RAL: an algebra for querying RDF

Abstract

To make the WWW machine-understandable there is a strong demand both for languages describing metadata and for languages querying metadata. The Resource Description Framework (RDF), a language proposed by W3C, can be used for describing metadata about (Web) resources. The RDF schema (RDFS) extends RDF by providing means for creating application specific vocabularies (ontologies). While the above two languages are widely acknowledged as a standard means for describing Web metadata, a standardized language for querying RDF metadata is still an open issue. Research groups from both industry and academia are presently involved in proposing several RDF query languages. Due to the lack of an RDF algebra such query languages use APIs to describe their semantics and optimization issues are mostly neglected. This paper proposes RAL, an RDF algebra suitable for defining (and comparing) the semantics of different RDF query languages and (at a future stage) for performing algebraic optimizations. After a definition of the data model we present the operators with which the model can be manipulated. The operators come in three flavors: extraction operators retrieve needed resources from the input RDF model, loop operators support repetition, and construction operators build the resulting RDF model.