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3.7 Service Components

The existing Web services standards do not provide for the definition of the business semantics of Web services. Today’s Web services are isolated and opaque. Overcoming this isolation means connecting Web services and specifying how to jointly use collections (compositions) of Web services to realize much more comprehensive and complex functionality—typically referred to as a business process. A business process specifies the potential execution order of operations from a collection of Web services, the data that is shared between these composed Web services, which partners are involved, and how they are involved in the business process, joint exception handling for collections of Web services, and so on. This composition especially allows the specification of long-running transactions between composed Web services. Consistency and reliability are increased for Web services applications. Breaking this opaqueness of Web services means specifying usage constraints of operations of a collection of Web services and their joint behavior. This, too, is similar to specifying business processes.

3.7.1 Composition of Web Services

Business Process Execution Language for Web services (WS-BPEL, often shortened to BPEL) provides a language to specify business processes and process states and how they relate to Web services. This includes specifying how a business process uses Web services to achieve its goal, and it includes specifying Web services that a business process provides. Business processes specified in BPEL are fully executable and are portable between BPEL-conformant tools and environments. A BPEL business process interoperates with the Web services of its partners, whether these Web services are realized based on BPEL or not. Finally, BPEL supports the specification of business protocols between partners and views on complex internal business processes.

BPEL supports the specification of a broad spectrum of business processes, from fully executable, complex business processes over more simple business protocols to usage constraints of Web services. It provides a long-running transaction model that allows increasing consistency and reliability of Web services applications. Correlation mechanisms are supported that allow identifying statefull instances of business processes based on business properties. Partners and Web services can be dynamically bound based on service references.

WS-BPEL is discussed in detail in Chapter 14, "Modeling Business Processes: BPEL."

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