Web Services Business Process Execution Language Version 0 Public Review Draft 02, 20 November, 2006

1. Introduction

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 request-response or uncorrelated one-way interactions.

Models for business interactions typically assume sequences of peer-to-peer message exchanges, both request-response and one-way, 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. An Abstract Process may be used to describe observable message exchange behavior of each of the parties involved, 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 observable behavior. Observable behavior must clearly be described in a platform independent manner and captures behavioral aspects that may have cross enterprise business significance.

The following concepts for describing business processes should be considered:

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  Business processes include data-dependent behavior. For example, a supply-chain process 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.
  
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  The ability to specify exceptional conditions and their consequences, including recovery sequences, is at least as important for business processes as the ability to define the behavior in the "all goes well" case.
  
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  Long-running interactions include multiple, often nested units of work, each with its own data requirements. Business processes frequently require cross partner coordination of the outcome (success or failure) of units of work at various levels of granularity.
  

A WS-BPEL Abstract Process is a partially specified process that is not intended to be executed and that must be explicitly declared as ‘abstract’. Whereas Executable Processes are fully specified and thus can be executed, an Abstract Process may hide some of the required concrete operational details expressed by an Executable artifact.

All the constructs of Executables Processes are made available to Abstract Processes; consequently, Executable and Abstract WS-BPEL Processes share the same expressive power. In addition to the features available in Executable Processes, Abstract Processes provide two mechanisms for hiding operational details: (1) the use of explicit opaque tokens and (2) omission. Although a particular Abstract Process definition might contain complete information that would render it Executable, its Abstract status states that any concrete realizations of it are permitted to perform additional processing steps that are not relevant to the audience to which it has been given.

Abstract Processes serve a descriptive role, with more than one use case. One such use case might be to describe the observable behavior of some or all of the services offered by an Executable Process. Another use case would be to define a process template that embodies domain-specific best practices. Such a process template would capture essential process logic in a manner compatible with a design-time representation, while excluding execution details to be completed when mapping to an Executable Process.

Regardless of the specific use case and purpose, all Abstract Processes share a common syntactic base. They have different requirements for the level of opacity and restrictions on which parts of a process definition may be omitted or hidden. Tailored uses of Abstract Processes have different effects on the consistency constraints and on the semantics of that process. Some of these required constraints are not enforceable by the XML Schema.

A common base specifies the features that define the syntactic universe of Abstract Processes. Given this common base, a usage profile provides the necessary specializations and semantics based on Executable WS-BPEL for a particular use of an Abstract Process.

As mentioned above it is possible to use WS-BPEL to define an Executable Business Process. While a WS-BPEL Abstract Process definition is not required to be fully specified, 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.

The continuity of the basic conceptual model between Abstract and Executable Processes in WS-BPEL makes it possible to export and import the public aspects embodied in Abstract Processes as process or role templates while maintaining the intent and structure of the observable behavior. This applies even where private implementation aspects use platform dependent functionality. This is a key feature for the use of WS-BPEL from the viewpoint of unlocking the potential of Web Services because it allows the development of tools and other technologies that greatly increase the level of automation and thereby lower the cost in establishing cross enterprise automated business processes.

In this specification, the description of Abstract Business Processes is presented after Executable. We clearly differentiate concepts required for Abstract Business Process description from the concepts for Executable in the section 13. WS-BPEL Abstract Processes.

WS-BPEL defines a model and a grammar for describing the behavior of a business process based on interactions between the process and its partners. The interaction with each partner occurs through Web Service interfaces, and the structure of the relationship at the interface level is encapsulated in what is called a partnerLink. The WS-BPEL process defines how multiple service interactions with these partners are coordinated to achieve a business goal, as well as the state and the logic necessary for this coordination. WS-BPEL also introduces systematic mechanisms for dealing with business exceptions and processing faults. Moreover, WS-BPEL introduces a mechanism to define how individual or composite activities within a unit of work are to be compensated in cases where exceptions occur or a partner requests reversal.

WS-BPEL utilizes several XML specifications: WSDL 1.1, XML Schema 1.0, XPath 1.0 and XSLT 1.0. WSDL messages and XML Schema type definitions provide the data model used by WS-BPEL processes. XPath and XSLT provide support for data manipulation. All external resources and partners are represented as WSDL services. WS-BPEL provides extensibility to accommodate future versions of these standards, specifically the XPath and related standards used in XML computation.

A WS-BPEL process is a reusable definition that can be deployed in different ways and in different scenarios, while maintaining a uniform application-level behavior across all of them. The description of the deployment of a WS-BPEL process is out of scope for this specification.