8.6. Summary The major points of this chapter are: Choreography is the attempt to build stateful, conversational, long-running, multiparticipant processes out of basic stateless, atomic web services operations. Orchestration is the coordination of web services within a single process. Choreography is coordination at the global level. Orchestration is subjective; choreography is subjective. The standards body behind the choreography effort is the W3C's Choreography Working Group. The W3C's other work on web services includes WSDL and SOAP, as well as the WS-* stack. WS-Choreography is one of its more recent contributions, and WS-CDL is its current albeit unfinishedstandard. Previous approaches, published on the W3C site but of diminishing importance, are WSCI and WSCL. WS-CDL is the officially endorsed W3C choreography language. A taut declarative language based on XML, WS-CDL presents a global view of a multiparty process and is especially suitable for B2B collaborations. The main concepts of WS-CDL are choreography, interaction, and channel. WS-CDL allows participants to pass around channels (web service-based communication links) in the spirit of the pi-calculus. A powerful feature of WS-CDL is that of guarded interactions, which initiate only if preconditions are met and can be configured to wait for all required data to become available before evaluating the precondition. WS-CDL is in its infancy. The specification is incomplete and is devoid of examples. WS-CDL's future is hard to predict, but it will likely face competition from BPEL. WS-CDL lacks strong industry backing. Oracle is the principal author, but Microsoft and IBM are in the BPEL camp. WSCI is an XML language that extends WSDL to model the orchestration of each participant in a process as well as the global model of interaction The former model, called interface, resembles BPML and supports the same set of P4 process patterns. The latter, the global model, is a declarative enumeration of links connecting interface actions. WSCI, though not exactly a BPM process language like BPEL, is often positioned against it. In the face of BPEL's enormous popularity and its powerful backing by IBM, Microsoft, and BEA, WSCI, supported by Sun and Oracle but not by its principal author BEA, has effectively died. WSCL, submitted by HP to the W3C in 2002, is the most obscure of the choreography languages. Its lack of popularity is regrettable, for its state-machine approach is compelling. A WSCL conversation, which represents one participant's view of a choreography, is a state machine consisting of interactions and transitions. An interaction, which can be conceived as a state, represents a web service operation. Special dummy start and end interactions, with no web services meaning, also exist to control the start and end of the conversation. The direction of a WSCL interaction, following the WSDL model, can be one-way receive or send or bidirectional send-receive or receive-send. A transition connects one interaction to another. Because a bidirectional interaction can have multiple outcomes (one success and multiple faults), a transition that connects such an interaction to another must identify which outcome to consider. WSCL's main weaknesses are its omission of a global model and the simplicity of its state model (e.g., no exception handling, no state hierarchy, and the weakness of the WSDL relationship).
|
