Query processing in RDF/S-based P2P database systems

Abstract

In Peer-to-peer (P2P) systems a very large number of autonomous computing nodes (the peers) pool together their resources and rely on each other for data and services. More and more P2P data management systems rely nowadays on intensional (i.e., schema) information for integrating and querying peer bases. Such information can be easily captured by emerging Semantic Web languages such as RDF/S. In this chapter, we present the SQPeer middleware for processing RQL queries over peer RDF/S bases (materialized or virtual), which are advertised using adequate RVL views. The novelty of SQPeer lies on the interleaved execution of the query routing and planning phases using intensional advertisements of peer bases under the form of RDF/S schema fragments (i.e., views). More precisely, routing is responsible for discovering peer views relevant to a specific query based on appropriate subsumption techniques of RDF/S schema fragments. On the other hand, query planning relies on the obtained data localization information, as well as compile and run-time optimization techniques. The generated plans are then executed in a fully distributed way by the involved peers for obtaining as fast as possible the first results of a query available in peer bases. This can be achieved by initially considering the peer bases that answer the whole query and at each iteration round, to route and evaluate smaller query fragments. The interleaved execution not only favors intra-peer processing, which is less expensive that the inter-peer one, but additionally takes benefit of a parallel execution of the query routing, planning and execution in different peers. Peers can exchange query plans and results, as well as, revisit established plans using appropriate communication channels. We finally demonstrate through examples the execution of two main query processing phases for two different architectural alternatives, namely a hybrid and a structured RDF/S schema-based P2P system. © 2006 Springer-Verlag Berlin Heidelberg.