Abstract: The characteristics of Internet such as open, autonomy and dynamic impose new challenges on share and composition over Internet resources. Meanwhile, researchs on runtime QoS-guarantee of Web service composition, service redundant technology and replanning arc very important fault-tolerant methods.But they are facing severe problems on high-cost, requirement of real-time and infcasibility of replanning strategy. Fault tolerance is a tradeoff between QoS-guarantee and extra-overhead. In order to reduce the fault tolerant cost, several decomposed principias were presented based on analyise of high-cost brought by fault tolerant technology. According to these principias, it is helpful for the designer to select the proper service granularity and reduce the degree of coupling of services. Simulation results indicate that the mechanism is effective to increase the reliability of composite services and reduce the fault tolerant cost.

Key words: Servcic composition, Service granularity, Service redundant, Replanning, Reliability, Fault tolerance