enclosing scope, the local variable will be used in local assignments and/or getVariableProperty functions.
The messageType, type or element attributes are used to specify the type of a variable. Exactly one of
these attributes must be used. Attribute messageType refers to a WSDL
message type definition. Attribute type refers to an XML Schema
type (simple or complex).an XML Schema
simple type. Attribute element refers to an XML Schema element. The infoset for a complexType variable consists of
a document information item that contains exactly one child, an element
information item which is pointed at by the document element property. The
properties of the document element, specifically the namespace name and local
name properties, are undefined by this specification, as such an implementation
MUST specify whatever legal property values it likes. However the children of
the document element MUST exclusively consist of the complexType values
assigned to the variable. An XML Schema
complex type must beassociated with an element to be used by a BPEL4WSWS-BPEL
variable.
An example of a variable declaration using a message type declared in a WSDL document with the targetNamespace "http://example.com/orders":
<variable xmlns:ORD="http://example.com/orders"
name="orderDetails" messageType="ORD:orderDetails"/>
Variables associated with message types can be specified as input or output variables for invoke, receive, and reply activities (see Invoking Web Service Operations and Providing Web Service Operations). When an invoke operation returns a fault message, this causes a fault in the current scope. The fault variable in the corresponding fault handler is initialized with the fault message received (see Scopes and Fault Handlers).
Each variable is declared within a scope and is said to belong to that scope. Variables that belong to the global process scope are called global variables. Variables may also belong to other, non-global scopes, and such variables are called local variables. Each variable is visible only in the scope in which it is defined and in all scopes nested within the scope it belongs to. Thus, global variables are visible throughout the process. It is possible to "hide" a variable in an outer scope by declaring a variable with an identical name in an inner scope. These rules are exactly analogous to those in programming languages with lexical scoping of variables.
A global variable is in an uninitialized state at the beginning of a process. A local variable is in an uninitialized state at the start of the scope it belongs to. Note that non-global scopes in general start and complete their behavior more than once in the lifetime of the process instance they belong to. Variables can be initialized by a variety of means including assignment and receiving a message. Variables can be partially initialized with property assignment or when some but not all parts in the message type of the variable are assigned values.
Values stored in variables can be mutated during the course of process execution. The validate activity can be used explicitly to ensure that values of variables are valid against their associated XML data definition, including XML Schema simple type, complex type, element definition and XML definitions of WSDL parts. The validate activity has a variables attribute, which has a NCNAMES value. The variables attribute points to the variables being validated. The syntax of the validate activity is:
<validate variables="ncnames" />
When one or more variables are invalid against their correponding XML definition, a standard fault of "bpws:invalidVariables" fault MUST be thrown.
A BPEL implementation MAY provide a mechanism to
turn on/off any explicit validation. E.g. validate activity.
9.3. Assignment
Copying data from one variable to another is a common task
within a business process. The assign activity can be used to copy data from one variable to another, as
well as to construct and insert new data using expressions. The use of
expressions is primarily motivated by the need to perform simple computation
(such as incrementing sequence numbers) that is required for describing business
protocol behavior. Expressions operate on message selections, properties, and
literal constants to produce a new value for a variable property or selection. This
activity can also be used to copy endpoint references to and from partner
links. Finally, it is possible to include extensible data manipulation
operations defined as extension elements under namespaces different from the
WS-BPEL namespace.Finally, this activity can also be used to copy
endpoint references to and from partner links.
The assign assign contains one or more elementary assignments.
<assign validate="yes|no"? standard-attributes>
standard-elements
(<copy> from-spec to-spec </copy> | <extensibleAssign> ...assign-element-of-other-namespace... </extensibleAssign>) +<copy>+
from-spec
to-spec
</copy>
</assign>
The assign activity allows copying a
type-compatible value from the source ("from-spec") to the
destination ("to-spec"), using the <copy> element.The assign
activity copies a type-compatible value from the source ("from-spec")
to the destination ("to-spec"). The from-spec MUST be
one of the following forms except for the opaque form available in abstract
processes:
<from variable="ncname" part="ncname"?/>
<from partnerLink="ncname" endpointReference="myRole|partnerRole"/>
<from variable="ncname" property="qname"/>
<from> <expression expressionLanguage="anyURI"?>general-expr</expression> </from>
<from> <literal> ... literal value ... </literal> </from><from> ... literal value ... </from><from><service-ref reference-scheme="anyURI"?> ... EPR value ... </service-ref></from>
The to-spec MUST be one of the following forms:
<to variable="ncname" part="ncname"?/>
<to partnerLink="ncname"/>
<to variable="ncname" property="qname"/>To-specs MUST identify Lvalues so that assignment is possible. A Lvalue, in the context of XPATH 1.0, is a node-list containing a single node from the value store, e.g. variable, partnerLink, property, etc., identified by the to-spec that is to be replaced by the assignment. If a to-spec does not identify a Lvalue then a bpws:selectionFailure MUST be thrown.To-specs MUST identify Lvalues so that assignment is possible. A Lvalue, in the context of XPATH 1.0, is a node-list containing a single node from the value store, e.g. variable, partnerLink, property, etc., identified by the to-spec that is to be replaced by the assignment. If a to-spec does not identify a Lvalue then a bpws:selectionFailure MUST be thrown.
In the first from-spec and to-spec variants the variable attribute provides the name of a variable. If
the type of the variable is a WSDL messge type the optional part attribute MAY
be used to provide the name of a part within that variable. When the variable
is defined using XML Schema simple type XML Schema types (simple or complex)or
element, the part attribute MUST NOT be used.
The second from-spec and to-spec variants allow dynamic manipulation of the endpoint references associated with partner links. The value of the partnerLink attribute is the name of a partnerLink declared in the process. In the case of from-specs, the role must also be specified because a process might need to communicate an endpoint reference corresponding to either its own role or the partner's role within the partnerLink. The value “myRole” means that the endpoint reference of the process with respect to that partnerLink is the source, while the value “partnerRole” means that the partner’s endpoint reference for the partnerLink is the source. For the to-spec, the assignment is only possible to the partnerRole, hence there is no need to specify the role. The type of the value used in partnerLink-style from/to-specs is always an endpoint reference (see Partner Link Types, Partner Links, and Endpoint References).
The third from-spec and to-spec variants allow explicit manipulation of message varialbe properties (see Message Propertiessee Varialbe Properties) occurring in variables. The
property value generated by the from-spec is generated in the same manner and
with the same faults as the value returned by the getVariableProperty()
function. The property forms are especially useful for abstract
processes, because they provide a way to clearly define how distinguished data
elements in messages are being used.
The fourth ("expression") from-spec variant allows processes to perform simple computations on properties and variables (for example, increment a sequence number).
The fifth from-spec variant allows a literal value to be given as the source value to assign to a destination. The type of the literal value MUST be the type of the destination (to-spec). The type of the literal value MAY be optionally indicated inline with the value by using XML Schema's instance type mechanism (xsi:type).
The fifth from-spec variant allows a literal value to be given as the source value to assign to a destination. The type of the literal value MUST be the type as the destination (to-spec). The literal value to be assigned is included within a <literal> element in order prevent conflicts with standard extensibility elements under <from>; note that the <literal> element itself does not allow standard extensibility. The type of the literal value MAY be optionally indicated inline with the value by using XML Schema's instance type mechanism (xsi:type).
The sixth ("service-ref") from-spec variant allows an endpoint reference value to be assign to a partnerLink directly, when used with the second variant of the to-spec. (see Partner Link Types, Partner Links, and Endpoint References).
In addition to <copy> specifications, other extensibility data manipulation elements MAY be included in an assign activity, inside an <extensibleAssign> element. The extensible assign elements MUST belong to a namespace different from the WS-BPEL namespace. Note also that the <extensibleAssign> element does not allow standard extensibility.
An optional validate attribute can be used with the assign activity. When validate is set to "yes", it can be seen as a convenient marco of a combination of assign and validate activities. With validate="yes", the assign activity will validate all the variables being modified by the activity. E.g. variables being referenced in the to-specs.
The logic related to the validate attribute and assignment logic is considered as one unit of operation. If the "validate" parts of the operation fail, the whole assignment operation is considered as a failure also. When one of the variables is invalid against its correponding XML definition, a standard fault of "bpws:invalidVariables" fault MUST be thrown. Please note that the default of the validate attribute is "no".
A BPEL implementation MAY provide a mechanism to turn on/off any explicit validation. E.g. validate attribute at assign.
9.3.1. Type Compatibility in Assignment
For an assignment to be valid, the data referred to by the from and to specifications MUST be of compatible types. The following points make this precise:
· The from-spec is a variable of a WSDL message type and the to-spec is a variable of a WSDL message type. In this case both variables MUST be of the same message type, where two message types are said to be equal if their qualified names are the same.
· The from-spec is a variable of a WSDL message type and the to-spec is not, or vice versa. This is not legal because parts of variables, selections of variable parts, or endpoint references cannot be assigned to/from variables of WSDL message types directly.
· In all other cases, the types of the source and destination are XML Schema types or elements, and the constraint is that the source value MUST possess the element or type associated with the destination. Note that this does not require the types associated with the source and destination to be the same. In particular, the source type MAY be a subtype of the destination type. In the case of variables defined by reference to an element, moreover, both the source and the target MUST be the same element.
The semantics of a process in which any of the matching constraints above is violated is undefined.
9.3.2. Assignment Example
The example assumes the following complex type definition in the namespace "http://tempuri.org/bpws/example":
<complexType name="tAddress">
<sequence>
<element name="number" type="xsd:int"/>
<element name="street" type="xsd:string"/>
<element name="city" type="xsd:string"/>
<element name="phone">
<complexType>
<sequence>
<element name="areacode" type="xsd:int"/>
<element name="exchange" type="xsd:int"/>
<element name="number" type="xsd:int"/>
</sequence>
</complexType>
</element>
</sequence>
</complexType>
<element name = “address” type = “tAddress”/>
Assume that the following WSDL message definition exists for the same target namespace:
<message name="person" xmlns:x="http://tempuri.org/bpws/example">
<part name="full-name" type="xsd:string"/>
<part name="address" element="x:address"/>
</message>
Also assume the following BPEL4WSWS-BPEL
variable declarations:
<variable name="c1" messageType="x:person"/>
<variable name="c2" messageType="x:person"/>
<variable name="c3" element="x:address"/>
The example illustrates copying one variable to another as well as copying a variable part to a variable of compatible element type:
<assign>
<copy>
<from variable="c1"/>
<to variable="c2"/>
</copy>
<copy>
<from variable="c1" part = “address”/>
<to variable="c3"/>
</copy>
</assign>
10. Correlation
The information provided so far suggests that the target for messages that are delivered to a business process service is the WSDL port of the recipient service. This is an illusion because, by their very nature, stateful business processes are instantiated to act in accordance with the history of an extended interaction. Therefore, messages sent to such processes need to be delivered not only to the correct destination port, but also to the correct instance of the business process that provides the port. The infrastructure hosting the process must do this in a generic manner, to avoid burdening every process implementation with the need to implement a custom mechanism for instance routing. Messages, which create a new business process instance, are a special case, as described in The Lifecycle of a Business Process.
In the object-oriented world, such stateful interactions are mediated by object references, which intrinsically provide the ability to reach a specific object (instance) with the right state and history for the interaction. This works reasonably well in tightly coupled implementations where a dependency on the structure of the implementation is normal. In the loosely coupled world of Web Services, the use of such references would create a fragile web of implementation dependencies that would not survive the independent evolution of business process implementation details at each business partner. In this world, the answer is to rely on the business data and communication protocol headers that define the wirelevel contract between partners and to avoid the use of implementation-specific tokens for instance routing whenever possible.
Consider the usual supply-chain situation where a buyer sends
a purchase order to a seller. Suppose that the buyer and seller have a stable
business relationship and are statically configured to send documents related
to the purchasing interaction to the URLs associated with the relevant WSDL
service ports. The seller needs to asynchronously return an acknowledgement for
the order, and the acknowledgement must be routed to the correct business process
instance at the buyer. The obvious and standard mechanism to do this is to
carry a business token in the order message (such as a purchase order number)
that is copied into the acknowledgement for correlation. The token can be in
the message envelope in a header or in the business document (purchase order)
itself. In either case, the exact location and type of the token in the
relevant messages is fixed and instance independent. Only the value of the
token is instance dependent. Therefore, the structure and position of the
correlation tokens in each message can be expressed declaratively in the
business process description. The BPEL4WSWS-BPEL
notion of correlation set, described in the following section, provides this
feature. The declarative information allows a BPEL4WSWS-BPEL-compliant
infrastructure to use correlation tokens to provide instance routing
automatically.
The declarative specification of correlation relies on declarative properties of messages. A property is simply a "field" within a message identified by a query—by default the query language is XPath 1.0. This is only possible when the type of the message part or binding element is described by using an XML Schema. The use of correlation tokens and endpoint references is restricted to message parts described in this way. To be clear, the actual wire format of such types can still be non-XML, for example, EDI flat files, based on different bindings for port types.
10.1. Message Correlation
During its lifetime, a business process instance typically holds one or more conversations with partners involved in its work. Conversations may be based on sophisticated transport infrastructure that correlates the messages involved in a conversation by using some form of conversation identity and routes them automatically to the correct service instance without the need for any annotation within the business process. However, in many cases correlated conversations involve more than two parties or use lightweight transport infrastructure with correlation tokens embedded directly in the application data being exchanged. In such cases, it is often necessary to provide additional application-level mechanisms to match messages and conversations with the business process instances for which they are intended.
