The solution presented in this paper is the model-driven approach to deal with failures encountered during an execution of CWS. Our proposition focuses on omission failures and uses the colored Petri Nets modeling
Figure 10: Communication between components in the Vacation planner CWS.
Figure 11: The deployment of the Vacation planner components.
language to infer about alternative paths to execute CWS. The main advantage of our solution is that we avoid interactions with components that are not working. This goal is achieved without reducing the autonomy of used components, which is very important in CWS and makes our proposition different from the others.
The main drawback of this concept is that alternatives must be prepared before execution can start, and the number of them may become high. Forn used WS we can have as many as 2n versions, because we may need a version for working and not working case of every WS. Because each version must be translated into BPEL and deployed on server, complex CWS may require substantial computation power and memory. To overcome this disadvantage we propose to use the Petri Nets model as an execution basis and perform the reachability analysis during execution. This was (partially) done by implementing an engine to execute CWS, based on the colored Petri Nets models. The implementation is a plugin to the BRITNeY tool [8], which allows us to add to the nets the code that invokes external WS (they are deployed on Axis 1.4 server) and which uses a simulation as the basis for executing CWS. Within this engine we can perform dynamically the reachability analysis using OE-graphs, which are built for every loaded CPN model of CWS. In this way we can avoid storing many versions of CWS and decide whether external WS should be invoked at the time of executing CWS.
The other problem of our proposition is the very limited number of solutions to overcome failures of used WS. e use only one way to do it: not to invoke the faulty WS. The result is that in case when the faulty web service is compulsory to execute successfully CWS (like FindFlight in the example) we are unable to overcome it. To make it possible, at the level of CWS, we can only try to find other web service that can perform requested operation. This can be done by maintaining a register of equivalent web services or if we use a dynamic discovery of the same WS. Another approach may be to use the semantic web services [16], thus find services that are semantically compatible.
The solution for overcoming failures of external web services presented above is based on the knowledge that the composite web services have during their definition and execution. It was shown that this solution is feasible to implement in the current standards (BPEL [26] and [34]), however we are still investigating possibilities of improving it.
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