Web
Services Business Process Execution Language
Working Draft 01, 08
September 2004
Document identifier:
wsbpel-specification-draft-01
Location:
http://www.oasis-open.org/apps/org/workgroup/wsbpel/
Editors:
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 Business Process Execution Language for
Web Services (abbreviated to BPEL4WS in the rest of this document). Processes
in BPEL4WS 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. BPEL4WS is meant to be used to model
the behavior of both executable and abstract processes.
BPEL4WS
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. BPEL4WS
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 1.0
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
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. Serializable 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.
BPEL4WS uses a notion of message
properties 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. BPEL4WS 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 BPEL4WS can
be applied in one of two ways. A BPEL4WS 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 BPEL4WS 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. BPEL4WS 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
BPEL4WS 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
BPEL4WS 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 BPEL4WS 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 BPEL4WS 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 BPEL4WS specification is
focused on defining the common core, and adds only the essential extensions
required for each usage pattern.
BPEL4WS 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 BPEL4WS
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. BPEL4WS also introduces systematic mechanisms
for dealing with business exceptions and processing faults. Finally, BPEL4WS 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.
BPEL4WS 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 BPEL4WS
processes. XPath provides support for data manipulation. All external resources
and partners are represented as WSDL services. BPEL4WS 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
BPEL4WS 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 BPEL4WS language. The BPEL4WS process model is layered on top
of the service model defined by WSDL 1.1. At the core of the BPEL4WS 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 BPEL4WS 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, BPEL4WS defines the message
exchange protocols followed by the business process of a specific role in the
interaction.
The definition of a BPEL4WS
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 BPEL4WS 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 BPEL4WS 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 BPEL4WS
process definition.
A BPEL4WS 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 BPEL4WS process is out of scope for
this specification.
Introduction of service reference
container (“bpws:service-ref”) is meant to avoid inventing a private BPEL4WS
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 1.0) all BPEL
implementations SHOULD be configurable such that they can participate in Basic
Profile 1.0 compliant interactions. A BPEL implementation MAY allow the Basic
Profile 1.0 configuration to be disabled, even for scenarios encompassed by the
Basic Profile 1.0.
4. What
Changed from BPEL4WS 1.0
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 BPEL4WS 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.
BPEL4WS 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 BPEL4WS, situations of undefined semantics always result in standard faults
in the BPEL4WS namespace. These cases will be described as part of the Extensions for Executable Processes in the
specification. However, it is important to note that BPEL4WS uses two standard internal
faults for its core control semantics, namely, bpws:forcedTermination and
bpws:joinFailure. These are the only two standard faults that play a role in
the core concepts of BPEL4WS. 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
BPEL4WS 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 BPEL4WS 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 BPEL4WS 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
BPEL4WS 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:portType name="pos:purchaseOrderPT"/>
</plnk:role>
</plnk:partnerLinkType>
<plnk:partnerLinkType name="invoicingLT">
<plnk:role name="invoiceService">
<plnk:portType name="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, 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 BPEL4WS, all faults, whether internal
or resulting from a service invocation, are identified by a qualified name. In
particular, each WSDL fault is identified in BPEL4WS 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, BPEL4WS 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 BPEL4WS 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 BPEL4WS 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>?
<partner name="ncname">+
<partnerLink name="ncname"/>+
</partner>
</partners>
<variables>?
<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"?>*
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">*
<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 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 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".
Note that: <documentation>
construct may be added to virtually all BPEL4WS 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>
·
<terminate>
·
<wait>
·
<empty>
·
<sequence>
·
<switch>
·
<while>
·
<pick>
·
<flow>
·
<scope>
·
<compensate>
·
<rethrow>
The syntax of each of these
elements, except <terminate>, <compensate> and <rethrow>, is
considered in the following paragraphs.
·
Although
<terminate> 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).
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<receive partnerLink="ncname" portType="qname"? operation="ncname"
variable="ncname"? createInstance="yes|no"?
standard-attributes>
standard-elements
<correlations>?
<correlation set="ncname" initiate="yes|rendezvous|no"?/>+
</correlations>
</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).
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<reply partnerLink="ncname" portType="qname"? operation="ncname"
variable="ncname"? faultName="qname"?
standard-attributes>
standard-elements
<correlations>?
<correlation set="ncname" initiate="yes|rendezvous|no"?/>+
</correlations>
</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" faultVariable="ncname"?
faultMessageType="qname"?>*
activity
</catch>
<catchAll>?
activity
</catchAll>
<compensationHandler>?
activity
</compensationHandler>
</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:
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<assign standard-attributes>
standard-elements
<copy>+
from-spec
to-spec
</copy>
</assign>
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"?>+
<correlations>?
<correlation set="ncname" initiate="yes|rendezvous|no"?/>+
</correlations>
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 <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 variableAccessSerializable="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>
<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>?
<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
BPEL4WS contains constructs that
are generally sufficient for expressing abstract and executable business
processes. In some cases, however, it might be necessary to “extend” the
BPEL4WS language with additional constructs from other XML namespaces.
BPEL4WS supports extensibility by
allowing namespace-qualified attributes to appear on any BPEL4WS element and by
allowing elements from other namespaces to appear within BPEL4WS defined
elements. This is allowed in the XML Schema specifications for BPEL4WS.