Correlation patterns can become quite complex. The use of a particular set of correlation tokens does not, in general, span the entire interaction between a service instance and a partner (instance), but spans a part of the interaction. Correlated exchanges may nest and overlap, and messages may carry several sets of correlation tokens. For example, a buyer might start a correlated exchange with a seller by sending a purchase order (PO) and using a PO number embedded in the PO document as the correlation token. The PO number is used in the PO acknowledgement by the seller. The seller might later send an invoice that carries the PO number, to correlate it with the PO, and also carries an invoice number so that future payment-related messages need to carry only the invoice number as the correlation token. The invoice message thus carries two separate correlation tokens and participates in two overlapping correlated exchanges.
BPEL4WSWS-BPEL
addresses correlation scenarios by providing a declarative mechanism to specify
correlated groups of operations within a service instance. A set of correlation
tokens is defined as a set of properties shared by all messages in the
correlated group. Such a set of properties is called a correlation set.
Correlation sets are declared within scopes and associated with them in a manner that is analogous to variable declarations. Each correlation set is declared within a scope and is said to belong to that scope. Correlation sets that belong to the global process scope are called global correlation sets. Correlation sets may also belong to other, non-global scopes, and such correlation sets are called local correlation sets. Each correlation set is only visible in the scope in which it is defined and in all scopes nested within the scope it belongs to. Thus, global correlation sets are visible throughout the process. It is possible to "hide" a correlation set in an outer scope by declaring a correlation set with an identical name in an inner scope.
A global correlation set is in an uninitiated state at the beginning of a process. A local correlation set is in an uninitiated state at the start of the scope it belongs to. Note that non-global scopes in general start and complete their behavior more than once in the lifetime of the process instance they belong to.
Correlation sets resemble late-bound constants rather than variables in their semantics. The binding of a correlation set is triggered by a specially marked message send or receive operation. A correlation set can be initiated only once during the lifetime of the scope it belongs to. Thus, a global correlation set can only be initiated at most once during the lifetime of the process instance. Its value, once initiated, can be thought of as an alias for the identity of the business process instance. A local correlation set is available for binding each time the corresponding scope starts, but once initiated must retain its value until the scope completes.
In multiparty business protocols, each participant process in a correlated message exchange acts either as the initiator or as a follower of the exchange. The initiator process sends the first message (as part of an operation invocation) that starts the conversation, and therefore defines the values of the properties in the correlation set that tag the conversation. All other participants are followers that bind their correlation sets in the conversation by receiving an incoming message that provides the values of the properties in the correlation set. Both initiator and followers must mark the first activity in their respective groups as the activity that binds the correlation set.
10.2. Defining and Using Correlation Sets
The examples in this section show correlation being used on
almost every messaging activity (receive, reply, and invoke). This is because BPEL4WSWS-BPEL
does not assume the use of any sophisticated conversational transport protocols
for messaging. In cases where such protocols are used, the explicit use of
correlation in BPEL4WSWS-BPEL
can be reduced to those activities that establish the conversational
connections.
Each correlation
set in BPEL4WSWS-BPEL
is a named group of properties that, taken together, serve to define a way of
identifying an application-level conversation within a business protocol instance.
A given message can carry multiple correlation sets. After a correlation set is
initiated, the values of the properties for a correlation set must be identical
for all the messages in all the operations that carry the correlation set and
occur within the corresponding scope until its completion. This correlation
consistency constraint MUST be observed in all cases of initiate values. The
legal values of the initiate
attribute are: "yes", "rendezvous", "no". The default value of the initiate attribute is "no".
· When the initiate attribute is set to "yes", the related activity MUST attempt to initiate the correlation set.
o If the correlation set is already initiated and the initiate attribute is set to "yes", the semantics is undefined.
· When the initiate attribute is set to "rendezvous", the related activity MUST attempt to initiate the correlation set, if the correlation set is NOT initiated yet.
o If the correlation set is already initiated and the initiate attribute is set to "rendezvous", the correlation consistency constraint MUST be observed. If the constraint is violated, the semantics is undefined.
· When the initiate attribute is set to "no", the related activity MUST NOT attempt to initiate the correlation set.
o If an activity with the "initiate" attribute set to "no" attempts to use a correlation set that has not been previously initiated, the semantics is undefined.
o If the correlation set is already initiated and the initiate attribute is set to "no", the correlation consistency constraint MUST to be observed. If the constraint is violated, the semantics is undefined.
If multiple correlation sets are used in a message activity, then the consistency constraint MUST be observed for all correlation sets used. For example, the correlation sets used in an inbound message activity (e.g. receive) must all match message for that message to be delivered to the activity in the given process instance. If one of initiated correlation set does NOT match with the message, the semantics is undefined.
As the following examples illustrate, a correlation set is initiated when the activity within which it is used applies the attribute initiate="yes" to the set.
<correlationSets>?
<correlationSet name="ncname" properties="qname-list"/>+
</correlationSets>
Following is an extended example of correlation. It begins by defining four message properties: customerID, orderNumber, vendorID and invoiceNumber. All of these properties are defined as part of the "http://example.com/supplyCorrelation.wsdl" namespace defined by the document.
<definitions name="properties"
targetNamespace="http://example.com/supplyCorrelation.wsdl"
xmlns:tns="http://example.com/supplyCorrelation.wsdl"
xmlns="http://schemas.xmlsoap.org/wsdl/">
<!-- define correlation properties -->
<bpws:property name="customerID" type="xsd:string"/>
<bpws:property name="orderNumber" type="xsd:int"/>
<bpws:property name="vendorID" type="xsd:string"/>
<bpws:property name="invoiceNumber" type="xsd:int"/>
</definitions>
Note that these properties are global names with known
(simple) XMLSchema types. They are abstract in the sense that their occurrence
in variablesmessages
needs to be separately specified (see Message Propertiessee Variable
Properties). The example continues by defining purchase order and
invoice messages and by using the concept of aliasing to map the abstract
properties to fields within the message data identified by selection.
<definitions name="correlatedMessages"
targetNamespace="http://example.com/supplyMessages.wsdl"
xmlns:tns="http://example.com/supplyMessages.wsdl"
xmlns:cor=http://example.com/supplyCorrelation.wsdl xmlns:po = “http://example.com/po.xsd”
xmlns="http://schemas.xmlsoap.org/wsdl/">
<!—define schema types for PO and invoice information -->
<types>
<xsd:schema targetNamespace = “http://example.com/po.xsd”>
<xsd:complexType name="PurchaseOrder">
<xsd:element name="CID" type="xsd:string"/>
<xsd:element name="order" type="xsd:int"/>
...
</xsd:complexType>
<xsd:complexType name="PurchaseOrderResponse">
<xsd:element name="CID" type="xsd:string"/>
<xsd:element name="order" type="xsd:int"/>
<xsd:element name="VID" type="xsd:string"/>
<xsd:element name="invNum" type="xsd:int"/>
...
</xsd:complexType>
<xsd:complexType name="PurchaseOrderRejectType">
<xsd:element name="CID" type="xsd:string"/>
<xsd:element name="order" type="xsd:int"/>
<xsd:element name="reason" type="xsd:string"/>
...
</xsd:complexType>
<xsd:complexType name="InvoiceType">
<xsd:element name="VID" type="xsd:string"/>
<xsd:element name="invNum" type="xsd:int"/>
</xsd:complexType> <xsd:element name = “PurchaseOrderReject” type= “po:PurchaseOrderRejectType”> <xsd:element name = “Invoice” type= “po:invoiceType”>
</xsd:schema>
</types>
<message name="POMessage">
<part name="PO" type="po:PurchaseOrder"/>
</message>
<message name="POResponse">
<part name="RSP" type="po:PurchaseOrderResponse"/>
</message>
<message name="POReject">
<part name="RJCT" element="po:PurchaseOrderReject"/>
</message>
<message name="InvMessage">
<part name="IVC" element = “po:Invoice"/>
</message>
<bpws:propertyAlias propertyName="cor:customerID"
messageType="tns:POMessage" part="PO">
<bpws:query> /PO/CID </bpws:query></bpws:propertyAlias>
<bpws:propertyAlias propertyName="cor:orderNumber"
messageType="tns:POMessage" part="PO">
<query> /PO/Order </query></bpws:propertyAlias>
<bpws:propertyAlias propertyName="cor:customerID"
messageType="tns:POResponse" part="RSP">
<bpws:query> /RSP/CID </bpws:query></bpws:propertyAlias>
<bpws:propertyAlias propertyName="cor:orderNumber"
messageType="tns:POResponse" part="RSP">
<query> /RSP/Order </query></bpws:propertyAlias>
<bpws:propertyAlias propertyName="cor:vendorID"
messageType="tns:POResponse" part="RSP">
<query> /RSP/VID </query></bpws:propertyAlias>
<bpws:propertyAlias propertyName="cor:invoiceNumber"
messageType="tns:POResponse" part="RSP">
<query> /RSP/InvNum </query></bpws:propertyAlias>
<bpws:propertyAlias propertyName="cor:vendorID"
messageType="tns:InvMessage" part="IVC">
<query> /IVC/VID </query></bpws:propertyAlias>
<bpws:propertyAlias propertyName="cor:invoiceNumber"
messageType="tns:InvMessage" part="IVC">
<query> /IVC/InvNum </query></bpws:propertyAlias>
...
</definitions>
Finally, the portType used is defined, in a separate WSDL document.
<definitions name="purchasingPortType"
targetNamespace="http://example.com/puchasing.wsdl"
xmlns:smsg="http://example.com/supplyMessages.wsdl"
xmlns="http://schemas.xmlsoap.org/wsdl/"> <! – import supplyMessage.wsdl -- >
<portType name="PurchasingPT">
<operation name="SyncPurchase">
<input message="smsg:POMessage"/>
<output message="smsg:POResponse"/>
<fault name="tns:RejectPO" message="smsg:POReject"/>
</operation>
<operation name="AsyncPurchase">
<input message="smsg:POMessage"/>
</operation>
</portType>
<portType name="BuyerPT">
<operation name="AsyncPurchaseResponse">
<input message="smsg:POResponse"/>
</operation>
<operation name="AsyncPurchaseReject">
<input message="smsg:POReject"/>
</operation>
</portType>
</definitions>Both the properties and their mapping to purchase order and invoice messages will be used in the following correlation examples.
<correlationSets
xmlns:cor="http://example.com/supplyCorrelation.wsdl">
<!-- Order numbers are particular to a customer,
this set is carried in application data -->
<correlationSet name="PurchaseOrder"
properties="cor:customerID cor:orderNumber"/>
<!-- Invoice numbers are particular to a vendor,
this set is carried in application data -->
<correlationSet name="Invoice"
properties="cor:vendorID cor:invoiceNumber"/>
</correlationSets>
Correlation set names are used in invoke, receive, and reply activities (see Invoking Web Service Operations and Providing Web Service Operations), in the onMessage branches of pick activities, and in the onEvent variant of event handlers (see Pick and Message Events). These sets are used to indicate which correlation sets (i.e., the corresponding property sets) occur in the messages being sent and received. The initiate attribute is used to indicate whether the set is being initiated. When the attribute is set to "yes" the set is initiated with the values of the properties occurring in the message being sent or received. (Please see the beginning of this section for details of of initiate attribute.) Finally, in the case of invoke, when the operation invoked is synchronous request/response, a pattern attribute is used to indicate whether the correlation applies to the outbound (request) message, the inbound (response) message, or both. These ideas are explained in more detail in the context of the use of correlation in the rest of this example.
A message can carry the tokens of one or more correlation sets. The first example shows an interaction in which a purchase order is received in a one-way inbound request and a confirmation including an invoice is sent in the asynchronous response. The PurchaseOrder correlationSet is used in both activities so that the asynchronous response can be correlated to the request at the buyer. The receive activity initiates the PurchaseOrder correlationSet. The buyer is therefore the initiator and the receiving business process is a follower for this correlationSet. The invoke activity sending the asynchronous response also initiates a new correlationSet Invoice. The business process is the initiator of this correlated exchange and the buyer is a follower. The response message is thus a part of two separate conversations, and forms the bridge between them.
In the following, the prefix SP: represents the namespace "http://example.com/puchasing.wsdl".
<receive partnerLink="Buyer" portType="SP:PurchasingPT"
operation="AsyncPurchase"
variable="PO">
<correlations>
<correlation set="PurchaseOrder" initiate="yes"/>
</correlations>
</receive>
<invoke partnerLink="Buyer" portType="SP:BuyerPT"
operation="AsyncPurchaseResponse" inputVariable="POResponse">
<correlations>
<correlation set="PurchaseOrder" initiate="no" pattern="out"/>
<correlation set="Invoice" initiate="yes" pattern="out"/>
</correlations>
</invoke>
Alternatively, the response might have been a rejection (such as an "out-of-stock" message), which in this case terminates the conversation correlated by the correlationSet PurchaseOrder without starting a new one correlated with Invoice. Note that the initiate attribute is missing. It therefore has the default value of "no".
<invoke partnerLink="Buyer" portType="SP:BuyerPT"
operation="AsyncPurchaseReject" inputVariable="POReject">
<correlations>
<correlation set="PurchaseOrder" pattern="out"/>
</correlations>
</invoke>
The use of correlation with synchronous Web Service invocation is illustrated by the alternative synchronous purchasing operation used by an invoke activity used in the buyer's business process.
