5 May 2003
Authors (listed alphabetically):
Tony Andrews, Microsoft
Francisco Curbera, IBM
Hitesh Dholakia, Siebel Systems
Johannes Klein, Microsoft
Frank Leymann, IBM
Kevin Liu, SAP
Dieter Roller, IBM
Doug Smith, Siebel Systems
Ivana Trickovic, SAP
Sanjiva Weerawarana, IBM
Copyright© 2002, 2003 BEA Systems, International Business Machines Corporation, , SAP AG, Siebel Systems. All rights reserved.
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http://www-106.ibm.com/developerworks/webservices/library/ws-bpel
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnbiz2k2/html/bpel1-1.asp
http://ifr.sap.com/bpel4ws/
http://www.siebel.com/bpel
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Abstract
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.
Status
This is a second public draft release of the BPEL4WS specification. BPEL4WS represents a convergence of the ideas in the XLANG and WSFL specifications. Both XLANG and WSFL are superseded by the BPEL4WS specification.
Contents
1 Introduction
2 Notational Conventions
3 Relationship with WSDL
4 What Changed from BPEL4WS 1.0
4.1.1 Core Concepts Clarification
4.1.2 Terminology Changes
4.1.3 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
6.3 Language Extensibility
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 Message Properties
8.1 Motivation
8.2 Defining Properties
9 Data Handling
9.1 Expressions
9.1.1 Boolean Expressions
9.1.2 Deadline-Valued Expressions
9.1.3 Duration-Valued Expressions
9.1.4 General Expressions
9.2 Variables
9.3 Assignment
9.3.1 Type Compatibility in Assignment
9.3.2 Assignment Example
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 Structured Activities
12.1 Sequence
12.2 Switch
12.3 While
12.4 Pick
12.5 Flow
12.5.1 Link Semantics
12.5.2 Dead-Path-Elimination (DPE)
12.5.3 Flow Graph Example
12.5.4 Links and Structured Activities
13 Scopes
13.1 Data Handling
13.2 Error Handling in Business Processes
13.3 Compensation Handlers
13.3.1 Defining a Compensation Handler
13.3.2 Invoking a Compensation Handler
13.4 Fault Handlers
13.4.1 Implicit Fault and Compensation Handlers
13.4.2 Semantics of Activity Termination
13.4.3 Handling Faults That Occur Inside Fault and Compensation Handlers
13.5 Event Handlers
13.5.1 Message Events
13.5.2 Alarm events
13.5.3 Enablement of Events
13.5.4 Processing of Events
13.5.4.1 Alarm Events
13.5.4.2 Message Events
13.5.5 Disablement of Events
13.5.6 Fault Handling Considerations
13.5.7 Concurrency Considerations
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
16.1.1 Service Description
16.1.2 Message Properties
16.1.3 Process
16.2 Loan Approval
16.2.1 Service Description
16.2.2 Process
16.3 Multiple Start Activities
16.3.1 Service Description
16.3.2 Process
17 Security Considerations
18 Acknowledgments
19 References
Appendix A – Standard Faults
Appendix B – Attributes and Defaults
Appendix C – Coordination Protocol
Coordination Protocol for BPEL4WS Scopes
Appendix D - XSD Schemas
BPEL4WS Schema
Partner Link Type Schema
Message Properties Schema
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, platform-independent 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 cross-enterprise 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 RFC2119 [13].
Namespace URIs of the general form "some-URI" represent some application-dependent or context-dependent URI as defined in RFC 2396 [14].
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 <!-- descriptions --> 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-schema1] [WSDL].
3 Relationship with WSDL
BPEL4WS depends on the following XML-based specifications: WSDL 1.1, XML Schema 1.0, 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 [16] is meant to avoid inventing a private BPEL4WS mechanism for web service endpoint references—such references are obviously a very general requirement in the usage of web services.
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.
4.1.1 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.
4.1.2 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.
4.1.3 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 as given in WS-Addressing [16].
- 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 the 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.
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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 Type Schema). 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).
|
<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/">
<import namespace="http://manufacturing.org/xsd/purchase"
location="http://manufacturing.org/xsd/purchase.xsd"/>
<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" type="xsd:string"/>
</message>
<message name="shippingRequestMessage">
<part name="customerInfo" type="sns:customerInfo"/>
</message>
<message name="shippingInfoMessage">
<part name="shippingInfo" type="sns:shippingInfo"/>
</message>
<message name="scheduleMessage">
<part name="schedule" type="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.
|
<process name="purchaseOrderProcess"
targetNamespace="http://acme.com/ws-bp/purchase"
xmlns="http://schemas.xmlsoap.org/ws/2003/03/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">
<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">
<source linkName="ship-to-invoice"/>
</invoke>
<receive partnerLink="shipping"
portType="lns:shippingCallbackPT"
operation="sendSchedule"
variable="shippingSchedule">
<source linkName="ship-to-scheduling"/>
</receive>
</sequence>
<sequence>
<invoke partnerLink="invoicing"
portType="lns:computePricePT"
operation="initiatePriceCalculation"
inputVariable="PO">
</invoke>
<invoke partnerLink="invoicing"
portType="lns:computePricePT"
operation="sendShippingPrice"
inputVariable="shippingInfo">
<target linkName="ship-to-invoice"/>
</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">
<target linkName="ship-to-scheduling"/>
</invoke>
</sequence>
</flow>
<reply partnerLink="purchasing"
portType="lns:purchaseOrderPT"
operation="sendPurchaseOrder"
variable="Invoice"/>
</sequence>
</process> |
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).
