Web
Services Business Process Execution Language
Working Draft 01, 30
July 2004
Document identifier:
wsbpel-specification-draft-01
Location:
http://www.oasis-open.org/apps/org/workgroup/wsbpel/
Editors:
Ben BlochSid
Askary <saskary@nuperus.comben_b54@hotmail.com>
Ben Bloch <ben_b54@hotmail.com>
Francisco Curbera,
IBM <curbera@us.ibm.com>
Francisco Curbera, IBM <curbera@us.ibm.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}
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
the very first draft version of the specification, converted to OASIS TC draft
format 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 © 2003
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
D. XSD
Schemas
E. Notices
F. Intellectual
Property Rights
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/"
·bpws – “http://schemas.xmlsoap.org/ws/2004/03/business-process/”http://schemas.xmlsoap.org/ws/2003/03/business-process/”
·
wsa -
"http://schemas.xmlsoap.org/ws/2003/03/addressing"
3. Relationship
with WSDLOther Specifications
BPEL4WS depends on the following
XML-based specifications: WSDL 1.1, XML Schema 1.0 and,
XPath 1.0 and WS-Addressing.
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.
The dependency on [WS-Addressing]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. —such
references are obviously a very general requirement in the usage of web
services.
With respect to [WS-I Basic Profile] (Basic
Profile
1.0) aIn
terms ll 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., even for
scenarios encompassed by Basic Profile P 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 (formerly service
references) are now defined manifested as
a
service reference container (“bpws:service-ref”). given in
[WS-Addressing]
·
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:xsd="http://www.w3.org/2001/XMLSchema"
xmlns="http://schemas.xmlsoap.org/wsdl/"
xmlns:plnk="http://schemas.xmlsoap.org/ws/2003/05/partner-link/">
<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:OrderFaulttype="xsd:string"/>
</message>
<message name="shippingRequestMessage">
<part name="customerInfo" typeelement="sns:customerInfo"/>
</message>
<message name="shippingInfoMessage">
<part name="shippingInfo" typeelement="sns:shippingInfo"/>
</message>
<message name="scheduleMessage">
<part name="schedule" typeelement="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/20043/03/business-process/"
xmlns:lns="http://manufacturing.org/wsdl/purchase">
<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>
<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"?
enableInstanceCompensation="yes|no"?abstractProcess="yes|no"?
xmlns="http://schemas.xmlsoap.org/ws/20032004/03/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>
<compensationHandler>?
activity
</compensationHandler>
<eventHandlers>?
<!-- Note: There must be at least one onMessage onEvent or onAlarm handler. -->
<onMessage onEvent partnerLink="ncname" portType="qname"?
operation="ncname" messageType="qname" variable="ncname"?>*
<correlations>?
<correlation set="ncname" initiate="yes|rendezvous|no"?/>+
</correlations>
activity
</onMessageonEvent>
<onAlarm for="duration-expr"? until="deadline-expr"? repeatEvery="duration-expr"?>*
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.
·enableInstanceCompensation.
This attribute determines whether the process instance as a whole can be
compensated by platform-specific means. The default for this attribute is
"no".
·
abstractProcess.
This attribute specifies whether the process being defined is abstract (rather
than executable). The default for this attribute is "no".
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 (for="duration-expr" | until="deadline-expr") standard-attributes>
standard-elements
</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 condition="bool-expr" standard-attributes> <condition expressionLanguage="anyURI"?> ... bool-expr ... </condition>
standard-elements
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="duration-expr"? | until="deadline-expr"? | repeatEvery="duration-expr"?)>*
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
</scope>1234567890123456789012345678901234567890123456789012345678901234567890
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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"?
joinCondition="bool-expr"?
where the default values are as
follows:
·
name. : No
default value (that is, the default is unnamed)
·joinCondition. The
logical OR of the liveness status of all links that are targeted at this
activity
·.:
No
·
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="bool-expr"?/>+* <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.
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 childrenchild
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 imported
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 an 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.
6.54. 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.
