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Under SOA, multiple services can be aggregated to create a new composite service based on some predefined workflow. The QoS of this composite service is determined by the cooperation of all these Web services. With workflow pipelining, it is unreasonable to improve the overall service performance by only considering individual services without considering the relationship among them. In this paper, we propose to allocate resources by tracing and predicting workloads dynamically with the pipelining of service requests in workflow graph. At any moment, there are a number of service requests being handled by different services. Firstly, we predict future workloads for any requests as soon as they arrive at any service in the workflow. Secondly, we allocate resources for the predicted workloads to enhance the performance by replicating more services to resources. Our target is to maximize the number of successful requests with the constraints of limited resources. Experiment shows that our dynamic resource allocation mechanism is more efficient for enhancing the global performance of composite service than static resource allocation mechanism in general.