| Abstract |
In recent years, the service-oriented computing research community has paid a lot of attention to Web service specification, discovery and composition. Web services can be seen as applications accessible to other applications over the Web. As Web services become commodities and their numbers increase, more efficient platforms supporting their orchestration and deployment in applications are needed. The ability to efficiently select and compose services on the Web in order to accomplish a complex user goal is of utmost importance. However, current Web service descriptions do not enable the efficient selection and composition of available services. Several proposals for addressing the Web service composition problem, which can also be seen as a workflow or an AI planning problem, have appeared in the literature. However,, service composition still lacks satisfactory solutions. Current Web service specifications provide services with the definition of preconditions and postconditions, i.e., conditions that must be satisfied in the initial and final state of the service. However, these types of conditions are not sufficient for Web service composition. Due to the need for composition of a set of services, several conditions and constraints that did not exist in the individual service specifications arise. For the definition of these conditions, we augment Web service specifications with invariant assertions. These assertions must be satisfied throughout the set of states of a complex service. In addition, a set of composition rules is proposed. Given specific preconditions, postconditions and invariants of the Web services that need to be composed, we use these rules to determine the effects of the complex Web service in the presence of complex operations. Composition rules enable reasoning about service composition and service interactions. They can be used to provide compositionality knowledge in composite service descriptions. The consideration of such rules in planning - especially in backward planning - can provide more efficient patterns for service selection as compared to the mere matching between inputs / outputs and preconditions / postconditions. These patterns are based on rule satisfiability proving, and provide the generation of a plan, the execution of which will produce the desired goal.
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