| Web services are distributed and self-described modular components based on network, which is used to perform specific tasks. It is logically and relatively independent business units and it performs specific functions. Generally, Web services are designed for special purposes based on the functionalities of their providers. Moreover, to decentralize and simplify the application logic and to improve the reusability of Web services, a single service should not be complex. However, in the real world, a user's requests may be so complex that it can not be satisfied by a single service. Therefore, it is necessary to compose some pre-existing available Web services to provide new value-added services.However, this procedure is called service composition. The number of Web services is dramatically increasing with the popularity of Web services technology. Although Web services'application is prospective, and its basic framework and main technology are also developing and growing rapidly, there are many other technical challenges that have to be handled before delivering to practical applications and operation through which many benefits can be brought up. Among these challenges, fault-tolerance is an important one.The availability of composed Web service is further studied in this thesis. The fault-tolerance ability of composed Web service is strengthened to improve its availability.This thesis mainly discusses Web service composition with fault-tolerance. The major work and contributions are as following:(1) The service will be composition problems in mathematics abstraction description, with constraints into a multi-objective optimization model of the composition. According to the optimization model and multi-objective genetic method NSGA-Ⅱ, this thesis designs Web services composition of multi-objective genetic algorithm's QoS for global optimal service selection algorithm (MOGA-WS),which gets multiple Pareto optimal composition service plan through the optimization of the algorithm calculated eventually. The composition scheme is used to select the final implementation scheme integrating candidate scheme.(2) MOGA-WS getting the optimal scheme is Pareto under constraint conditions, with multiple identical basic services, on the same objective distribution characteristics of consecutiveness. Using this feature, this thesis puts forward the composition scheme of the concept of compatible scheme. That compatibility scheme is composition plan execution of accident can replace the execution after Pareto optimal solutions, meeting Pareto optimality. In order to calculate the composition of service, and put forward compatibility scheme, which can be expressed composition service for composition relations of the hierarchical-tree structure.Based on hierarchy-tree structure, this thesis puts forward the methods used in service compatibility calculation.(3) With these compatible solutions, expectant value of success rate of successfully completing composite Web service is forecasted, which is an important index for composite Web service's execution in the dynamic scene where the basic service is unstable. Expectant value of success rate is used to assist selecting the finally decisional solution in addition to user's predilection and availability, which made the composite Web service with high availability and high completing success rate, as well as meeting individual needs, is selected. Finally, the higher expectant value of success rate of successfully completing composite Web service which means more alternative paths can be switched when more basic services occur failure, which also means more backup solutions can be switched in dynamic running environments. Therefore the execution of the whole composite Web service can be achieved; fault-tolerant ability of Composite Web services at run-time is enhanced. |
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