A peer mediator system (PMS) is a decentralized mediator system based on the P2P paradigm, where mediators integrate data sources and other mediators through views defined in a multi-mediator query language. In a PMS mediator peers compose views in terms of views in other peers - mediators and sources, or directly pose queries in the multi-mediator query language to some peer. All peers are fully autonomous and there is no central catalog or controller. Each peer in a PMS must provide an interface to its data and meta-data sufficient to allow the cooperative processing of queries by the PMS. We analyze the computational capabilities and meta-data that a software system has to export in order to participate as a peer in a PMS. For the analysis we identify and compare six classes of peer interfaces with increasing complexity. For each class we investigate the performance and scalability implications that result from the available capabilities and required meta-data. Our results are two-fold: i) we provide guidelines for the design of mediator peers that can make best use of the interfaces provided by the data sources, and ii) we analyze the tradeoffs in the design of inter-mediator interfaces so that mediator peers can efficiently cooperate to process queries against other composed mediators. Finally we describe the choices made in a concrete implementation of a PMS.
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