Abstract:
Real-scale Semantic Web applications, such as Knowledge Portals and E-Marketplaces, require the management of voluminous repositories of resource metadata. At the same time, personalized access and content syndication involving diverse conceptual representations of information resources are key challenges for such applications. The Resource Description Framework (RDF) enables the creation and exchange of metadata as any other Web data and constitutes nowadays the core language for creating and exchanging resource descriptions worldwide. Although large volumes of RDF descriptions are already appearing, sufficiently expressive declarative query languages for RDF and full-fledged view definition languages are still missing . In this talk, we present RQL, a new query language adopting the functionality of semistructured or XML query languages to the peculiarities of RDF, but also extending this functionality in order to uniformly query both RDF descriptions and schemas. RQL is a typed language, following a functional approach (a la OQL) and relies on a formal graph model that permits the interpretation of superimposed resource descriptions created using one or more schemas. We illustrate the syntax, semantics and type systems of RQL and report on the performance of RSSDB, our persistent RDF Store, for storing and querying voluminous RDF metadata. We also propose $\RVL$, a view definition language capable of creating not only virtual resource descriptions, but also virtual RDF/S schemas from (meta)classes, properties, as well as, resource descriptions available on the Semantic Web. $\RVL$ exploits the functional nature and type system of the RQL query language in order to navigate, filter and restructure complex RDF/S schema and resource description graphs. Last, but not least, we address the problem of integrating legacy data sources using SWIM, a Datalog-based framework for mediating high-level queries to relational and/or XML sources using ontologies expressed in RDF/S.