Recently, there have been several proposals that consider the integration of information and the computation of queries in an open-ended network of distributed peers (Bernstein, Giunchiglia, Kementsietsidis, Mylopulos, Serafini, & Zaihrayen, 2002; Calvanese, De Giacomo, Lenzerini, & Rosati, 2004; Franconi, Kuper, Lopatenko, & Zaihrayeu, 2003) as well as the problem of schema mediation and query optimization in P2P (peerto- peer) environments (Gribble, Halevy, Ives, Rodrig, & Suciu, 2001; Halevy, Ives, Suciu, & Tatarinov, 2003; Madhavan & Halevy, 2003; Tatarinov & Halevy, 2004). Generally, peers can both provide or consume data and the only information a peer participating in a P2P system has is about neighbors, that is, information about the peers that are reachable and can provide data of interest. More specifically, each peer joining a P2P system exhibits a set of mapping rules, in other words, a set of semantic correspondences to a set of peers that are already part of the system (neighbors). Thus, in a P2P system, the entry of a new source, or peer, is extremely simple as it just requires the definition of the mapping rules. By using mapping rules as soon as it enters the system, a peer can participate and access all data available in its neighborhood, and through its neighborhood it becomes accessible to all the other peers in the system.
Query optimization in deductive databases by magic-sets transformation.
