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The recent outburst of context-dependent knowledge on the Semantic Web (SW)has led to the realization of the importance of the quads in the SW community.Quads, which extend a standard RDF triple, by adding a new parameter of the`context' of an RDF triple, thus informs a reasoner to distinguish between theknowledge in various contexts. Although this distinction separates the triplesin an RDF graph into various contexts, and allows the reasoning to be decoupledacross various contexts, bridge rules need to be provided for inter-operatingthe knowledge across these contexts. We call a set of quads together with thebridge rules, a quad-system. In this paper, we discuss the problem of queryanswering over quad-systems with expressive forall-existential bridge rules. Itturns out the query answering over quad-systems is undecidable, in general. Wederive a decidable class of quad-systems, namely context-acyclic quad-systems,for which query answering can be done using forward chaining. Tight bounds fordata and combined complexity of query entailment has been established for thederived class.

Query Answering over Contextualized RDF/OWL Knowledge with Forall-Existential Bridge Rules: Attaining Decidability using Acyclicity (full version)