<invoke partnerLink="Seller" portType="SP:PurchasingPT"
operation="SyncPurchase"
inputVariable="sendPO"
outputVariable="getResponse">
<correlations>
<correlation set="PurchaseOrder" initiate="yes" pattern="out"/>
<correlation set="Invoice" initiate="yes" pattern="in"/>
</correlations>
<catch faultName="SP:RejectPO" faultVariable="POReject" faultMessageType="smsg:POReject">
<!-- handle the fault -->
</catch>
</invoke>
Note that an invoke consists of two messages: an outgoing request message and an incoming reply message. The correlation sets applicable to each message must be separately considered because they can be different. In this case the PurchaseOrder correlation applies to the outgoing request that initiates it, while the Invoice correlation applies to the incoming reply and is initiated by the reply. Because the PurchaseOrder correlation is initiated by an outgoing message, the buyer is the initiator of that correlation but a follower of the Invoice correlation because the values of the correlation properties for Invoice are initiated by the seller in the reply received by the buyer.
11. Basic Activities
11.1. Standard Attributes for Each Activity
Each activity has optional standard attributes: a name and an indicator whether a join fault should be suppressed if it occurs. See Flow for a full discussion of these two attributes.
name="ncname"?
suppressJoinFailure="yes|no"?>
When the suppressJoinFailure attribute is not specified for an activity, it inherits its value from its closest enclosing activity or from the process if no enclosing activity specifies this attribute.
11.2. Standard Elements for Each Activity
Each BPEL4WSWS-BPEL
activity has nested standard elements <source> and <target> within
the optional containers <sources> and <targets>. The
use of these elements is required for establishing synchronization
relationships through links (see Flow). Each
link is defined independently and given a name. The link name is used as value
of the linkName attribute of the <source> element. An activity MAY declare
itself to be the source of one or more links by including one or more
<source> elements. Each <source> element MUST use a distinct link
name. Similarly, an activity MAY declare itself to be the target of one or more
links by including one or more <target> elements. Each <source>
element associated with a given activity MUST use a link name distinct from all
other <source> elements at that activity. Each <target> element
associated with a given activity MUST use a link name distinct from all other
<target> elements at that activity. Each <source> element MAY
optionally specify a transition condition that functions as a guard for
following this specified link (see Flow). If the transition condition is
omitted, it is deemed to be present with the constant value true.
<sources>? <source linkName="ncname">+<transitionCondition expressionLanguage="anyURI"?>?
... bool-expr ... </transitionCondition> </source></sources>
<targets>? <joinCondition expressionLanguage="anyURI"?>? ... bool-expr ... </joinCondition> <target linkName="ncname"/>+</targets>?
The value of the <joinCondition> element is a Boolean-valued expression in the expression language indicated by the expressionLanguage attribute, or in the default expression language for this process (see Expressions). If no join condition is specified, the join condition is the logical OR of the link status of all incoming links of this activity (see 12.5.1 Link semantics).
11.3. Invoking Web Service Operations
Web Services provided by partners (see Partner
Link Types, Partner Links, and Endpoint References) can be used to
perform work in a BPEL4WSWS-BPEL
business process. Invoking an operation on such a service is a basic activity.
Recall that such an operation can be a synchronous request/response or an
asynchronous one-way operation. BPEL4WSWS-BPEL
uses the same basic syntax for both with some additional options for the
synchronous case.
An asynchronous invocation requires only the input variable of the operation because it does not expect a response as part of the operation (see Providing Web Service Operations). A synchronous invocation requires both an input variable and an output variable. One or more correlation sets can be specified to correlate the business process instance with a stateful service at the partner’s side (see Correlation). However, these attributes are both syntactically optional since they are absolutely required only in executable processes.
In the case of a synchronous invocation, the operation might
return a WSDL fault message. This results in a BPEL4WSWS-BPEL
fault. Such a fault can be caught locally by the activity, and in this case the
specified activity will be performed. If a fault is not caught locally by the
activity it is thrown to the scope that encloses the activity (see Scopes and Fault Handlers).
Note that a WSDL fault is identified in BPEL4WSWS-BPEL
by a qualified name formed by the target namespace of the corresponding
portType and the fault name. This uniform naming mechanism must be followed
even though it does not accurately match WSDL’s faultnaming model.
Because WSDL does not require that fault names be unique within the namespace
where the service operation is defined, all faults sharing a common name and defined
in the same namespace are indistinguishable in BPEL4WSWS-BPEL.
In WSDL 1.1 it is necessary to specify a portType name, an operation name, and
the fault name to uniquely identify a fault. This limits the ability to use
fault-handling mechanisms to deal with invocation faults. This is an important
shortcoming of the WSDL fault model that will be removed in future versions of
WSDL.
Implementer's Note: WS-BPEL treats faults based on abstract WSDL 1.1 operation definitions, without reference to binding details. Normally, when sending or receiving a fault, a WS-BPEL process only deals with the fault information in the abstract fault message and a WSDL 1.1 binding is required to transform the abstract fault message data to or from specific communication media. In the case of SOAP bindings this would mean providing transformation between abstract fault message data and the sub elements of of the SOAP Fault element, namely the faultcode, faultstring, faultactor and detail elements. However the WSDL 1.1 standard SOAP binding explicitly precludes mapping any information from an abstract fault message to a SOAP Fault other than the contents of the detail element. In other words, there is no standard way to relate the faultcode, faultstring and faultactor sub-elements of a SOAP Fault element to data visible to a WS-BPEL process. This specification does not provide a resolution for this problem.
Finally, an activity can be associated with another activity that acts as its compensation action. This compensation handler can be invoked either explicitly or by the default compensation handler of the enclosing scope (see Scopes and Compensation Handlers).
Semantically, the specification of local fault and/or compensation handlers is equivalent to the presence of an implicit scope immediately enclosing the activity and providing those handlers. The name of such an implicit scope is always the same as the name of the activity it encloses.
<invoke partnerLink="ncname" portType="qname"? operation="ncname"
inputVariable="ncname"? outputVariable="ncname"?
standard-attributes>
standard-elements
<correlation set="ncname" initiate="yes|no"?
pattern="in|out|out-in"/>+
</correlations>
<catch faultName="qname"faultName=“qname”? faultVariable="ncname"? faultMessageType="qname"? faultElement="qname"?>*
activity
</catch>
<catchAll>?
activity
</catchAll>
<compensationHandler>?
activity
</compensationHandler><toPart part="ncname" fromVariable="ncname"/>*<fromPart part="ncname" toVariable="ncname"/>*
See Correlation for an explanation of the correlation semantics. The following example shows an invocation with a nested compensation handler. Other examples are shown throughout the specification.
<invoke partnerLink="Seller" portType="SP:Purchasing"
operation="SyncPurchase"
inputVariable="sendPO"
outputVariable="getResponse">
<compensationHandler>
<invoke partnerLink="Seller" portType="SP:Purchasing"
operation="CancelPurchase"
inputVariable="getResponse"
outputVariable="getConfirmation">
</compensationHandler>
</invoke>
If the WSDL operation used to send and/or receive a message in an invoke activity is defined as a message containing exactly one part which itself is defined using an element then a BPEL variable of the same element type as used to define the part MAY be submitted directly to the invoke activity. The result of submitting a BPEL variable in the previously defined circumstance MUST be the equivalent of declaring an anonymous temporary WSDL message variable based on the associated WSDL message type. In the case of an inputVariable the value of the submitted BPEL variable will be used to set the value of the part in the anonymous temporary WSDL message variable. In the case of an outputVariable the value of the received part in the temporary WSDL message variable will be used to set the value of the submitted BPEL variable.
<toPart> elements provide another short cut that makes it easier to create outgoing multi-part WSDL messages from the contents of BPEL variables. The inputVariable attribute MUST NOT be used on an Invoke activity that also contains a <toPart> element. The <toPart> elements, as a group, act as a single virtual assign where each <toPart> is turned into a copy in the virtual assign. The destination of all the copies is an anonymous temporary WSDL variable, of the type specified by the relevant WSDL operation, which will then be used to send the actual message created by the virtual assign. Each <toPart> is turned into a copy in which data from the variable indicated in the “fromVariable” attribute is copied into the part of the anonymous temporary WSDL variable referenced in the “part” attribute of the <toPart> element.
The virtual assign MUST follow the same semantics and use the same faults as a real assign. When this mechanism is used in an Invoke, it is not required that there be a <toPart> for every part in the WSDL message definition, nor is the order in which parts are specified relevant. Parts not explicitly represented by <toPart> elements result in empty parts in the target anonymous WSDL variable used by the <invoke> activity.
<fromPart> elements are a similar short cut to <toPart> elements, but in the inverse. <fromPart> elements are used to pull data out of an incoming multi-part WSDL message and place it into individual BPEL variables. The outputVariable attribute MUST NOT be used on an Invoke activity that also contains a <fromPart> element. The <fromPart> elements, as a group, act as a single virtual assign. When a WSDL message is received on an <invoke> activity that uses <fromPart> the message is placed in an anonymous temporary WSDL variable, of the type specified by the relevant WSDL operation. The <fromPart> elements are then gathered together to form a single virtual assign where each <fromPart> is turned into a copy. Each <fromPart> is turned into a copy in which the data at the part of the anonymous temporary WSDL variable referenced in the <fromPart> is copied into the BPEL variable indicated in the “toVariable” attribute. The virtual assign MUST follow the same semantics and use the same faults as a real assign. When this mechanism is used in an Invoke, it is not required that there be a <fromPart> for every part in the WSDL message definition, nor is the order in which parts are specified relevant. Parts not explicitly represented by <fromPart> elements are simply not copied from the anonymous WSDL variable to a BPEL variables.
A single <invoke> can contain any combination of <toPart> elements, <fromPart> elements, outputVariable and inputVariable with the exception, as previously specified, that if <toPart> elements are used then the inputVariable attribute cannot be used and if <fromPart> elements are used then the outVariable attribute cannot be used. The virtual assign created as a consequence of the use of input or output elements occurs as part of the scope of the <invoke> activity and therefore any faults that are thrown can be caught by the invoke’s inline fault handler. Also note that it is legal to use <toPart>/<fromPart> with WSDL messages that only have a single part.
11.4. Providing Web Service Operations
A business process provides services to its partners through
receive activities and corresponding reply activities. A receive activity
specifies the partner link it expects to receive from, and the port type
(optional) and operation that it expects the partner to invoke. In
addition, it may specify a variable, using the variable attribute, that is to
be used to receive the message data received.In addition, it may specify
a variable that is to be used to receive the message data received.
However, this attribute is syntactically optional since it is absolutely
required only in executable processes. An alternative to the
variable attribute are <fromPart> elements. The syntax and semantics of
the <fromPart> elements as used on the receive activity are the same as
specified in section 11.3 for the invoke activity. This includes the
restriction that if <fromPart> elements are used on a receive activity
then the variable attribute MUST NOT be used on the same activity.
In addition, receive activities play a role in
the lifecycle of a business process. The only way to instantiate a business
process in BPEL4WSWS-BPEL is
to annotate a receive activity with the createInstance attribute set to "yes" (see Pick
for a variant). The default value of this attribute is "no". A
"start activity" is a receive/pick
activity that is annotated with a createInstance="yes"
attribute. NO other "non-start activities" but scope, flow, sequence or empty
activities may potentially be performed prior to or simultaneously with a
"start activity". The logical order of performing activities is
determined by static analysis.A receive activity annotated in this way MUST be an initial
activity in the process, that is, the only other basic activities may
potentially be performed prior to or simultaneously with such a receive activity MUST be similarly annotated receive
activities.
It is permissible to have the createInstance attribute set to "yes" for a set of
concurrent initial activities. It is permissible to
have multiple start activities. As specified in section 6.5, the
initial start activity must complete execution before any other start activities
are allowed to execute. In this case the intent is to express the
possibility that any one of a set of required inbound messages can create the
process instance because the order in which these messages arrive cannot be
predicted. If a process has multiple start activities then all
the start activities MUST use the same correlation sets and the pattern for all
the correlation sets MUST be set to "rendezvous" (see Correlation).All such receive activities MUST use the same correlation sets (see
Correlation). Compliant
implementations MUST ensure that only one of the inbound messages carrying the
same correlation set tokens actually instantiates the business process (usually
the first one to arrive, but this is implementation dependent). The other
incoming messages in the concurrent initial set MUST be delivered to the
corresponding receive activities in the already created instance.
The following example is illegal, since assign activity cannot be a start activity:
<flow>
<receive ... createInstance="yes" />
<assign ... />
</flow>
The following example is legal, since the assign activity will not be performed prior to or simultaneously with the receive activity:
<flow>
<links>
<link name="RecvToAssign"/>
</links>
<sequence>
<receive ... createInstance="yes">
<sources> <source linkName="RecvToAssign" /> <sources>
</receive>
...
</sequence>
<sequence>
<assign>
<targets> <target linkName="RecvToAssign" /> </targets>
...
