This chapter addresses the problem of data integration in a P2P environment, where each peer stores schema of its local data, mappings between the schemas, and some schema constraints. The goal of the integration is to answer queries formulated against a chosen peer. The answer must consist of data stored in the queried peer as well as data of its direct and indirect partners. The chapter focuses on defining and using mappings, schema constraints, query propagation across the P2P system, and query answering in such scenario. Schemas, mappings, constraints (functional dependencies) and queries are all expressed using a unified approach based on tree-pattern formulas. The chapter discusses how functional dependencies can be exploited to increase information content of answers (by discovering missing values) and to control merging operations and propagation strategies. The chapter proposes algorithms for translating high-level specifications of mappings and queries into XQuery programs, and it shows how the discussed method has been implemented in SixP2P (or 6P2P) system.
A mediator-based data integration system for query answering using an optimized extended inverse rules algorithm
