Monday, February 06, 2006

Modeling Web Services Choreography with New Eclipse Tool


Choreography is the dark continent of Web services: few onlookers have traveled there, and many question whether there are any riches to be brought home from the trip. In the first place, choreographies bear such a striking resemblance to business processes that the novice might think that the two types of artifacts are indistinguishable.

After all, isn't choreography just a way to describe what a business process does (i.e., it choreographs the actions of its participants)? And then there is the dearth of choreography tools; until recently choreography was a topic learned by reading, not by hands-on experimentation. This article takes the trip. It describes how, in the ideal set of design tools, not only are choreographies and processes treated as entirely different artifacts (with different development life cycles), but that special modeling techniques are available to fully accentuate the nuances of choreographies.

Choreography and Process
Contrary to what many Web services and business process people think, it is terminologically incorrect to say that a process choreographs its services. Choreography describes the global protocol that governs how individual processes interact with one another. Each process offers its own services and uses services of partner processes. It is correct to say that a process orchestrates these services, but the view from one process is only the behavior of that process in terms of its partners. Choreography presents the unified global view, depicting all of the processes and their required interactions.

Web Services Choreography Description Language (WS-CDL) is the leading choreography language, and Business Process Execution Language (BPEL) is the dominant process orchestration language. Though both XML-based languages feature a similar flow-oriented design style, only BPEL is meant to have an actual run-time platform: BPEL processes run, and WS-CDL choreographies are formal specifications documenting rules to guide interprocess exchange. There are no traffic cops in this laissez faire world, only traffic laws and law-abiding drivers.

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