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.
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:
|
<process name="ncname" targetNamespace="uri"
queryLanguage="anyURI"?
expressionLanguage="anyURI"?
suppressJoinFailure="yes|no"?
enableInstanceCompensation="yes|no"?
abstractProcess="yes|no"?
xmlns="http://schemas.xmlsoap.org/ws/2003/03/business-process/">
<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"?>*
activity
</catch>
<catchAll>?
activity
</catchAll>
</faultHandlers>
<compensationHandler>?
activity
</compensationHandler>
<eventHandlers>?
<!-- Note: There must be at least one onMessage or onAlarm handler. -->
<onMessage partnerLink="ncname" portType="qname"
operation="ncname" variable="ncname"?>
<correlations>?
<correlation set="ncname" initiate="yes|no"?>+
<correlations>
activity
</onMessage>
<onAlarm for="duration-expr"? until="deadline-expr"?>*
activity
</onAlarm>
</eventHandlers>
activity
</process>
|
The top-level attributes are as follows:
- queryLanguage. This attribute specifies the XML query language used for selection of nodes in assignment, property definition, and other uses. 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.
- 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".
- 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>
The syntax of each of these elements, except <terminate>, 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. The <receive> construct allows the business process to do a blocking wait for a matching message to arrive.
|
<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>
|
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.
|
<reply partnerLink="ncname" portType="qname" operation="ncname"
variable="ncname"? faultName="qname"?
standard-attributes>
standard-elements
<correlations>?
<correlation set="ncname" initiate="yes|no"?>+
</correlations>
</reply>
|
The <invoke> construct allows the business process to invoke a one-way or request-response operation on a portType offered by a partner.
|
<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"?>*
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:
|
<assign standard-attributes>
standard-elements
<copy>+
from-spec
to-spec
</copy>
</assign>
|
The <throw> construct generates a fault from inside the business process.
|
<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.
|
<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.
|
<empty standard-attributes>
standard-elements
</empty>
|
The <sequence> construct allows you to define a collection of activities to be performed sequentially in lexical order.
|
<sequence standard-attributes>
standard-elements
activity+
</sequence>
|
The <switch> construct allows you to select exactly one branch of activity from a set of choices.
|
<switch standard-attributes>
standard-elements
<case condition="bool-expr">+
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.
|
<while condition="bool-expr" standard-attributes>
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.
|
<pick createInstance="yes|no"? standard-attributes>
standard-elements
<onMessage partnerLink="ncname" portType="qname"
operation="ncname" variable="ncname"?>+
<correlations>?
<correlation set="ncname" initiate="yes|no"?>+
</correlations>
activity
</onMessage>
<onAlarm (for="duration-expr" | until="deadline-expr")>*
activity
</onAlarm>
</pick>
|
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.
|
<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.
|
<scope variableAccessSerializable="yes|no" standard-attributes>
standard-elements
<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>
|
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.
|
<compensate scope="ncname"? standard-attributes>
standard-elements
</compensate>
|
Note that the "standard-attributes" referred to above are:
|
name="ncname"?
joinCondition="bool-expr"?
suppressJoinFailure="yes|no"?
|
where the default values are as follows:
- name. No default value (that is, unnamed)
- joinCondition. The logical OR of the liveness status of all links that are targeted at this activity
- suppressJoinFailure. No
and that the "standard-elements" referred to above are:
|
<target linkName="ncname"/>*
<source linkName="ncname" transitionCondition="bool-expr"?/>*
|
where the default value of the "transitionCondition" attribute is "true()", the truth-value function from the default expression language XPath 1.0.
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 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 Multiple Start Activities, for 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 a Service Instance). In this case the termination is abnormal.
If a compensation handler is specified for the business process as a whole (see Compensation), 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".
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 [16] 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:
|
<partnerLinkType name="BuyerSellerLink"
xmlns="http://schemas.xmlsoap.org/ws/2003/05/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:
|
<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.
|
<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.
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.
|
<partner name="SellerShipper"
xmlns="http://schemas.xmlsoap.org/ws/2003/05/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:
|
<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 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 [16]. 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.
8 Message Properties
8.1 Motivation
The data in a message consists conceptually of two parts: application data and protocol-relevant data, where the protocols can be business protocols or infrastructure protocols providing higher quality of service. An example of business protocol data is the correlation tokens that are used in correlation sets (see Correlation). Examples of infrastructure protocols are security, transaction, and reliable messaging protocols. The business protocol data is usually found embedded in the application-visible message parts, whereas the infrastructure protocols almost always add implicit extra parts to the message types to represent protocol headers that are separate from application data. Such implicit parts are often called message context because they relate to security context, transaction context, and other similar middleware context of the interaction. Business processes might need to gain access to and manipulate both kinds of protocol-relevant data. The notion of message properties is defined as a general way of naming and representing distinguished data elements within a message, whether in application-visible data or in message context. For a full accounting of the service description aspects of infrastructure protocols, it is necessary to define notions of service policies, endpoint properties, and message context. This work is outside the scope of 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/">
<!-- 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 th