</assign>
</sequence>
</flow>
If the WSDL operation used to receive or send a message in a request or a reply activity is defined as a message containing exactly one part which itself is defined using an element then a BPEL variable of the same element type as used to define the part MAY be submitted directly to the request/reply activity. The result of submitting a BPEL variable in the previously defined circumstance MUST be the equivalent of declaring an anonymous temporary WSDL message variable based on the associated WSDL message type. In the case of a receive activity, the incoming part’s value will be used to set the value of the submitted BPEL variable. In the case of a reply activity the value of the submitted BPEL variable will be used to set the value of the part in the anonymous temporary WSDL message variable that is sent out. In the case of a reply that is sending a fault the same logic applies but using a fault name rather than an operation name. Aletrnatively, it is possible to use <fromPart> elements in a receive activity to indicate how the data from a received message is to be directly copied to BPEL variables from a corresponding anonymous WSDL message variable. Similarly, in the case of a reply activity, <toPart> elements may be used to have data from BPEL variables directly copied into an anonymous WSDL message used by the reply activity.
A business process instance MUST NOT simultaneously enable two or more receive activities for the same partnerLink, portType, operation and correlation set(s). Note that receive is a blocking activity in the sense that it will not complete until a matching message is received by the process instance. The semantics of a process in which two or more receive actions for the same partnerLink, portType, operation and correlation set(s) may be simultaneously enabled is undefined. For the purposes of this constraint, an onMessage clause in a pick and an onEvent event handler are equivalent to a receive (see Pick and Message Events).
It is worth pointing out that race conditions may occur in business process execution. Messages that target a particular process instance may arrive before the corresponding receive activity is started. For example, it is perfectly reasonable to model a process that receives a series of messages in a while loop where all the messages use the same correlation. At run time, the messages will arrive independent of the iterations of the loop. But the fact that the correlation is already initiated should enable the runtime engine and messaging platform to recognize that these messages are correlated to the process instance and handle those messages appropriately. For another example, a process may invoke a remote service, initiate a correlation set for an expected callback message, and the callback message may arrive before the corresponding receive activity is started for a variety of reasons. The correlation data in the arriving message should enable the engine to recognize the message is targeted for this process instance and handle it appropriately. Process engines may employ different mechanisms to handle such race conditions. This specification does not mandate any specific mechanism. For the purposes of handling race conditions, an onMessage clause in a pick and an onMessage event handler are equivalent to a receive (see Pick and Message Events).
<receive partnerLink="ncname" portType="qname"? operation="ncname"
variable="ncname"? createInstance="yes|no"?
messageExchange="ncname"?
standard-attributes>
standard-elements
<correlations>?
<correlation set="ncname" initiate="yes|no"?/>+
</correlations> <fromPart part="ncname" toVariable="ncname"/>*
</receive>
A reply activity is used to send a response to a request previously accepted through a receive activity. Such responses are only meaningful for synchronous interactions. An asynchronous response is always sent by invoking the corresponding one-way operation on the partner link. A reply activity may specify a variable that contains the message data to be sent in reply. However, this attribute is syntactically optional since it is absolutely required only in executable processes. An alternative to the variable attribute are <toart> elements. The syntax and semantics of the <toPart> elements as used on the reply activity are the same as specified in section 11.3 for the invoke activity. This includes the restriction that if <toPart> elements are used on a receive activity then the variable attribute MUST NOT be used on the same activity.
The correlation between a request and the corresponding reply is based on the constraint that more than one outstanding synchronous request from a specific partner link for a particular portType, operation and correlation set(s) MUST NOT be outstanding simultaneously. The semantics of a process in which this constraint is violated is undefined. For the purposes of this constraint, an onMessage clause in a pick is equivalent to a receive (see Pick). Moreover, a reply activity must always be preceded by a receive activity for the same partner link, portType and (request/response) operation, such that no reply has been sent for that receive activity. The semantics of a process in which this constraint is violated is undefined.
<reply partnerLink="ncname" portType="qname"? operation="ncname"
variable="ncname"? faultName="qname"?
messageExchange="ncname"?
standard-attributes>
standard-elements
<correlations>?
<correlation set="ncname" initiate="yes|no"?/>+
</correlations> <toPart part="ncname" fromVariable="ncname"/>*
</reply>
Note that the <reply> activity corresponding to a given
request has two potential forms. If the response to the request is normal, the faultName attribute
is not used and the variable attribute, when present, will indicate a variable of the normal
response message type. If, on the other hand, the response indicates a fault,
the faultName attribute is used and the variable attribute, when present, will
indicate a variable of the message type for the corresponding fault. Observe
that WS-BPEL treats faults based on abstract WSDL 1.1
operation definitions, without reference to binding details. This limits
te ability of a WS-BPEL process to determine the information transmitted when
faults are returned over a SOAP binding. We refer
to the Implementer’s Note in Section 11.3 for additional
details.
The
optional messageExchange attribute is used to associate a <reply>
activity with an inbound message activity, such as, <receive>,
<onMessage> and <onEvent>, when
there are multiple simultaneously
incomplete inbound message
activities which requires a reply message to
complete.
A reply activity is associated with a inbound message activity based on the tuple - partnerLink, operation, and messageExchange. The value defined in messageExchange is scoped to the combination of partnerLink and operation. That is, it is legal to use the same messageExchange value in multiple simultaneously incomplete receive activities so long as the combination of partnerLink and operation on the receives are all different from each other. An incomplete inbound message activity describes the state of a BPEL process from the point that a request/response receive activity starts execution until its associated reply begins execution.
If there should ever be multiple simultaneous incomplete inbound message activities which have the same partnerLink, operation and messageExchange tuple then the bpws:conflictingRequest fault MUST be thrown within the BPEL process on the conflicting inbound message activities subsequent to the execution of the successful incomplete receive.
If a reply activity cannot be associated with an incomplete receive activity by matching the tuples and this error situation is not caught during static analysis, then bpws:missingRequest fault MUST be thrown within the BPEL process on the reply activity. Because conflicting requests should have been rejected at the time inbound message activity is executed, there cannot be more than one corresponding inbound message activity at the time <reply> is executed.
If the messageExchange attribute is not specified on a receive then its value is taken to be empty. For matching purposes two empty messageExchange values with the same partnerLink and operation values are said to match. Empty value does not match with other non-empty values.
11.5. Updating Variable Contents
Variable update occurs through the assignment activity, which is described in Assignment.
11.6. Signaling Faults
The throw activity can be used when a business process needs to signal an internal fault explicitly. Every fault is required to have a globally unique QName. The throw activity is required to provide such a name for the fault and can optionally provide a variable of data that provides further information about the fault. A fault handler can use such data to analyze and handle the fault and also to populate any fault messages that need to be sent to other services.
BPEL4WSWS-BPEL
does not require fault names to be defined prior to their use in a throw
element. An application or process-specific fault name can be directly used by
using an appropriate QName as the value of the faultName attribute and
providing a variable with the fault data if required. This provides a very
lightweight mechanism to introduce application-specific faults.
<throw faultName="qname" faultVariable="ncname"? standard-attributes>
standard-elements
</throw>
A simple example of a throw activity that does not provide a variable of fault data is:
<throw xmlns:FLT="http://example.com/faults" faultName="FLT:OutOfStock"/>
11.7. Waiting
The wait activity allows a business process to specify a delay for a certain period of time or until a certain deadline is reached (see Expressions for the grammar of duration expressions and deadline expressions).
<wait standard-attributes>
standard-elements ( <for expressionLanguage="anyURI"?>duration-expr</for> | <until expressionLanguage="anyURI"?>deadline-expr</until> )
</wait>
A typical use of this activity is to invoke an operation at a certain time (in this case a constant, but more typically an expression dependent on process state):
<sequence>
<wait><until>'2002-12-24T18:00+01:00'</until></wait>
<invoke partnerLink="CallServer" portType="AutomaticPhoneCall"
operation="TextToSpeech"
inputVariable="seasonalGreeting">
</invoke>
</sequence>
11.8. Doing Nothing
There is often a need to use an activity that does nothing, for example when a fault needs to be caught and suppressed. The empty activity is used for this purpose. The syntax is obvious and minimal.
<empty standard-attributes>
standard-elements
</empty>
12. Structured Activities
Structured activities prescribe the order in which a collection of activities take place. They describe how a business process is created by composing the basic activities it performs into structures that express the control patterns, data flow, handling of faults and external events, and coordination of message exchanges between process instances involved in a business protocol.
The structured activities of BPEL4WSWS-BPEL
include:
· Ordinary sequential control between activities is provided by sequence, switch, and while.
· Concurrency and synchronization between activities is provided by flow.
· Nondeterministic choice based on external events is provided by pick.
The set of structured activities in BPEL4WSWS-BPEL
is not intended to be the minimal required set. There are cases where one
activity can replace another. For example, the sequence activity, used to structure
sequential processing, may be emulated by a properly configured flow with
additional links. The purpose in providing what are, strictly speaking,
redundant activities is to make it easier for BPEL process designers to both
read and write BPEL processes using familiar, even if functionally redundant,
activity constructs.
Structured activities can be used recursively in the usual way. A key point to understand is that structured activities can be nested and combined in arbitrary ways. This provides a somewhat unusual but very attractive free blending of the graph-like and program-like control regimes that have traditionally been seen as alternatives rather than orthogonal composable features. A simple example of such blended usage is found in the Initial Example.
It is important to emphasize that the word activity is used throughout the following to include both basic and structured activities.
12.1. Sequence
A sequence activity contains one or more activities that are performed sequentially, in the order in which they are listed within the <sequence> element, that is, in lexical order. The sequence activity completes when the final activity in the sequence has completed.
<sequence standard-attributes>
standard-elements
activity+
</sequence>
Example:
<sequence>
<flow>
...
</flow>
<scope>
...
</scope>
<pick>
...
</pick>
</sequence>
12.2. Switch
The switch structured activity supports conditional behavior in a pattern that occurs quite often. The activity consists of an ordered list of one or more conditional branches defined by case elements, followed optionally by an otherwise branch. The case branches of the switch are considered in the order in which they appear. The first branch whose condition holds true is taken and provides the activity performed for the switch. If no branch with a condition is taken, then the otherwise branch is taken. If the otherwise branch is not explicitly specified, then an otherwise branch with an empty activity is deemed to be present. The switch activity is complete when the activity of the selected branch completes.
<switch standard-attributes>
standard-elements
<case>+ <condition expressionLanguage="anyURI"?> ... bool-expr ... </condition>
activity
</case>
<otherwise>?
activity
</otherwise>
</switch>
Example:
<switch xmlns:inventory="http://supply-chain.org/inventory"
xmlns:FLT="http://example.com/faults">
<case> <condition> bpws:getVariableProperty(stockResult,level) > 100 </condition>
<flow>
<!-- perform fulfillment work -->
</flow>
</case>
<case> <condition> bpws:getVariableProperty(stockResult,level) >= 0 </condition>
<throw faultName="FLT:OutOfStock"
variable="RestockEstimate"/>
</case>
<otherwise>
<throw faultName="FLT:ItemDiscontinued"/>
</otherwise>
</switch>
12.3. While
The while activity supports repeated performance of a specified iterative activity. The iterative activity is performed as long as the given Boolean while condition holds true at the beginning of each iteration.
<while standard-attributes>
standard-elements <condition expressionLanguage="anyURI"?> ... bool-expr ... </condition>
activity
</while>
Example:
...
<variable name="orderDetails" type="xsd:integer"/>
...
<while> <condition>
bpws:getVariableData(orderDetails) > 100 </condition>
<scope>
...
</scope>
</while>
12.4. Pick
The pick activity awaits the occurrence of one of a set of events and then performs the activity associated with the event that occurred. The occurrence of the events is often mutually exclusive (the process will either receive an acceptance message or a rejection message, but not both). If more than one of the events occurs, then the selection of the activity to perform depends on which event occurred first. If the events occur almost simultaneously, there is a race and the choice of activity to be performed is dependent on both timing and implementation.
The form of pick is a set of branches of the form event/activity, and exactly one of the branches will be selected based on the occurrence of the event associated with it before any others. Note that after the pick activity has accepted an event for handling, the other events are no longer accepted by that pick. The possible events are the arrival of some message in the form of the invocation of an inbound one-way or request/response operation, or an "alarm" based on a timer (in the sense of an alarm clock).
A special form of pick is used when the creation of an instance of the business process could occur as a result of receiving one of a set of possible messages. In this case, the pick itself has a createInstance attribute with a value of yes (the default value of the attribute is no). In such a case, the events in the pick must all be inbound messages and each of those is equivalent to a receive with the attribute "createInstance=yes". No alarms are permitted for this special case.
Each pick activity MUST include at least one onMessage event.
The semantics of the onMessage event are identical
to a receive activity regarding the optional nature of the variable attribute,
the handling of race conditions, the single element-based part message short
cut, the constraint regarding simultaneous enablement of conflicting receive
actions and the optional use of <fromPart> elements. Each pick activity
MUST include at least one onMessage event. The
semantics of the onMessage event is identical to a receive activity regarding
the optional nature of the variable attribute, the handling of race conditions,
and the constraint regarding simultaneous enablement of conflicting receive
actions. For the last case, recall that the semantics of a process
in which two or more receive actions for the same partner link, portType, operation and correlation
set(s) may be simultaneously enabled is undefined (see Providing
Web Service Operations). Enablement of each onMessage handler is
equivalent to enablement of the corresponding receive activity for the
purposes of this constraint. The optional messageExchange attribute is used to
associate an <onMessage> activity with a <reply> activity. (For
details, see Providing Web Service Operations).
<pick createInstance="yes|no"? standard-attributes>
standard-elements
<onMessage partnerLink="ncname" portType="qname"?
operation="ncname" variable="ncname"?
messageExchange="ncname"? >+
<correlations>?
