Introduction

Ontop is a Virtual Knowledge Graph system. It 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.

Ontop translates SPARQL queries (opens new window) expressed over the knowledge graphs into SQL queries executed by the relational data sources. It relies on R2RML mappings (opens new window) and can take advantage of lightweight ontologies.

Versions

This documentation is for Ontop 3.0 and more recent versions.

For earlier versions, please refer to our previous Wiki (opens new window).

Most recent version:

• Stable: Ontop 5.2.1, released on November 24, 2024.

See release notes for more details.

Main features

• Uses RDF 1.1 as graph data model
• Supports RDFS and OWL 2 QL ontologies
• Supports R2RML and Ontop mappings
• Supports the majority of SPARQL 1.1 features, including the main SPARQL aggregation functions (since 4.0.0) and GeoSPARQL functions (since 4.1.0)
• Can be deployed as a SPARQL endpoint and as a predefined query endpoint (since 4.1.0)
• Produces efficient SQL queries by applying many optimizations
• Supports the main database systems: PostgreSQL, MySQL, MariaDB (since 5.0.0), SQL Server, Oracle, DB2, Snowflake (since 5.0.0), Databricks (since 5.0.0), Google BigQuery (since 5.0.2), AWS Redshift (since 5.0.2), DuckDB (since 5.0.2), and AWS DynamoDB (since 5.1.0)
• Supports database federators such as Denodo, Dremio (since 4.1.0), Teiid (since 4.1.1), Apache Spark (since 4.2.0) and Trino / PrestoDB / AWS Athena (since 5.0.2)
• Supports lenses which are "virtual views" specified outside of the data sources (since 4.2.0)
• Can materialize virtual graphs into RDF files
• Provides a plugin for editing and testing mappings in the Protégé ontology editor (opens new window)

Organizations

Ontop is backed by the Free University of Bozen-Bolzano (opens new window) and Ontopic s.r.l. (opens new window). It also receives regular important contributions from University of Bergen (opens new window) and Birkbeck, University of London (opens new window). See the community section for more details.

Licenses

Ontop is available under the Apache 2.0 (opens new window) license.

All the documentation is licensed under the Creative Commons (Attribution) (opens new window) license.

Social

You can find us on the following social platforms:

• Twitter (ontop4obda) (opens new window)
• Mastodon (@ontop@fosstodon.org)
• Facebook (obdaontop) (opens new window)
• Google Group (ontop4obda) (opens new window)
• GitHub (ontop/ontop) (opens new window)

Citations

• If you use Ontop in your work, please cite one of the following articles describing the system.

  • Guohui Xiao, Davide Lanti, Roman Kontchakov, Sarah Komla-Ebri, Elem Güzel-Kalayci, Linfang Ding, Julien Corman, Benjamin Cogrel, Diego Calvanese, and Elena Botoeva. The Virtual Knowledge Graph System Ontop (opens new window). In: International Semantic Web Conference (Resource Track), 2020.
  • Diego Calvanese, Benjamin Cogrel, Sarah Komla-Ebri, Roman Kontchakov, Davide Lanti, