| After Web service appears, reliability issues different from central systems emerge due to loosely coupling property and autonomy of service composition system. Service composition system is connected by compters in around world web. Components may often encounter exceptions and undecidable activities which slow down performance and ruin consistency of the composition. Atomic transactions are techniques to hold consistency for exceptions. Its famous ACID property guarantee that even if the application is very complex, no matter what happens, consistency of states hold. This is a very useful technique especially when dealing with multiple resources. However, service composition becomes a method to integrate process and application across enterprises, tranditional transactional processes are apparently outdated. Reliable service composition is different because its execution duration is much wider.This paper proposed a service composition net(SCN) to describe web services and related compositions based on SPN. Definition and implementation of web service is given according to autonomy. This paper defines five basic compositive relations after analyzing similar structures in BPEL: sequential, exclusive, anyorder, recursive and concurrent and calculate performance factors by analyzing the corresponding Markov chain. This paper defined two classes of exceptions:local exception and global exception and used AND/OR tree to deal with global exception and proposed local exception handling based on compensation which adds a compensate service to each service to keep failure atomicity when system encounters unsolvable exception. Finally, this paper shows how to build the application to calculate factors of reliable service composition automatically. |
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