Hybrid reasoning on OWL RL

Both materialization and backward-chaining as different modes of performing inference have complementary advantages and disadvantages. Materialization enables very efficient responses at query time, but at the cost of an expensive up front closure computation, which needs to be redone every time the knowledge base changes. Backward-chaining does not need such an expensive and change-sensitive precomputation, and is therefore suitable for more frequently changing knowledge bases, but has to perform more computation at query time. Materialization has been studied extensively in the recent semantic web literature, and is now available in industrial-strength systems. In this work, we focus instead on backward-chaining, and we present a general hybrid algorithm to perform efficient backward-chaining reasoning on very large RDF datasets. To this end, we analyze the correctness of our algorithm by proving its completeness using the theory developed in deductive databases and we introduce a number of techniques that exploit the characteristics of our method to execute efficiently (most of) the OWL RL rules. These techniques reduce the computation and hence improve the response time by reducing the size of the generated proof tree and the number of duplicates produced in the derivation. We have implemented these techniques in an experimental prototype called QueryPIE and present an evaluation on both realistic and artificial datasets of a size that is between five and ten billion of triples. The evaluation was performed using one machine with commodity hardware and it shows that (i) with our approach the initial precomputation takes only a few minutes against the hours (or even days) necessary for a full materialization and that (ii) the remaining overhead introduced by reasoning still allows atomic queries to be processed with an interactive response time. To the best of our knowledge our method is the first that demonstrates complex rule-based reasoning at query time over an input of several billion triples and it takes a step forward towards truly large-scale reasoning by showing that complex and large-scale OWL inference can be performed without an expensive distributed hardware architecture.