·If a compensation
handler is specified for the business process as a whole (see Compensation Handlers), a business process
instance can be compensated after normal completion by
platform-specific means. This functionality is enabled by setting the enableInstanceCompensation attribute of the process to "yes".
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 executedperformed and no corresponding reply activity has been executedperformed. A fault
bpws:missingReply is
thrown Iif the process instance reaches
the end of its executionbehavior, and one or more receive activities remain open,
then the status of the instance becomes undefined. This faultcondition 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”), [WS-Addressing] 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/20032004/0503/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/"
xmlns:plnk="http://schemas.xmlsoap.org/ws/2003/05/partner-link/"
...
<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 static 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/20032004/0503/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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1 2 3 4 5 6
<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.
BPEL4WS uses the notion of
endpoint reference defined in [WS-Addressing]. 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:bpws="http://schemas.xmlsoap.org/ws/2003/03/business-process/"
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/20032004/03/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"
xmlns:bpws="http://schemas.xmlsoap.org/ws/2003/03/business-process/">
<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:bpws="http://schemas.xmlsoap.org/ws/2003/03/business-process/"
xmlns="http://schemas.xmlsoap.org/wsdl/">
<!-- define a correlation property -->
<bpws:property name="taxpayerNumber" type="txtype:SSN"/>
...
<bpws:propertyAlias propertyName="tns:taxpayerNumber"
messageType="txmsg:taxpayerInfo" part="identification">
<query> ="/txtype: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"
...
xmlns:bpws="http://schemas.xmlsoap.org/ws/2003/03/business-process/">
<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, and query
attributes 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/2004/3/03/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 and same
messageType/type/element as a variable defined in an enclosing
scope, the local variable will be used in local assignments and/or
getVariableProperty functions. It is not permitted to have variables with same
name but different messageType/type/element within an enclosing scope
hierarchy. The behavior of such variables is not defined.
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="general-expr"/>
<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. The semantics of a process
in which this consistency constraint is violated is undefined. Similarly
undefined is the semantics of a process in which an activity with the initiate attribute set to no attempts to use a
correlation set that has not been previously initiated. 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:bpws="http://schemas.xmlsoap.org/ws/2003/03/business-process/"
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:bpws="http://schemas.xmlsoap.org/ws/2003/03/business-process/"
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="potns:PurchaseOrder"/>
</message>
<message name="POResponse">
<part name="RSP" type="potns:PurchaseOrderResponse"/>
</message>
<message name="POReject">
<part name="RJCT" elementtype="potns:PurchaseOrderReject"/>
</message>
<message name="InvMessage">
<part name="IVC" type="tnselement = “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"/>
<fault name="tns:RejectPO" message="smsg:POReject"/>
</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 onMessage 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, a join condition,
and an indicator whether a join fault should be suppressed if it occurs. A join
condition is used to specify requirements about concurrent paths reaching at an
activity. See Flow for a full
discussion of these last two attributes. 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 The default value of suppressJoinFailure is no.
name="ncname"?
joinCondition="bool-expr"?
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.
The
value of the joinCondition attribute is a Boolean-valued expression in the
expression language indicated for this document (see Expressions). The
default value of the join condition for the default expression language XPath
is the logical OR of the link status of all incoming links of this activity.
11.2. Standard
Elements for Each Activity
Each BPEL4WS activity has optional
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=”bool-expr”? > <joinCondition expressionLanguage="anyURI"?>? ... bool-expr ... </joinCondition> < target linkName="ncname"/>+*</targets>?
The value of the <joinCondition> elementattribute is a Boolean-valued expression
in the expression language indicated by
the expressionLanguage attribute,
or in the default expression language for
this processdocument (see Expressions). If no
join condition is specifiedthis attribute is
not present,
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 onMessage 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 (for="duration-expr" | until="deadline-expr") standard-attributes>
standard-elements
</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'"/>
<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 BPELprogrammersprocess designers to both read and write BPELprogramsprocessesby making availableusing familiar, even if functionally redundant,programmingactivity 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