The Ontop Framework for Ontology Based Data Access

Ontology Based Data Access (OBDA) [4] is a paradigm of accessing data trough a conceptual layer. Usually, the conceptual layer is expressed in the form of an RDF(S) [10] or OWL [15] ontology, and the data is stored in relational databases. The terms in the conceptual layer are mapped to the data layer using mappings which associate to each element of the conceptual layer a (possibly complex SQL) query over the data sources. The mappings have been formalized in the recent R2RML W3C standard [6]. This virtual graph can then be queried using an RDF query language such as SPARQL [7]. Formally, an OBDA system is a triple O = 〈T,S,M〉, where: – T is the intensional level of an ontology. We consider ontologies formalized in description logics (DLs), hence T is a DL TBox. – S is a relational database representing the sources. – M is a set of mapping assertions, each one of the form Φ(x) ← Ψ(x) where • Φ(x) is a query over S, returning tuples of values for x • Ψ(x) is a query over T whose free variables are from x. The main functionality of OBDA systems is query answering. A schematic descrip-tion of the query transformation process (usually SPARQL to SQL) performed by a typical OBDA system is provided in Figure 1. In such an architecture, queries posed over a conceptual layer are translated into a query language that can be handled by the data layer. The translation is independent of the actual data in the data layer. In this way, the actual query evaluation can be delegated to the system managing the data sources. 2 The Ontop Framework Ontop is an open-source OBDA framework released under the Apache license, developed at the Free University of Bozen-Bolzano1 and currently acts as the query transformation module of the EU project Optique2