<correlation set="ncname" initiate="yes|no"?/>+
</correlations> <fromPart part="ncname" toVariable="ncname"/>*
activity
</onMessage>
<onAlarm>* ( <for expressionLanguage="anyURI"?>duration-expr</for> | <until expressionLanguage="anyURI"?>deadline-expr</until> )? <repeatEvery expressionLanguage="anyURI"?>duration-expr</repeatEvery>?
activity
</onAlarm>
</pick>
The pick activity completes when one of the branches is triggered by the occurrence of its associated event and the corresponding activity completes. The following example shows a typical usage of pick. Such a pick activity can occur in a loop that is accepting line items for a large order, but a completion action is enabled as an alternative event.
<pick>
<onMessage partnerLink="buyer"
portType="orderEntry"
operation="inputLineItem"
variable="lineItem">
<!-- activity to add line item to order -->
</onMessage>
<onMessage partnerLink="buyer"
portType="orderEntry"
operation="orderComplete"
variable="completionDetail">
<!-- activity to perform order completion -->
</onMessage>
<!-- set an alarm to go after 3 days and 10 hours -->
<onAlarm> <for>'P3DT10H'</for>
<!-- handle timeout for order completion -->
</onAlarm>
</pick>
12.5. Flow
The flow construct provides concurrency and synchronization. The grammar for flow is:
<flow standard-attributes>
standard-elements
<links>?
<link name="ncname">+
</links>
activity+
</flow>
The standard attributes and standard elements for activities nested within a flow are especially significant because the standard attributes and elements primarily exist to provide flow-related semantics to activities.
The most fundamental semantic effect of grouping a set of activities in a flow is to enable concurrency. A flow completes when all of the activities in the flow have completed. Completion of an activity in a flow includes the possibility that it will be skipped if its enabling condition turns out to be false (see Dead-Path-Elimination). Thus the simplest use of flow is equivalent to a nested concurrency construct. In the following example, the two invoke activities are enabled to start concurrently as soon as the flow is started. The completion of the flow occurs after both the seller and the shipper respond (assuming the invoke operations were synchronous request/response). The bank is invoked only after the flow completes.
<sequence>
<flow>
<invoke partnerLink="Seller" .../>
<invoke partnerLink="Shipper" .../>
</flow>
<invoke partnerLink="Bank" .../>
</sequence>
More generally, a flow activity creates a set of concurrent activities directly nested within it. It further enables expression of synchronization dependencies between activities that are nested directly or indirectly within it. The link construct is used to express these synchronization dependencies. A link has a name and all the links of a flow activity MUST be defined separately within the flow activity. The standard source and target elements of an activity are used to link two activities. The source of the link MUST specify a source element specifying the link's name and the target of the link MUST specify a target element specifying the link's name. The source activity MAY also specify a transition condition by including a <transitionCondition> element under the <source> element. If the transition condition is omitted, it is deemed to be present with a value of "true". Targets MAY also specify a joinCondition by including the <joinCondition> element. This is described further in 12.5.1. Every link declared within a flow activity MUST have exactly one activity within the flow as its source and exactly one activity within the flow as its target. Moreover, at most one link may be used to connect two activities; that is, two different links MUST NOT share the same source and target activities. Finally, the source and target of a link MAY be nested arbitrarily deeply within the (structured) activities that are directly nested within the flow, except for the boundary-crossing restrictions.
The following example shows that links can cross the boundaries of structured activities. There is a link named "CtoD" that starts at activity C in sequence Y and ends at activity D, which is directly nested in the enclosing flow. The example further illustrates that sequence X must be performed prior to sequence Y because X is the source of the link named "XtoY" that is targeted at sequence Y.
<flow>
<links>
<link name="XtoY"/>
<link name="CtoD"/>
</links>
<sequence name="X"> <sources> <source linkName="XtoY"/> </sources> <invoke name="A" .../>
<invoke name="B" .../>
</sequence>
<sequence name"Y"> <targets> <target linkName="XtoY"/> </targets> <receive name="C" ...> <sources> <source linkName="CtoD"/> </sources>
</receive>
<invoke name="E" .../>
</sequence>
<invoke partnerLink="D" ...> <targets> <target linkName="CtoD"/> </targets>
</invoke>
</flow>
A link is said to cross the boundary of a syntactic construct
if the source or target activity for the link is nested within the construct
while the link is declared outside the construct. Note that it is possible for
a link to cross the boundary of a syntactic construct even in those cases where
both the source and the target activities are nested within the same construct
(so long as the link is declared outside that construct).A link MUST NOT cross
the boundary of a while activity, an isolatedserializable
scope, an event handler or a compensation handler (see Scopes
for the specification of event, fault and compensation handlers). In addition,
a link that crosses a fault-handler boundary MUST be outbound, that is, it MUST
have its source activity within the fault handler and its target activity
within a scope that encloses the scope associated with the fault handler.
Finally, a link MUST NOT create a control cycle, that is, the source activity
must not have the target activity as a logically preceding activity, where an
activity A logically precedes an activity B if the initiation of B semantically
requires the completion of A. Therefore, directed graphs created by links are
always acyclic.
12.5.1. Link Semantics
In the rest of this section, the links for which activity A is the source will be referred to as A's outgoing links, and the links for which activity A is the target will be referred to as A's incoming links. If activity X is the target of a link that has activity Y as the source, X has a synchronization dependency on Y.
Every activity that is the target of a link has an implicit or explicit “join condition” associated with it. This applies even when an activity has exactly one incoming link. Explicit join conditions are provided by the <joinCondition> element under the <targets> element. If the explicit join condition is missing, the implicit condition requires the status of at least one incoming link to be positive (see below for an explanation of link status). A join condition is a Boolean expression (see Expressions). The expression for a join condition for an activity MUST be constructed using only Boolean operators and the bpws:getLinkStatus function (see Expressions) applied to incoming links at the activity.
Without considering links, the semantics of business processes, scopes, and structured activities determine when a given activity is ready to start. For example, the second activity in a sequence is ready to start as soon as the first activity completes. An activity that defines the behavior of a branch in a switch is ready to start if and when that branch is chosen. Similarly, an activity nested directly within a flow is ready to start as soon as the flow itself starts, because flow is fundamentally a concurrency construct.
If an activity that is ready to start in this sense has incoming links, then it does not start until the status of all its incoming links has been determined and the (implicit or explicit) join condition associated with the activity has been evaluated.
The link status is a tri-state flag associated with each declared link. This flag may be in the following three states: "positive", "negative", or "unset". Each time a certain flow activity is activated, the link status of all the links declared in that activity is "unset", that is the lifetime of the status of a link is exactly the lifetime of the flow activity within which it is declared.
The precise semantics of link status evaluation are as follows:
When activity A completes, the following steps are performed to determine the effect of the synchronization links on other activities:
· Determine the status of all outgoing links for A. The status will be either positive or negative. To determine the status for each link its transitionCondition is evaluated. Note that the evaluation is carried out with the actual values of the variables referenced in the transition condition expression. If some of the variables are modified in a concurrent behavior path, the result of the transition condition evaluation may depend nondeterministically on the timing of behavior among concurrent activities. If the value is true the status is positive, otherwise it is negative.
NOTE: The transition condition is evaluated after the activity has completed. If an error occurs while evaluating the transition condition, that error does not affect the completion status of the activity and is handled by the source activity's enclosing scope. If the target of the link is outside the source activity's enclosing scope then the status of the link is negative (see the Dead-Path-Elimination section below). If the target is within the enclosing scope the status is irrelevant since the scope has faulted. It is important to keep in mind that in the case of source activities that are themselves scopes, completion does not necessarily imply successful completion. A scope may suffer an internal fault and yet complete (unsuccessfully) if there is a corresponding fault handler associated with the scope and that fault handler completes without throwing a fault. In the case of a link L with a scope S as its source activity, a fault resulting from an error in evaluating the transition condition for L would be propagated to the enclosing scope for S.
· For each activity B that has a synchronization dependency on A, check whether:
o B is ready to start (except for its dependency on incoming links) in the sense described above.
o The status of all incoming links for B has been determined.
· If both these conditions are true, then evaluate the join condition for B. If the join condition evaluates to false, a standard bpws:joinFailure fault is thrown, otherwise activity B is started.
If, during the performance of structured activity S, the semantics of S dictate that activity X nested within S will not be performed as part of the behavior of S, then the status of all outgoing links from X is set to negative. An example is an activity within a branch that is not taken in a switch activity, or activities that were not completed in a scope in which processing was halted due to a fault, including a bpws:joinFailure (see Scopes and Compensation Handlers).
Note that onEvent event handlers are permitted to have several simultaneously active instances. A private copy of all process data and control behavior defined within an event handler is provided to each instance of an event handler. This includes the behavior of links defined within an event handler. Each instance of the event handler must independently evaluate the status of the link as needed.
Note that in general multiple target activities will be enabled based on the completion of an activity with multiple outgoing links; because of this, such an activity is often called a fork activity.
When a flow activity is nested within another flow activity, the inner flow activity may define a link with the same name as in the enclosing flow activity. When this happens, a source or target reference to such link from an activity matches the innermost link visible to the activity and hides all other links with the same name. Consider the following example:
<flow name="F1">
<links>
<link name="L1"/> <!-- L1 is defined here and ... -->
</links>
<sequence name="S1"><flow name="F2">
<links>
<link name="L1"/> <!-- ... here -->
</links><sequence name="S2">
<receive name="R">
<sources>
<source linkName="L1"/> <!—This matches F2.L1 and not F1.L1 --> </sources> </receive><invoke name="I" .../>
</sequence>
... </flow> ... </sequence> ...
</flow>A link with the name “L1” is defined in the flow “F1” as well in its nested flow “F2”. The source reference to link with the name L1 from the receive activity R, matches the link L1 defined in F2 as it is the inner most link with that name visible to the activity.
12.5.2. Dead-Path-Elimination (DPE)
In cases where the control flow is largely defined by networks
of links, the normal interpretation of a false join condition for activity A is
that A should not be performed, rather than that a fault has occurred.
Moreover, there is a need to propagate the consequences of this decision by
assigning a negative status to the outgoing links for A. BPEL4WSWS-BPEL
makes it easy to express these semantics by using an attribute suppressJoinFailure
on an activity. A value of "yes" for this attribute has the effect of
suppressing the bpws:joinFailure fault for the activity and all nested activities, except where the
effect is overridden by using the suppressJoinFailure attribute with a value of
"no" in a nested activity. Suppressing the bpws:joinFailure is
equivalent to the fault being logically caught by a special default handler
attached to an implicit scope that immediately encloses just the activity with
the join condition. The default handler behavior is an empty activity, that
is, the handler suppresses the fault and does nothing about it. However,
because the activity with the join condition was not performed, its outgoing
links are automatically assigned a negative status according to the rules of Link Semantics. Thus within an activity with the value
of the suppressJoinFailure attribute set to "yes", the semantics of a join condition
that evaluates to false are to skip the associated activity and to set the
status of all outgoing links from that activity to negative. This is called
dead-pathelimination because in a graph-like interpretation of networks of
links with transition conditions, these semantics have the effect of
propagating negative link status transitively along entire paths formed by
consecutive links until a join condition is reached that evaluates to true.
Note that the name of the implicit scope (created to suppress the bpws:joinFailure) that immediately encloses an activity with a join condition is exactly the same as the name of the activity itself. In case this is an invoke activity (see Invoking Web Service Operations) with an inlined fault or compensation handler, the implicit scope for the fault and compensation handlers is merged with the implicit scope described here, which adds an additional fault handler for the bpws:joinFailure.
The default value of the suppressJoinFailure attribute of the global process element is "no". This is to avoid unexpected behavior in simple use cases where complex graphs are not involved and links without transition conditions are used for synchronization. The designers of such use cases are likely to be naive about link semantics and are likely to be surprised by the consequences of a default interpretation that suppresses a well-defined fault. For example, consider the interpretation of the Initial Example with the suppressJoinFailure attribute set to "yes". Suppose further that the invocations of the shippingProvider are enclosed in a scope that provides a fault handler (see Scopes and Fault Handlers). If one of these invocations were to fault, the status of the outgoing link from the invocation would be negative, and the (implicit) join condition at the target of the link would be false, but the resulting bpws:joinFailure would be implicitly suppressed and the target activity would be silently skipped within the sequence instead of causing the expected fault.
If universal suppression of the bpws:joinFailure is desired, it is easy to achieve by using the suppressJoinFailure attribute with a value of "yes" in the overall process element at the root of the business process definition.
12.5.3. Flow Graph Example
In the following example, the activities with the names getBuyerInformation, getSellerInformation, settleTrade, confirmBuyer, and confirmSeller are nodes of a graph defined through the flow activity. The following links are defined:
· The link named buyToSettle starts at getBuyerInformation (specified through the corresponding source element nested in getBuyerInformation) and ends at settleTrade (specified through the corresponding target element nested in settleTrade).
· The link named sellToSettle starts at getSellerInformation and ends at settleTrade.
· The link named toBuyConfirm starts at settleTrade and ends at confirmBuyer.
· The link named toSellConfirm starts at settleTrade and ends at confirmSeller.
Based on the graph structure defined by the flow, the activities getBuyerInformation and getSellerInformation can run concurrently. The settleTrade activity is not performed before both of these activities are completed. After settleTrade completes the two activities, confirmBuyer and confirmSeller are performed concurrently again.
