Abstract
The database (DB) landscape has been significantly diversified during the last decade, resulting in the emergence of a variety of non-relational (also called NoSQL) DBs, e.g., xml and json-document DBs, key-value stores, and graph DBs. To enable access to such data, we generalize the well-known ontology-based data access (OBDA) framework so as to allow for querying arbitrary data sources using sparql. We propose an architecture for a generalized OBDA system implementing the virtual approach. Then, to investigate feasibility of OBDA over non-relational DBs, we compare an implementation of an OBDA system over MongoDB, a popular json-document DB, with a triple store. This article is an extended and revised version of an article that appeared in the proceedings of the 17th International Conference of the Italian Association for Artificial Intelligence (AI IA) [4].