Key Research On Technologies Of Web Services Composition And Availability

With the ability to realize the interoperation of applications on the internet, Web service emerges rapidly as a kind of promising integrating technology. A basic requirement for Web service is services composition. That is, to compose different existed web services to realize new value-added functions. Services composition has become a hot topic in the research field of Web service. Some specifications have been put forward as proposals for web services composition, such as BPEL4WS and WSCI.Substitution and recovery mechanism are important issues for availability of web services which participant composition. Current researches for substitution and recovery neglect characterizations of web services, such as autonomy, heterogeneous and loosely-coupled. These characterizations play important roles for substitution and recovery.In this paper, based on formal description of web service and services composition, we analyze constraints of correct composition and develop the algorithm to check compatibility of component services. Besides, we offer conditions of context-independent substitution and design a kind of recovery mechanism. Our main contributions are:1. Give a model for web services composition. Firstly, we formalize message flow of a single web service; then give a model for asynchronous composition of web services. To acquire correct composition, we use a bottom-up method to analyze compability of web services in asynchronous communication environment. Some constraints are put forward to make analysis of compatiliby decidable and an algorithm is provided for verification of compability.2. Offer constraints for substitution of Web service. Services are substituted in case of failure. We give a conformance relation to restrict services which can be used as substitution. In this way, correctness of composition can be guaranteed. Besides, substitution is context-independent.3. Design a recovery mechanism. To keep characterizations of web services, we design recovery mechanism for the decentralized composition mode. Recovery job are performed by different composition engines. Exactly-once execution semantics can be guaranteed. We also build a model to analyze costs of different mechanisms that deal with failures of composition, thus we can tell the cases that our recovery mechanism is more suitable.4. We develop the tool based on the composition model and the constraitns of substitution analysis. The above results expatiate on the key technologies of web services composition and availability, which can be not only applied to the B2B business applications, but also can be widely used in the scientific research fields, such as bioinformatics, weather forecasting, distributed computing and the grid service.This work is partially supported by the National Basic Research Program (973) under grant No.2005CB321905.