Weakly-coupled ontology integration of P2P database

We present the basic elements of a new semantics that captures the modular structure of a data-intensive Peer-to-peer (P2P) system, and opens up the possibility of effective query answering techniques. One of the basic characteristics of peers in our approach is that they (possibly) encapsulate an ontology of a data integration system in order to deal with incomplete Web-based information, and to offer a rich ontology interface to their clients. Each peer acts at the same level with respect to the others, with no direct mapping between them, and no unifying structure above them. Based on the above ideas, we define a general framework for P2P systems, and we describe an incremental algorithm for answering conjunctive queries posed to one peer. The contributions to the query answer of any peer are given by the certain answers that such peer provides. We show that the algorithm computes its answers according to a generalized epistemic modal S5 logic, where frames are partitioned according to the structure of peers. This new semantics for P2P systems models a kind of intensional mapping that is fundamentally different from the GLAV (Global and Local As View) extensional mappings, traditionally adopted in data integration