Ontop is a Virtual Knowledge Graph (VKG) system. A VKG system exposes the content of arbitrary relational databases as knowledge graphs. These graphs are virtual, which means that data remains in the data sources instead of being moved to another database, and are realized through a conceptual representation of the domain of interest, provided in terms of an ontology, to which the data sources are mapped.

Ontop translates SPARQL queries expressed over the knowledge graphs into SQL queries executed by the relational data sources. It relies on R2RML mappings and OWL 2 QL ontologies.

Key features of Ontop are:

• its solid theoretical foundations, based on the VKG approach, which avoids materializing triples and is implemented through the query rewriting technique;
• its compliance to all relevant W3C recommendations (including SPARQL queries, R2RML mappings, and OWL 2 QL and RDFS ontologies), and its support for all major relational databases.
• Its good performance in query answering, by extensive optimizations exploiting all elements of the OBDA architecture;

Ontop supports applications which want to get the advantage of the VKG technology in the following ways:

• through a stand-alone SPARQL endpoint, which can be easily setup also through a dedicated docker image.
• through a Command Line Interface (CLI);

Ontop also comes in the form of a plugin for Protégé, providing a number of functionalities such as mappings bootstrapping and creation, or SPARQL query answering over the virtual knowledge graph.