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Web Service Choreography

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Web Service Choreography (WS-Choreography) is a specification by the W3C defining a XML-based business process modeling language that describes collaboration protocols of cooperating Web Service participants, in which services act as peers, and interactions may be long-lived and stateful. (Orchestration is another term with a very similar, but still different meaning.)

The main effort to get a choreography, The W3C Web Services Choreography Working Group, was closed on the 10th July 2009[1] leaving WS-CDL as a Candidate Recommendation.

Service Choreography

Service choreography is a form of service composition in which the interaction protocol between several partner services is defined from a global perspective[2]. The intuition underlying the notion of service choreography can be summarised as follows:

“Dancers dance following a global scenario without a single point of control"

That is, at run-time each participant in a service choreography executes its part of it (i.e. its role) according to the behavior of the other participants[3]. A choreography's role specifies the expected messaging behavior of the participants that will play it in terms of the sequencing and timing of the messages that they can consume and produce[4].

Service choreography is better understood through the comparison with another paradigm of service composition: Service orchestration. On one hand, in service choreographies the logic of the message-based interactions among the participants are specified from a global perspective. In Service orchestration, on the other hand, the logic is specified from the local point of view of one single participant, called the orchestrator. In the Service orchestration language BPEL, for example, the specification of the service orchestration (e.g. the BPEL process file) can be deployed on the service infrastructure (for example a BPEL execution engine like Apache ODE). The deployment of the service orchestration specification creates the composed service.

In a sense, service choreography and orchestrations are two flips of the same coin. On one hand, the roles of a service choreography can be extracted as service orchestrations through a process called projection (see for example [5]). Through projection it is possible to realize skeletons, i.e. incomplete service orchestrations that can be used as baselines to realize the web services that participate to the service choreography. On the other hand, already existing service orchestrations may be composed in service choreographies.

It is important to notice that unlike service orchestrations, service choreographies are not executed: they are enacted. A service choreography is enacted when its participants execute their roles[6]. That is, unlike Service orchestration, service choreographies are not run by some engine on the service infrastructure, but they ``happen when their roles are executed. This is because the logic of the service choreography is specified from a global point of view, and thus it is not realized by one single service like in service orchestration.

History

"Many presentations at the W3C Workshop on Web services of 11-12 April 2001 pointed to the need for a common interface and composition language to help address choreography. The Web Services Architecture Requirements Working Draft created by the Web Services Architecture Working Group also lists the idea of Web service choreography capabilities as a Critical Success Factor, in support of several different top-level goals for the nascent Web services architecture"[1].

The problem of choreography was (and still is) of great interest to the industry during that time efforts such as WSCL (Web Service Conversation Language) and WSCI (Web Service Choreography Interface) were submitted to W3C and were published as Technical Notes. Moreover complementary efforts were launched[2]:

  • BPML, now BPMN
  • BPSS by ebXML[3]
  • WSFL by IBM
  • XLANG by Microsoft
  • BPEL4WS by IBM, Microsoft and BEA

"In June 2002, Intalio, Sun, BEA and SAP released a joint specification named Web Services Choreography Interface (WSCI). This specification was also submitted to W3C as a note in August 2002. W3C has since formed a new Working Group called Web Services Choreography Working Group within the Web services Activity. The WSCI specification is one of the primary inputs into the Web Services Choreography Working Group which published a Candidate Recommendation on WS-CDL version 1.0 on November 9th, 2005"[4]. "XLang, WSFL and WSCI are no longer being supported by any standard organization or companies BPEL replaced Xlang and WSFL WSCI was superseded by WS-CDL"[5].

The upcoming Business Process Modeling Notation version 2.0 will introduce diagrams for specifying service choreographies[7]. The academic field has put forward other service choreography languages, for example Let's Dance[8] and BPEL4Chor[9].

References

  1. ^ Web Services Choreography Working Group at W3
  2. ^ S-Cube Knowledge Model: Service Choreography
  3. ^ Chris Peltz: Web Services Orchestration and Choreography. IEEE Computer (COMPUTER) 36(10):46-52 (2003)
  4. ^ Zongyan Qiu, Xiangpeng Zhao, Chao Cai, Hongli Yang: Towards the theoretical foundation of choreography. WWW 2007:973-982
  5. ^ Hongli Yang, Xiangpeng Zhao, Chao Cai, Zongyan Qiu: Exploring the Connection of Choreography and Orchestration with Exception Handling and Finalization/Compensation. FORTE 2007:81-96
  6. ^ Howard Foster, Sebastián Uchitel, Jeff Magee, Jeff Kramer: Model-Based Analysis of Obligations in Web Service Choreography. AICT/ICIW 2006:149
  7. ^ ack Vaughan: BPMN 2.0 adds notation to handle BPM choreography. SearchSOA.com
  8. ^ Johannes Maria Zaha, Alistair P. Barros, Marlon Dumas, Arthur H. M. ter Hofstede: Let's Dance: A Language for Service Behavior Modeling. OTM Conferences 2006:145-162
  9. ^ Gero Decker, Oliver Kopp, Frank Leymann, Mathias Weske: BPEL4Chor: Extending BPEL for Modeling Choreographies. ICWS 2007:296-303

See also

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