Service composition is one of the most promising advantages of the service-oriented paradigm. In a service market scenario, given a functional description of a service, different providers may offer diverse service implementations that match such a functional description but differ for some QoS attributes. A key point for the construction of a suitable composition is the selection of the services that best meet the QoS requirements of the composite service users. In this paper, we consider a broker-based architecture for service composition, focusing on the service selection problem and assuming that the broker supports different QoS classes. We formulate the service selection as a constrained optimization problem, where each QoS class is modeled by suitable constraints. Differently from most of the existing approaches to service selection, in our approach the broker optimizes the overall QoS of a flow of requests rather than of a single request.