Distributed RDF Query Processing and Reasoning in Peer-to-Peer Networks

With the interest in Semantic Web applications rising rapidly, the Resource Description Framework (RDF) and its accompanying vocabulary description language, RDF Schema (RDFS), have become one of the most widely used data models for representing and integrating structured information in the Web. RDF provides a simple and abstract knowledge representation for resources on the Web, while RDFS defines the terms that will be used in RDF statements and gives specific meaning to them. The Linked Data initiative, which aims at connecting data sources on the Web, has already become very popular and has exposed many datasets using RDF and RDFS. With the vast amount of available RDF data sources on the Web increasing rapidly, there is an urgent need for RDF data management. RDF storage, query processing and reasoning have been at the center of attention during the last years in the Semantic Web community and more recently in other research fields as well. This thesis presents results that advance the state-of-the-art in the research area of distributed RDF query processing and reasoning in peer-to-peer (P2P) networks. We fully design and implement a P2P system, called Atlas, for the distributed query processing and reasoning of RDF and RDFS data. Atlas is built on top of distributed hash tables (DHTs), a popular case of P2P networks. The indexing scheme we deploy in our system is the triple indexing algorithm originally presented in RDFPeers where each RDF triple is indexed in the DHT three times. An important aspect of our indexing scheme is that data and schema information is handled uniformly. Although other distributed approaches assume that each node keeps all RDF schema information, we adopt a more generic approach where no global knowledge about the schema is required. In this way, our system can also handle scenarios with very big ontologies where other systems might not scale. Initially, we study algorithms for distributed RDFS reasoning in peer-to-peer networks. We design and develop distributed forward and backward chaining algorithms on top of a DHT. Our backward chaining algorithm is the first distributed top-down