Extensions MUST NOT change the
semantics of any element or attribute from the BPEL4WS 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 BPEL4WS Language
Extensibility mechanism.
6.4.
Document Linking
A BPEL4WS 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 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 BPEL4WS 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 BPEL4WS 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.
BPEL4WS, builds on WSDL by assuming that all external interactions of the
business process occur through Web Service operations. However, BPEL4WS
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 BPEL4WS
business process definition can be thought of as a template for creating
business process instances.
The creation of a process instance
in BPEL4WS 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." 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 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 terminate 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 BPEL4WS 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. BPEL4WS 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
BPEL4WS 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 BPEL4WS. 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 BPEL4WS 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
BPEL4WS. However, it is also possible to select and assign actual partner
services dynamically, and BPEL4WS 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. BPEL4WS 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, BPEL4WS 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
BPEL4WS to dynamically select a provider for a particular type of service and
to invoke their operations. BPEL4WS 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 BPEL4WS 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 BPEL4WS 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.
When BPEL4WS users interact and
manipulate endpoint references of partnerLinks, The “bpws:service-ref” element
are NOT exposed to BPEL4WS 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
literal-form of from-spec, the “bpws:service-ref” is visible to BPEL4WS users.
8. Message
Properties
8.1. Motivation
The data in a message consists
conceptually of two parts: application data and protocolrelevant 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 BPEL4WS.
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.
8.2. 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 context.
A typical use for a property in
BPEL4WS 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>
In 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.
The WSDL extensibility mechanism
is used to define properties so that the target namespace and other useful
aspects of WSDL are available.
The BPEL4WS 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).
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 BPEL4WS. 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. BPEL4WS 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 message
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
BPEL4WS 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)
BPEL4WS 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
BPEL4WS 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:
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.
BPEL4WS 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 BPEL4WS 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 BPEL4WS document at the location of the expression.
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
Properties). 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 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 BPEL4WS-defined extension
functions are available for use within all XPath 1.0 expressions.
The syntax of XPath 1.0 expressions
for BPEL4WS 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 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 simple type.
Attribute element refers to an XML Schema element. An XML Schema complex type must
beassociated with an element to be used by a BPEL4WS 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.
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. 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 standard-attributes>
standard-elements
<copy>+
from-spec
to-spec
</copy>
</assign>
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 value ... </from>
The to-spec MUST be one of the following forms:
<to variable="ncname" part="ncname"?/>
<to partnerLink="ncname"/>
<to variable="ncname" property="qname"/>
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 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 properties (see Message
Properties) occurring in variables. 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).
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 BPEL4WS
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 BPEL4WS
notion of correlation set, described in the following section, provides this
feature. The declarative information allows a BPEL4WS-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.
BPEL4WS 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 BPEL4WS 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 BPEL4WS can be reduced to those
activities that establish the conversational connections.
Each correlation set in BPEL4WS 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 messages needs to be separately specified (see Message 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: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: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 BPEL4WS 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 BPEL4WS 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. BPEL4WS 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
BPEL4WS 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 BPEL4WS 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 BPEL4WS. 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.
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
<correlations>?
<correlation set="ncname" initiate="yes|no"?
pattern="in|out|out-in"/>+
</correlations>
<catch faultName="qname" faultVariable="ncname"? faultMessageType="qname"?>*
activity
</catch>
<catchAll>?
activity
</catchAll>
<compensationHandler>?
activity
</compensationHandler>
</invoke>
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>
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 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.
In addition, receive activities
play a role in the lifecycle of a business process. The only way to instantiate
a business process in BPEL4WS 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 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. 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. 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.
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"?
standard-attributes>
standard-elements
<correlations>?
<correlation set="ncname" initiate="yes|no"?/>+
</correlations>
</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.
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"?
standard-attributes>
standard-elements
<correlations>?
<correlation set="ncname" initiate="yes|no"?/>+
</correlations>
</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.
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.
BPEL4WS 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
BPEL4WS 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 BPEL4WS 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 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.
<pick createInstance="yes|no"? standard-attributes>
standard-elements
<onMessage partnerLink="ncname" portType="qname"?
operation="ncname" variable="ncname"?>+
<correlations>?
<correlation set="ncname" initiate="yes|no"?/>+
</correlations>
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, a serializable 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 acitivity.
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. BPEL4WS 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 variableAccessSerializable="yes|no" standard-attributes>
standard-elements <partnerLinks>? ...
</partnerLinks>
<variables>?
...
</variables>
<correlationSets>?
...
</correlationSets>
<faultHandlers>?
...
</faultHandlers>
<compensationHandler>?
...
</compensationHandler>
<eventHandlers>?
...
</eventHandlers>
activity
</scope>
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]. BPEL4WS provides a variant of such a
compensation protocol by providing the ability for flexible control of the
reversal. BPEL4WS 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 BPEL4WS.
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 BPEL4WS 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
serializable 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 BPEL4WS. 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.
If the 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 scopes.
The <compensate/> form, in which the scope name is omitted
in a compensate activity, causes this default 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 scopes. 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 BPEL4WS 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 obvious name and data aspects based on
the definition of the fault in the WSDL operation. A programmatic throw activity is
another source, again with explicitly given name and data. The core concepts
and exexutable pattern extensions of BPEL4WS 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"?>*
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.
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 a terminate activity had been performed.
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. 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">
&nbs