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One of the major current trends in service-oriented systems is the emphasis given to the need of introducing runtime adaptation features, so that the system can meet its QoS requirements in a volatile operating environment. In this paper we present the design and implementation of a service broker that supports the QoS-driven runtime adaptation of SOA applications offered as composite services to users. We describe the functionalities provided by the broker components and present their design and implementation according to two different versions we have developed and that are both based on open source products. The components of the first version have been developed in Java as Web services, while the second version takes advantage of OpenESB. Since the broker needs to sustain a traffic of requests generated by several concurrent users, we also present the replicated architectures of the two broker versions. We discuss the design tradeoffs and the lesson we have learned in developing the broker.