<flow suppressJoinFailure="yes">
<links>
<link name="buyToSettle"/>
<link name="sellToSettle"/>
<link name="toBuyConfirm"/>
<link name="toSellConfirm"/>
</links>
<receive name="getBuyerInformation">
<sources> <source linkName="buyToSettle"/>
</sources>
</receive>
<receive name="getSellerInformation">
<sources> <source linkName="sellToSettle"/>
</sources>
</receive>
<invoke name="settleTrade" <targets>
<joinCondition> bpws:getLinkStatus('buyToSettle') and
bpws:getLinkStatus('sellToSettle') </joinCondition>
<target linkName="buyToSettle"/>
<target linkName="sellToSettle"/> </targets>
<sources> <source linkName="toBuyConfirm"/>
<source linkName="toSellConfirm"/>
</sources>
</invoke>
<reply name="confirmBuyer">
<targets> <target linkName="toBuyConfirm"/>
</targets>
</reply>
<reply name="confirmSeller">
<targets> <target linkName="toSellConfirm"/>
</targets>
</reply>
</flow>12.5.4. Links and Structured Activities
Links can cross the boundaries of structured activities. When this happens, care must be taken to ensure the intended behavior of the business process. The following example illustrates the behavior when links target activities within structured constructs.
The following flow is intended to perform the sequence of activities A, B, and C. Activity B has a synchronization dependency on the two activities X and Y outside of the sequence, that is, B is a target of links from X and Y. The join condition at B is missing, and therefore implicitly assumed to be the default, which is the disjunction of the status of the links targeted to B. The condition is therefore true if at least one of the incoming links has a positive status. In this case that condition reduces to the Boolean condition P(X,B) OR P(Y,B) based on the transition conditions on the links.
In the flow, the sequence S and the two receive activities X and Y are all concurrently enabled to start when the flow starts. Within S, after activity A is completed, B cannot start until the status of its incoming links from X and Y is determined and the implicit join condition is evaluated. When activities X and Y complete, the join condition for B is evaluated.
Suppose that the expression P(X,B) OR P(Y,B) evaluates to false. In this case, the standard fault bpws:joinFailure will be thrown, because the environmental attribute suppressJoinFailure is set to "no". Thus the behavior of the flow is interrupted and neither B nor C will be performed.
If, on the other hand, the environmental attribute suppressJoinFailure is set to "yes", then B will be skipped but C will be performed because the bpws:joinFailure will be suppressed by the implicit scope associated with B.
<flow suppressJoinFailure="no">
<links>
<link name="XtoB"/>
<link name="YtoB"/>
</links>
<sequence name="S">
<receive name="A" ...>
...
</receive>
<receive name="B" ...>
<targets> <target linkName="XtoB"/>
<target linkName="YtoB"/>
</targets>
...
</receive>
<receive name="C" ...>
...
</receive>
</sequence>
<receive name="X" ...>
<sources>
<source linkName="XtoB"> <transitionCondition>P(X,B)</transitionCondition> </source>
</sources> ...
</receive>
<receive name="Y" ...>
<sources>
<source linkName="YtoB">
<transitionCondition>P(Y,B)</transitionCondition> </source>
</sources>
...
</receive>
</flow>
Finally, assume that the preceding flow is slightly rewritten by linking A, B, and C through links (with transition conditions with constant truth-value of "true") instead of putting them into a sequence. Now, B and thus C will always be performed. Because the join condition is a disjunction and the transition condition of link AtoB is the constant "true", the join condition will always evaluate to "true", independent from the values of P(X,B) and P(Y,B).
<flow suppressJoinFailure="no">
<links>
<link name="AtoB"/>
<link name="BtoC"/>
<link name="XtoB"/>
<link name="YtoB"/>
</links>
<receive name="A">
<sources> <source linkName="AtoB"/>
</sources>
</receive>
<receive name="B">
<targets> <target linkName="AtoB"/>
<target linkName="XtoB"/>
<target linkName="YtoB"/> </targets> <sources>
<source linkName="BtoC"/> </sources>
</receive>
<receive name="C">
<targets> <target linkName="BtoC"/>
</targets>
</receive>
<receive name="X">
<sources><source linkName="XtoB">
<transitionCondition>P(X,B)</transitionCondition></source>
</sources>
</receive>
<receive name="Y">
<sources>
<source linkName="YtoB">
<transitionCondition>P(Y,B)</transitionCondition> </source>
</sources>
</receive>
</flow>
13. Scopes
The behavior context for each activity is provided by a scope. A scope can provide fault handlers, event handlers, a compensation handler, data variables, partner links, and correlation sets.
All scope elements are syntactically optional and some have default semantics when omitted. The syntax and semantics of scopes are explained in detail below.
<scope isolatedvariableAccessSerializable="yes|no" standard-attributes>
standard-elements <partnerLinks>? ...
</partnerLinks>
<variables>?
...
</variables>
<correlationSets>?
...
</correlationSets>
<faultHandlers>?
...
</faultHandlers>
<compensationHandler>?
...
</compensationHandler> <terminationHandler>? ... </terminationHandler>
<eventHandlers>?
...
</eventHandlers>
activity
</scope>
All handlers on a scope are lexically subordinate to the scope and so can access all variables, partnerLinks and correlation sets defined on the scope and its linear ancestors subject to any restrictions unique to the handler type specified elsewhere in this document.
Each scope has a primary activity that defines its normal behavior. The primary activity can be a complex structured activity, with many nested activities within it to arbitrary depth. The scope is shared by all the nested activities. In the following example, the scope has a primary flow activity, which contains two concurrent invoke activities. Either of the invoke activities can receive one or more types of fault responses. The fault handlers for the scope are shared by both invoke activities and can be used to catch the faults caused by the possible fault responses.
<scope>
<faultHandlers>?
...
</faultHandlers>
<flow>
<invoke partnerLink="Seller" portType="Sell:Purchasing"
operation="SyncPurchase"
inputVariable="sendPO"
outputVariable="getResponse"/>
<invoke partnerLink="Shipper"
portType="Ship:TransportOrders"
operation="OrderShipment"
inputVariable="sendShipOrder"
outputVariable="shipAck"/>
</flow>
</scope>
13.1. Data Handling and Partner Links
A scope can declare variables and parter links that live only within the scope. For further information see the chapter about Data Handling and Partner Links respectively.
13.2. Error Handling in Business Processes
Business processes are often of long duration and use
asynchronous messages for communication. They also manipulate sensitive
business data in back-end databases and line-of-business applications. Error
handling in this environment is both difficult and business critical. The use
of ACID transactions is usually limited to local updates because of trust
issues and because locks and isolation cannot be maintained for the long
periods during which technical and business errors and fault conditions can
occur in a business process instance. As a result, the overall business
transaction can fail or be cancelled after many ACID transactions have been
committed during its progress, and the partial work done must be undone as best
as possible. Error handling in business processes therefore relies heavily on the
well-known concept of compensation, that is, application-specific activities that attempt to reverse the
effects of a previous activity that was carried out as part of a larger unit of
work that is being abandoned. There is a long history of work in this area
regarding the use of Sagas [Sagas] and open nested transactions [Trends]. BPEL4WSWS-BPEL
provides a variant of such a compensation protocol by providing the ability for
flexible control of the reversal. BPEL4WSWS-BPEL
achieves this by providing the ability to define fault handling and
compensation in an application-specific manner, resulting in a feature called
Long-Running (Business) Transactions (LRTs).
It is important to understand that the notion of LRT described
here is meant to be used purely within a platform-specific implementation.
There is no prescribed requirement that the business process be distributed or
span multiple vendors and platforms. Additionally, it is important to
understand that the notion of LRT described here is purely local and occurs
within a single business process instance. There is no distributed coordination
regarding an agreed-upon outcome among multiple-participant services. The
achievement of distributed agreement is an orthogonal problem outside the scope
of BPEL4WSWS-BPEL.
As an example of an LRT, consider the planning and fulfillment
of a travel itinerary. This can be viewed as an LRT in which individual service
reservations can use nested transactions within the scope of the overall LRT.
If the itinerary is cancelled, the reservation transactions must be compensated
for by cancellation transactions, and the corresponding payment transactions
must be compensated accordingly. For ACID transactions in databases the
transaction coordinator(s) and the resources that they control know all of the
uncommitted updates and the order in which they must be reversed, and they are
in full control of such reversal. In the case of business transactions, the
compensation behavior is itself a part of the business logic and protocol, and
must be explicitly specified. For example, there might be penalties or fees
applied for cancellation of an airline reservation depending on the class of
ticket and the timing. If a payroll advance has been given to pay for the
travel, the reservation must be successfully cancelled before the payroll
advance for it can be reversed in the form of a payroll deduction. This means
the compensation actions might need to run in the same order as the original
transactions, which is not the standard or default in most transaction systems.
Using activity scopes as the definition of logical units of work, the LRT
feature of BPEL4WSWS-BPEL
addresses these requirements.
13.3. Compensation Handlers
Scopes can delineate a part of the behavior that is meant to be reversible in an applicationdefined way by a compensation handler. Scopes with compensation and fault handlers can be nested without constraint to arbitrary depth.
13.3.1. Defining a Compensation Handler
A compensation handler is simply a wrapper for a compensation activity as shown below.
<compensationHandler>?
activity
</compensationHandler>
As explained in Invoking Web Service Operations, there is a special shortcut for the invoke activity to inline a compensation handler rather than explicitly using an immediately enclosing scope. For example:
<invoke partnerLink="Seller" portType="SP:Purchasing"
operation="SyncPurchase"
inputVariable="sendPO"
outputVariable="getResponse">
<correlations>
<correlation set="PurchaseOrder" initiate="yes"
pattern="out"/>
</correlations>
<compensationHandler>
<invoke partnerLink="Seller" portType="SP:Purchasing"
operation="CancelPurchase"
inputVariable="getResponse"
outputVariable="getConfirmation">
<correlations>
<correlation set="PurchaseOrder" pattern="out"/>
</correlations>
</invoke>
</compensationHandler>
</invoke>
In this example, the original invoke activity makes a purchase and in case that purchase needs to be compensated, the compensationHandler invokes a cancellation operation at the same port of the same partnerLink, using the response to the purchase request as the input.
In standard syntax (without the invoke shortcut) this example would be equivalently expressed as follows:
<scope>
<compensationHandler>
<invoke partnerLink="Seller" portType="SP:Purchasing"
operation="CancelPurchase"
inputVariable="getResponse"
outputVariable="getConfirmation">
<correlations>
<correlation set="PurchaseOrder" pattern="out"/>
</correlations>
</invoke>
</compensationHandler>
<invoke partnerLink="Seller" portType="SP:Purchasing"
operation="SyncPurchase"
inputVariable="sendPO"
outputVariable="getResponse">
<correlations>
<correlation set="PurchaseOrder" initiate="yes"
pattern="out"/>
</correlations>
</invoke>
</scope>
Note that the variable getResponse is not local to the scope to which the compensation handler is attached and can be reused later for other purposes before compensation for this scope is invoked. The current state of non-local variables is available in compensation handlers as explained more fully below. But assuming the compensation handler needs the specific response to the invoke operation that is being reversed, that response would most conveniently be stored in a variable local to the scope, e.g., by making getResponse local to the scope.
13.3.2. Process State Usage in Compensation Handlers
Compensation handlers always use the current state of the process,
specifically the state of variables declared in their associated scope and all
enclosing scopes. The variables include partnerLinks at the process
scope. Compensation handlers are able to both get and set the values of
all such variables. Other parts of the process will see the changes made
to shared variables by compensation handlers, and conversely, compensation
handlers will see changes made to shared variables by other parts of the
process, including situations where a compensation handler runs concurrently
with other parts of the process. Compensation handlers will need to use isolatedserializable
scopes when they touch state in enclosing scopes to avoid interference if
concurrency is expected.
The current state of the process consists of the current local state of all
scopes that have been started. This includes scopes that have completed
successfully but for which the associated compensation handler has not been
invoked. For successfully completed uncompensated scopes their current
local state is the state as it was at the time of completion. Such scopes are
in suspended animation because their compensation handlers are still available
and therefore their execution may continue in compensation mode. Note
that a scope may have been executed several times in a loop, and the current
state of the process includes the state of each successfully completed (and
uncompensated) iteration through the scope. We refer to the preserved
state of a successfully completed uncompensated scope as a scope snapshot.
The behavior of a compensation handler can be thought of as an optional
continuation of the behavior of the associated scope and as such its usage of
variables is similar to the usage that occurred in the body of the scope
itself, including update actions. This includes variables in both
the local scope and all enclosing scopes. For the variables in the local
scope, the compensation handler starts with the scope snapshot. Note that
the compensation handler may itself have been called from an enclosing
compensation handler. It will then share the continuation of the state of
the enclosing scope that its caller is using. In the picture below
showing three nested scopes S1, S2 and S3, and their compensation handlers C1,
C2, C3, and failure handlers F1 and F2, we may have an error handling call
stack F1->C2->C3. In that case C3 will share the state of S2 as it
is being seen and used by C2, and the current state of the uncompleted scope
S1.
13.3.3. Invoking a Compensation Handler
The compensation handler can be invoked by using the compensate activity, which names the scope for which the compensation is to be performed, that is, the scope whose compensation handler is to be invoked. A compensation handler for a scope is available for invocation only when the scope completes normally. Invoking a compensation handler that has not been installed is equivalent to the empty activity (it is a no-op)—this ensures that fault handlers do not have to rely on state to determine which nested scopes have completed successfully. The semantics of a process in which an installed compensation handler is invoked more than once is undefined.
