Given a composition request, the formation of possible Web Service Composition Graphs (WSCGs) depends on the set of available services. Since the availability of Web services is dynamic, at any time, a new service can join or an existing service can leave the set of available services. A change in the set may bring the structural change in a previously formed WSCG. However, it is not always the case that a structural change in the WSCG brings the semantic change. In this paper, our aim is to verify the compositional equivalence between two WSCGs formed before and after the structural change caused by the change in the set of available services. Our proposed solution is based on an algebraic formalism, and by using the formalism, directed acyclic WSCGs are formed for a given composition request. Then, by using WSCGs, we propose the concept of composition expression and canonical composition expression. On the basis of the proposed concept of canonical composition expression, we verify compositional equivalence between two WSCGs. The advantage of our approach is that it reduces the equivalence verification to the subsumption checking between two algebraic expressions instead of directly using the WSCGs and solving a subgraph matching problem. The proposed mechanism is implemented and evaluated for the exhaustive possibilities in a travel agency case study with respect to a given composition request.
INTRODUCTIONService-oriented computing is a well-established computing paradigm developed over time and still passing through phases of technological refinements day by day. Nowadays, automatic and dynamic Web service composition is in focus of the service-oriented computing research. 1,2 As far as automatic and dynamic service composition is concerned, the dynamic reconfiguration of services is a key factor worth considering because the availability of candidate Web services cannot be specified before the runtime. Thus, the dynamic availability of services greatly affects the formation of a composition plan (made for a composition request). In this paper, our focus is on the graphical composition plans (say, Web service composition graph).In general, by means of a Web Service Composition Graph (WSCG), we refer to the graphical representation of how a set of Web services compose with each other in order to realize a composition request (composition request is formally defined in the Section 4). A WSCG depends on the set of available services. Therefore, a change in the set may bring the structural change in the previously formed WSCG. However, this is not always the case that a structural change in the WSCG brings the semantic change. For an illustration of the problem of compositional equivalence between two WSCGs, let us consider three services TA1, TA2, and HB1, where TA1 and TA2 are composite services and HB1 is a basic service. The all three services accept request for hotel booking. Suppose a user requests TA1 for the hotel booking (written as ⟨TA1, H_Book⟩). Since TA1 is a composite service, it has to take the he...
