τOWL: A Framework for Managing Temporal Semantic Web Documents

The World Wide Web Consortium (W3C) OWL 2 Web Ontology Language (OWL 2) recommendation is an ontology language for the Semantic Web. It allows defining both schema (i.e., entities, axioms, and expressions) and instances (i.e., individuals) of ontologies. OWL 2 ontologies are stored as Semantic Web documents. However, OWL 2 lacks explicit support for time-varying schema or for time-varying instances. Hence, knowledge engineers or maintainers of semantics-based Web resources have to use ad hoc techniques in order to specify OWL 2 schema for time-varying instances. In this paper, for a disciplined and systematic approach to the temporal management of Semantic Web documents, we propose the adoption of a framework called Temporal OWL 2 (τOWL), which is inspired by the τXSchema framework defined for XML data. In a way similar to what happens in τXSchema, τOWL allows creating a temporal OWL 2 ontology from a conventional (i.e., non-temporal) OWL 2 ontology and a set of logical and physical annotations. Logical annotations identify which elements of a Semantic Web document can vary over time; physical annotations specify how the time-varying aspects are represented in the document. By using annotations to integrate temporal aspects in the traditional Semantic Web, our framework (i) guarantees logical and physical data independence for temporal schemas and (ii) provides a lowimpact solution since it requires neither modifications of existing Semantic Web documents, nor extensions to the OWL 2 recommendation and Semantic Web standards. Keywords–Semantic Web; Ontology; OWL 2; τXSchema; Logical annotations; Physical annotations; Temporal database; XML Schema; XML