Note that in case an invoke activity has a compensation handler defined inline, the name of the activity is the name of the scope to be used in the compensate activity.
<compensate scope="ncname"? standard-attributes>
standard-elements
</compensate>
The ability to explicitly invoke the compensate activity is the
underpinning of the application-controlled error-handling framework of BPEL4WSWS-BPEL.
This activity can be used only in the following parts of a business process:
· In a fault handler of the scope that immediately encloses the scope for which compensation is to be performed.
· In the compensation handler of the scope that immediately encloses the scope for which compensation is to be performed.
Example:
<compensate scope="RecordPayment"/>
If a scope being compensated by name was nested in a loop, the instances of the compensation handlers in the successive iterations are invoked in reverse order.
In order for a compensation handler to be invoked by name, all scopes and activities directly nested in a scope MUST be uniquely named. If the value of the scope attribute specified on a compensate activity does not resolve to a unique scope or activity name, the behavior of the process is undefied.
If the explicit compensation handler for a
scope is absent, the default compensation handler invokes the compensation
handlers for the immediately enclosed scopes in the reverse order of the
completion of those scopesfor the scope is
invoked.
The <compensate/> form, in which the scope name is omitted in a compensate activity,
causes this the default
compensation behavior for the current scope behavior
to be invoked explicitly. This is useful when an enclosing fault
or compensation handler needs to perform additional work, such as updating
variables or sending external notifications, in addition to performing default
compensation for inner the associated scopescopes.
Note that the <compensate/> activity in a fault or compensation handler attached to scope S causes
the default-order invocation of compensation handlers for completed scopes
directly nested within S. The use of this activity can be mixed with any other
user-specified behavior except the explicit invocation of <compensate scope="Sx"/> for scope Sx nested directly within S. Explicit invocation of
compensation for such a scope nested within S disables the availability of
default-order compensation, as expected.
13.4. Fault Handlers
Fault handling in a business process can be thought of as a
mode switch from the normal processing in a scope. Fault handling in BPEL4WSWS-BPEL
is always treated as "reverse work" in that its sole aim is to undo
the partial and unsuccessful work of a scope in which a fault has occurred. The
completion of the activity of a fault handler, even when it does not rethrow
the fault handled, is never considered successful completion of the attached
scope and compensation is never enabled for a scope that has had an associated
fault handler invoked.
The optional fault handlers attached to a scope provide a way to define a set of custom fault-handling activities, syntactically defined as catch activities. Each catch activity is defined to intercept a specific kind of fault, defined by a globally unique fault QName and a variable for the data associated with the fault. If the fault name is missing, then the catch will intercept all faults with the right type of fault data. The fault variable is specified using the faultVariable attribute in a catch handler. The variable is deemed to be declared by virtue of being used as the value of this attribute and is local to the fault handler. It is not visible or usable outside the fault handler in which it is declared. The fault variable is optional because a fault might not have additional data associated with it.
A fault response to an invoke activity is one
source of faults, with name and data aspects based on the definition of the
fault in the WSDL operation A fault response to an
invoke activity is one source of faults, with obvious name and data aspects
based on the definition of the fault in the WSDL operation(please refer to
the Implementer’s Note in Section 11.3 for important
details regarding access to fault information when
faults are transmitted using SOAP). A
programmatic throw activity is another source, again with explicitly given name and data.
The core concepts and execxutable pattern
extensions of BPEL4WSWS-BPEL
define several standard faults with their names and data, and there might be
other platform-specific faults such as communication failures that can occur in
a business process instance. A catchAll clause can be added to catch any fault not caught by a more specific
catch handler.
<faultHandlers>?
<!-- there must be at least one fault handler or default -->
<catch faultName="qname"? faultVariable="ncname"? faultMessageType="qname"? faultElement="qname"?>*
activity
</catch>
<catchAll>?
activity
</catchAll>
</faultHandlers>
The
faultMessageType is technically redundant when the faultName used is derived
from a WSDL 1.1 fault message defined as a fault response to a synchronous
operation, since the WSDL fault response has a defined message type. However,
the faultName may reflect a purely internal custom fault in a process, or the
faultName may be missing. In such cases, the faultVariable, which is local to
the fault handler and declared by its occurrence in the catch element, will not
have a defined type. To avoid this possibility, although the faultName,
faultVariable and faultMessageType attributes are all optional, the
faultVariable and faultMessageType declarations go together, i.e., they
must either both be present or both absent. Moreover, the faultName and
faultVariable attributes cannot both be absent. Data
thrown with a fault can be a WSDL message type or a XML schema element. Each
catch which specifies a faultName can only catch a fault of a single type. This
one-to-one relationship is necessary in order to guarantee proper typing. If
the data to be caught is a WSDL message type then the faultMessageType
attribute MUST be used to specify the message type’s qname. If the data
to be caught is a XML element definition then the faultElement attribute MUST
be used to specify the element definition’s qname.
The faultName may reflect a purely internal custom fault in a process, or the faultName may be missing. In such cases, the faultVariable, which is local to the fault handler and declared by its occurrence in the catch element, will not have a defined type. To avoid this possibility faultVariable MUST only be used if either the faultMessageType or faultElement attributes, but not both, accompany it. faultMessageType and faultElement MUST NOT be used unless accompanied by faultVariable.
Because of the flexibility allowed in expressing
the faults that a catch activity can handle, it is possible for a fault to
match more than one fault handler. The following rules are used to select the catch activity
that will process a fault:
·If the
fault has no associated fault data, a catch activity
that specifies a matching faultName value and no
faultVariable attribute will be selected if
present. Otherwise, the default catchAll handler
is selected if present.
·If the
fault has associated fault data, a catch activity
specifying a matching faultName value and a faultVariable whose type (WSDL message type) matches the type of
the fault’s data will be selected if present. Otherwise, a catch activity
with no specified faultName and with a faultVariable whose type matches the type of the fault data will
be selected if present. Otherwise, the default catchAll handler is selected if
present.
If no catch or catchall is
selected, the fault is not caught by the current scope and is rethrown to the
immediately enclosing scope (see Implicit Fault and
Compensation Handlers for a more complete description of the default
fault and compensation handling behavior). If the fault occurs in (or is
rethrown to) the global process scope, and there is no matching fault handler
for the fault at the global level, the process terminates abnormally, as though
an terminateexit activity had been performed.
Because of the flexibility allowed in expressing the faults that a catch activity can handle, it is possible for a fault to match more than one fault handler.
In the case of faults thrown without associated data the fault will be caught as follows:
· If there is a catch activity with a matching faultName value that does not specify a faultVariable attribute then the fault is passed to the identified catch activity.
· Otherwise if there is a catchAll handler then the fault is passed to the catchAll handler.
· Otherwise the fault is thrown to the immediately enclosing scope.
In the case of faults thrown with associated data the fault will be caught as follows:
· If there is a catch activity with a matching faultName value that has a faultVariable whose type matches the type of the fault data then the fault is passed to the identified catch activity.
· Otherwise if the fault data is a WSDL message type where the message contains a single part defined by an element and there exists a catch activity with a matching faultName value that has a faultVariable whose type matches the type of the element used to define the part then the fault is passed to the identified catch activity with the faultVariable initialized to the value in the single part’s element.
· Otherwise if there is a catch activity without a faultName attribute that has a faultVariable whose type matches the type of the fault data then the fault is passed to the identified catch activity.
· Otherwise if the fault data is a WSDL message type where the message contains a single part defined by an element and there exists a catch activity without a faultName attribute that has a faultVariable whose type matches the type of the element used to define the part then the fault is passed to the identified catch activity with the faultVariable initialized to the value in the single part’s element.
· Otherwise if there is a catchAll handler then the fault is passed to the catchAll handler.
· Otherwise the fault is thrown to the immediately enclosing scope.
If the fault occurs in (or is rethrown to) the global process scope, and there is no matching fault handler for the fault at the global level, the process terminates abnormally, as though an exit activity had been performed. See Implicit Fault and Compensation Handlers for a more complete description of the default fault and compensation handling behavior.
Consider the following example:
<faulthandlers>
<catch faultName="x:foo">
<empty/>
</catch>
<catch faultVariable="bar" faultMessageType="tns:barType">
<empty/>
</catch>
<catch faultName="x:foo" faultVariable="bar" faultMessageType="tns:barType">
<empty/>
</catch>
<catchAll>
<empty/>
</catchAll>
</faulthandlers>
Assume that a fault named ”x:foo” is thrown. The first catch will be selected if the fault carries no fault data. If there is fault data associated with the fault, the third catch will be selected if and only if the type of the fault’s data matches the type of variable “bar”, otherwise the default catchall handler will be selected. Finally, a fault with a fault variable whose type matches the type of “bar” and whose name is not “x:foo” will be processed by the second catch. All other faults will be processed by the default catchall handler.
A fault caught by a <catchAll> handler or by a custom fault handler that does not specify a faultName, may need to be rethrown. However the <throw> activity that requires a faultName can not be used here as the faultName is not available. Hence all fault handlers are allowed to rethrow the original fault with a <rethrow> activity that is defined to be an empty element. In essence <rethrow> can be considered a macro for a <throw> that throws the fault caught by the corresponding fault handler.
An example is shown below:
<faulthandlers>
<catch faultName="x:foo">
<empty/>
</catch>
<catch faultVariable="bar" faultMessageType="y:myMsgType">
…. <rethrow/> <!- rethow of original fault -->
</catch>
<catchAll>
…. <rethrow/> <!- rethow of original fault -->
</catchAll>
</faulthandlers>
Although the use of compensation can be a key aspect of the behavior of fault handlers, each handler performs an arbitrary activity, which can even be <empty/>. When a fault handler is present, it is in charge of handling the fault. It might rethrow the same fault or a different one, or it might handle the fault by performing cleanup and allowing normal processing to continue in the enclosing scope.
A scope in which a fault occurred is considered to have ended abnormally, whether or not the fault was caught and handled without rethrow by a fault handler. A compensation handler is never installed for a scope in which a fault occurred.
When a fault handler for scope S handles a fault that occurred in S without rethrowing, links that have S as the source will be subject to regular evaluation of status after the fault has been handled, because processing in the enclosing scope is meant to be continued.
As explained in Invoking Web Service Operations, there is a special shortcut for the invoke activity to inline fault handlers rather than explicitly using an immediately enclosing scope. For example:
<invoke partnerLink="Seller"
portType="SP:Purchasing"
operation="SyncPurchase"
inputVariable="sendPO"
outputVariable="getResponse">
<catch faultName="SP:POFault" faultVariable="POFault" faultMessageType="lns:orderFaultType">
<!-- handle the fault -->
</catch>
</invoke>
In this example, the original invoke makes a purchase and a fault handler is inlined to handle the case where the purchase request results in a fault response. In standard syntax (without the invoke shortcut), this example would be equivalently expressed as follows:
<scope>
<faultHandlers>
<catch faultName="SP:POFault" faultVariable="POFault">
faultMessageType="lns:orderFaultType">
<!-- handle the fault -->
</catch>
</faultHandlers>
<invoke partnerLink="Seller"
portType="SP:Purchasing"
operation="SyncPurchase"
inputVariable="sendPO"
outputVariable="getResponse">
</invoke>
</scope>
The compensation handler for scope C becomes available for invocation by the fault and compensation handlers for its immediately enclosing scope exactly when scope C completes normally. A fault handler for scope C is available for invocation exactly when C has commenced but has not been completed. If the scope faults before completion, then the appropriate fault handler gets control and all other fault handlers are uninstalled. It is never possible to run more than one fault handler for the same scope under any circumstances.
Note that availability also applies to Implicit Fault and Compensation Handlers.
The behavior of a fault handler for scope C begins by implicitly terminating all activities that are currently active and directly enclosed within C (see Semantics of Activity Termination). The termination of these activities occurs before the specific behavior of a fault handler is started. This also applies to the implicit fault handlers described below. The activity of a fault handler is deemed to occur in the scope to which the fault handler is attached.
13.4.1. Implicit Fault and Compensation Handlers
Because the visibility of scope names and therefore of
compensation handlers is limited to the next enclosing scope, the ability to
compensate a scope would be lost if the enclosing scope did not have a
compensation handler or was missing a fault handler for some fault. Because
many faults are not programmatic or the result of operation invocation, it is
not reasonable to expect an explicit handler for every fault in every scope. BPEL4WSWS-BPEL
therefore provides default compensation and fault handlers when these are
missing. The behavior of these implicit handlers is to
run available compensation handlers in the reverse order of completion of the
corresponding scopes. This is defined in
more precise terms below.
Whenever a fault handler (for any fault) or the compensation handler is missing for any given scope, they are implicitly created with the following behavior:
Fault handler:
·
Run all available compensation handlers for
all directly and indirectly enclosed scopes in any
order consistent with the rules for default compensation order defined below.
Note that a compensation handler is available for a scope exactly when that
scope has completed successfullyRun all available
compensation handlers for immediately enclosed
scopes in the reverse
order of completion of the
corresponding scopes. .
· Rethrow the fault to the next enclosing scope.
Compensation handler:
·
Run all available compensation handlers for
all directly and indirectly enclosed scopes in any
order consistent with the rules for default compensation order defined below.
Note that a compensation handler is available for a scope exactly when that
scope has completed successfullyRun all available
compensation handlers for immediately enclosed scopes in the reverse order of
completion of the corresponding scopes.
