The What-To-Ask Problem for Ontology-Based Peers

The issue of cooperation, integration, and coordination between information peers has been addressed over the years both in the context of the Semantic Web and in several other networked environments, including data integration, Peer-to-Peer and Grid computing, service-oriented computing, distributed agent systems, and collaborative data sharing. One of the main problems arising in such contexts is how to exploit the mappings between peers in order to answer queries posed to one peer. We address this issue for peers managing data through ontologies and in particular focus on ontologies specified in logics of the DL-Lite family. Our goal is to present some basic, fundamental results on this problem. In particular, we focus on a simplified setting based on just two interoperating peers, and we investigate how to solve the so-called “What-To-Ask” problem: find a way to answer queries posed to a peer by relying only on the query answering service available at the queried peer and at the other peer. We show both a positive and a negative result. Namely, we first prove that a solution to this problem always exists when the ontology is specified in, and we provide an algorithm to compute it. Then, we show that for the case of the problem may have no solution. We finally illustrate that a solution to our problem can still be found even for more general networks of peers, and for any language of the DL-Lite family, provided that we interpret mappings according to an epistemic semantics, rather than the usual first-order semantics.