Reliable SPARQL queries with consistent results over P2P-shared RDF storage

One aim of the semantic web is to build large knowledge bases distributed over the internet. Knowledge management systems that gather, merge and make available the information physically stored in multiple locations suffer from consistency and data fragmentation issues due to node failures. In this paper we address such problems and we present an architecture for managing reliable SPARQL queries with consistent results over a P2P-shared RDF storage. The RDF-storage is composed of peer nodes organized in a ring topology based on a Distributed Hash Table (DHT) where each node provides an entry point that enables clients outside the network to query the knowledge base using atomic, disjunctive, and conjunctive SPARQL queries. The consistency of the results is increased using a data redundancy algorithm that replicates each RDF triple in multiple nodes so that, in the case of peer failure, other peers can retrieve the data needed to solve the queries. Additionally a load distribution algorithm is used to maintain a uniform distribution of the data among the participating peers by dynamically changing the key space assigned to each node in the DHT. The performance of this approach is then evaluated by monitoring the effectiveness of the load balancing and redundancy algorithm and the overhead introduced on the network load in both a static (only join events) and a dynamic scenario.