13.4.2 Default Compensation Order
There are two rules for default compensation order that address different aspects of the order relation. Note that they are additive, i.e., they MUST both be obeyed in every case in performing default compensation.
Informally, Rule 1 states that default compensation must respect the forward order of execution for the scopes being compensated, but only in so far as that order is mandated by the process definition. In cases where concurrency is permitted as a result of the use of the <flow> construct, and not otherwise constrained by links, any actual temporal order during execution is not a part of the constraint defined by the first rule. More formally, we state the rule based on a precise notion of control dependency.
Definition: Control Dependency. If an activity A must complete before activity B begins, as a result of the existence of a control path from A to B in the process definition, then we say that B has a control dependency on A. Note that control dependencies may occur due to control links in a <flow> as well as due to constructs like <sequence>. Control flow due to explicit <throw> is not considered a control dependency.
Rule 1: Consider scopes A and B such that B has a control dependency on A. Assuming both A and B completed successfully and both must be compensated as part of default compensation behavior, the compensation handler of B must run to completion before the compensation handler of A is started.
This rule permits scopes that executed concurrently on the forward path to also be compensated concurrently in the reverse path. Of course, if one follows the strict reverse order of completion, then that necessarily respects control dependencies and is also consistent with this rule. The rule imposes a constraint on the order in which compensation handlers run during default compensation, and is not meant to be fully prescriptive about the exact order and concurrency.
Informally, the second rule is needed as a result of the fact that all scopes are not isolated. It is syntactically possible for two scopes to have links crossing from activities within one to activities within the other, and moreover such links may cross in both directions. If both such scopes were isolated, this would be illegal because the semantics of links crossing isolated scope boundaries would imply that such bidirectional links constitute a cycle. The simple way to state the intent of Rule 2 is that we should treat all scopes as if they were isolated, but only for purposes of cycle detection regarding links crossing scope boundaries. This allows us to apply Rule 1 to any pair of scopes to decide unambiguously if there is a control dependency between them, and if so, in which direction. Formally, we need three definitions to state the rule precisely.
Definition: Peer-Scopes. Two scopes S1 and S2 are said to be peer scopes if they are both directly nested within the same parent scope (including process scope).
Definition: Scope-controlled set. An activity A is within the scope-controlled set of activities of scope S if either A is S itself, or A is nested within S, at any depth.
Definition: Peer-Scope Dependency. If S1 and S2 are peer scopes then we say that S2 has a direct peer-scope dependency on S1 if there is an activity B within the scope-controlled set of S2 and an activity A within the scope-controlled set of S1 such that B has a control dependency on A. The peer-scope dependency relation is the transitive closure of the direct peer-scope dependency relation.
Rule 2: The peer-scope dependency relation MUST NOT include cycles. In other words, WS-BPEL forbids a process in which there are peer scopes S1 and S2 such that S1 has a peer-scope dependency on S2 and S2 has a peer-scope dependency on S1.
One of the side effects of Rule 2 is to permit a depth-first traversal of the lexical scope tree for default compensation, respecting the control dependency relation among peer scopes as dictated by Rule 1. Thus, we no longer have any difficulty in defining default compensation of any scope, since depth-first order implies that such compensation is only dependent on the compensation of its nested scopes. A related point is also worth pointing out. The default compensation order mandated by the rules here is consistent with strict reverse order of completion, but not in depth-first order. Strict reverse order of completion applied to compensation of all scopes may require interleaving of nested compensations across peer scopes.
13.4.3. Compensation handlers and Isolated Scopes
Given that compensation handlers may run concurrently
with other activities including other compensation handlers, it is necessary to
allow compensation handlers to use isolation scope semantics. Compensation
handlers do not run within the isolation domain of their associated scopes, but
fault handlers do. This creates difficulties in the isolation semantics of
compensation handlers for scopes nested inside an serializableisolated
scope. Such handlers cannot use serializableisolated
scopes themselves because serializableisolated
scopes cannot be nested. However, their isolation environment is uncertain
because they may be invoked from either a fault handler within the isolation
domain of their enclosing scope or within the compensation handler of the same
enclosing scope which is not in that isolation domain.
In order to ensure
consistency of behavior, WS-BPEL mandates
that the compensation handler of an serializableisolated
scope will itself have serializableisolated
behavior implicitly, although it will create a separate isolation domain from
that of its associated scope.
13.4.24. Semantics
of Activity Termination
As stated above, the behavior of a fault handler for scope C
begins by implicitly terminating all activities directly enclosed within C that
are currently active. The following paragraphs define what this means for all BPEL4WSWS-BPEL
activity types.
The assign activities are sufficiently short-lived that they are allowed to
complete rather than being interrupted when termination is forced. The
evaluation of expressions when already started is also allowed to complete.
Each wait, receive, reply and invoke activity is interrupted and terminated prematurely. When a synchronous
invoke activity (corresponding to a request/reply operation) is interrupted
and terminated prematurely, the response (if received) for such a terminated
activity is silently discarded. The notion of termination does not apply to empty, terminateexit, and throw.
All structured activity behavior is interrupted. The iteration of while is interrupted and termination is applied to the loop body activity. If switch has selected a branch, then the termination is applied to the activity of the selected branch. The same applies to pick. If either of these activities has not yet selected a branch, then the switch and the pick are terminated immediately. The sequence and flow constructs are terminated by terminating their behavior and applying termination to all nested activities currently active within them.
Scopes provide the ability to control the semantics of forced termination to some degree. When the activity being terminated is in fact a scope, the forced termination of a scope begins by terminating all activities directly enclosed within its associated scope that are currently active. Following this, the custom termination handler for the scope, if present, is run. If the custom termination handler is missing, the default termination handler performs compensation of all successfully completed nested scopes in the same order as in the case of a default fault handler.
Forced termination for a scope applies only if the scope is in normal processing mode. If the scope has already experienced an internal fault and invoked a fault handler, then the termination handler is uninstalled, and the forced termination has no effect. The already active fault handler is allowed to complete.
The termination handler for a scope is permitted to use the same range of activities as a fault handler, including the <compensate/> activity. However, a termination handler cannot throw any fault. Even if an uncaught fault occurs during its behavior, it is not rethrown to the next enclosing scope. This is because the enclosing scope has already either faulted or is in the process of being terminated, which is what is causing the forced termination of the nested scope.
Forced termination of nested scopes occurs in
innermost-first order as a result of the rule (stated above) that the
termination handler is run after terminating all activities (including scope
activities) directly enclosed within its associated scope that are currently
active.Scopes provide the
ability to control the semantics of forced termination to some degree. When the
activity being terminated is in fact a scope, the behavior of the scope is
interrupted and the fault handler for the standard bpws:forcedTermination fault is run. Note that this applies only if the
scope is in normal processing mode. If the scope has already experienced an
internal fault and invoked a fault handler, then as stated above, all other
fault handlers including the handler for bpws:forcedTermination are uninstalled, and the forced termination has no
effect. The already active fault handler is allowed to complete.
The fault handler for
the bpws:forcedTermination fault is designed like other fault handlers, but
this fault handler cannot rethrow any fault. Even if an uncaught fault occurs
during its behavior, it is not rethrown to the next enclosing scope. This is
because the enclosing scope has already faulted, which is what is causing the
forced termination of the nested scope.
In other respects
this is a normal fault handler. Its behavior begins by implicitly (recursively)
terminating all activities directly enclosed within its associated scope that
are currently active. It can invoke compensate activities. And when it is
missing, it is provided by using the same implicit behavior that is used for
all other implicit fault handlers.
Note that forced
termination of nested scopes occurs in innermost-first order as a result of the
rule (quoted above) that the behavior of any fault handler begins by implicitly
(recursively) terminating all activities directly enclosed within its
associated scope that are currently active.
13.4.35. Handling
Faults That Occur Inside Fault and Compensation Handlers
Compensation handlers are always invoked directly or indirectly as part of the processing of some fault handler E. The behavior of a compensation handler invoked by E can cause a fault to be thrown. Such a fault, if uncaught by scopes within the chain of compensation handlers invoked by E, is treated as being a fault within E.
If a fault occurs in a fault handler E for a scope C, the fault can be caught through the use of a scope within E. If the fault is not caught by a scope within E, it is immediately thrown to the parent scope of C and the behavior of E terminates prematurely. In effect, no distinction is made between faults that E rethrows deliberately and faults that occur as undesired faults in E.
13.5. Event Handlers
The whole process as well as each scope can be associated with a set of event handlers that are invoked concurrently if the corresponding event occurs. The actions taken within an event handler can be any type of activity, such as sequence or flow, but invocation of compensation handlers using the <compensate/> activity is not permitted. As stated earlier, the <compensate/> activity can only be used in fault and compensation handlers. There are two types of events. First, events can be incoming messages that correspond to a request/response or one-way operation in WSDL. For instance, a status query is likely to be a request/response operation, whereas a cancellation may be a oneway operation. Second, events can be alarms, that go off after user-set times. The grammar for the set of event handlers associated with a scope or process is
<eventHandlers>?
<!-- there must be at least one onEvent or onAlarm handler -->
<onEvent partnerLink="ncname" portType="qname"?
operation="ncname" messageType="qname"
variable="ncname" (messageType="qname" | element="qname")?
messageExchange="ncname"? >*
<correlations>?
<correlation set="ncname" initiate="yes|no"/>+
</correlations>
activity
</onEvent>
<onAlarm>* ( <for expressionLanguage="anyURI"?>duration-expr</for> | <until expressionLanguage="anyURI"?>deadline-expr</until> )? <repeatEvery expressionLanguage="anyURI"?>duration-expr</repeatEvery>?
activity
</onAlarm>
</eventHandlers>
The portType attribute on <onEvent> is optional. If the portType attribute is included for readability, the value of the portType attribute MUST match the portType value implied by the combination of the partnerLink specified and the associated role. All instances of onEvent MUST use either the messageType or element attribute but not both.
It is important to emphasize that event handlers are considered a part of the normal behavior of the scope, unlike fault and compensation handlers.
13.5.1. Message Events
The onEvent tag
indicates that the specified event waits for a message to arrive. The
interpretation of this tag and its attributes is very similar to a receive
activity. The partnerLink attribute defines the partner link on which the
request is expected to arrive; the partnerLink must be defined in the
partnerLinks section. The portType and operation attributes define the appropriate
port type and operation that is invoked by the partner in order to cause the
event. The variable attribute identifies a variable local to the eventHandler
that will contain the message received from the partner. The messageType attribute specifies
the type of the variable by referencing a message type definition using its
QName. The type of the variable (as specified by the messageType attribute)
must be the same as the type of the input message defined by operation
referenced by the operation attribute. The variable and messageType attributes
constitute the declaration of a variable of that name and type within an
implicit scope associated with the event handler. The
messageType attribute specifies the type of the variable by referencing a
message type definition using its QName. The type of the variable (as specified
by the messageType attribute) must be the same as the type of the input message
defined by operation referenced by the operation attribute. Optionally the
messageType attribute may be omitted and instead the element attribute
substituted if the message to be received has a single part and that part is
defined with an element type that matches the element type referenced by the
element attribute. The variable and messageType/element attribute constitute
the declaration of a variable of that name and type within an implicit scope
associated with the event handler. If an element attribute is used then the
binding of the incoming message to the variable declared in the onEvent handler
occurs as specified for the receive activity in section 11.4.
Upon receipt of the input message the event handler assigns the input message to the variable before proceeding to perform the event handler activity. Since the variable is declared within a scope associated with the event handler, each instance of the event handler (whether executed serially or concurrently relative to other instances) contains a private copy of the variable, which is not shared with other instances.
Note that the operation specified in the onEvent handler may be either an asynchronous (oneway) or a synchronous (request/response) operation. In the latter case the event handler is expected to use a reply activity to send the response. The usage and interpretation of correlation is exactly the same as for receive activities. It should also be noted that an event cannot create a process instance.
The semantics of the onEvent event is identical to a receive
activity regarding the optional nature of the variable attribute, the handling
of race conditions, and the constraint regarding simultaneous enablement of
conflicting receive actions. For the last case, recall that the semantics of a
process in which two or more receive actions for the same partner link, portType, operation and correlation
set(s) may be simultaneously enabled is undefined (see Providing
Web Service Operations). Enablement of each onEvent event
handler is equivalent to enablement of the corresponding receive activity for
the purposes of this constraint.
The optional messageExchange attribute is used to associate an <onEvent>
activity with a <reply> activity. (For details, see Providing Web
Service Operations).
As specified in the grammar above, event handlers for message events are not permitted to carry the createInstance attribute. A business process instance cannot be created by a message event. This is because the event handler cannot be enabled until the instance is created.
When the message constituting an event arrives, the activity specified in the corresponding handler is carried out. The key point to understand is that the business process is enabled to receive such messages concurrently with the normal activity of the scope to which the event handler is attached. This allows such events to occur (or not occur) at arbitrary times and an arbitrary number of times while the corresponding scope (which may be the entire business process instance) is active.
The following example shows the usage of an event handler to support the termination of a process instance through an external message. Alternatively, the event handler could throw a fault to cause the ongoing work to be undone and compensated.
<process name=“orderCar”>
...
<eventHandlers>
<onEvent partnerLink=“buyer”
portType=“ns:car”
operation=“cancel” messageType=“ns:cancelOrder”
variable=“cancelDetails”>
<terminateexit/>
</onEvent>
...
</eventHandlers>
...
</process>
In this example, if the buyer invokes the cancel operation on
the port type car, the terminateexit
activity