Tuesday, May 19, 2009

Choreography comes of age

A choreography, as defined in the WS-CDL spec, "describes peer-to-peer collaborations of parties by defining, from a global viewpoint, their common and complementary observable behavior; where ordered message exchanges result in accomplishing a common business goal."

WS-CDL was ground breaking as a standard for two reasons:

1) Normally a standardisation process occurs when a number of proprietary de-facto standards, supported by different groups within industry, need to be merged to provide a single true standard that all parties are happy with.

No such proprietary standard existed for choreography, and therefore the working group were in the rare position of having to create something from scratch.

2) For introducing the concept of a 'global' model - a global model being a description of the interactions between parties from a neutral or global (i.e. service independent) perspective.

Much respect to Nickolas Kavantzas of ORACLE for dreaming up the idea, as this has spawned a lot of new research, and therefore if WS-CDL has achieved nothing else, it has made a significant contribution to computer science. Pity ORACLE didn't follow up on Nick's excellent contribution by providing an implementation of the standard, especially as they were also the co-chair of the working group.


However WS-CDL has its downsides.

In an attempt to provide a language with a strong formal type system, it has become complex to use. It is perceived by many to be web service specific, due to its affiliation with W3C. However the most damaging aspect is the long running misconception that it is competitive to BPEL.

As mentioned above, WS-CDL is an example of a 'global' model, and therefore provides a service independent view of interactions between participants.

Abstract BPEL is an example of a 'local' model, i.e. it is a service specific view of interactions between that service and its directly associated participants. Therefore the standards are actually complementary, they should be able to work together harmoniously in their separate roles.

The problem is that one of the main disadvantages with WS-CDL is that it does not define its own 'local' model representation that would be fully compatible with the 'global' model language. Although Abstract BPEL can be used to fulfill this role, it is not ideal for the job.


Now choreography has come of age. We have a new champion emerging on the scene in the form of BPMN2.

The popular graphical notation has taken a number of significant steps forward with version 2, including the introduction of a choreography 'global' model to accompany the existing process 'local' model notation.

The benefit of this approach is that it is (1) not perceived to be web service specific, (2) there is a compatible notation between the 'global' and 'local' model perspectives, and (3) it is being incorporated into a standard that is already very popular, and therefore should make it easier for users to adopt the choreography concept.

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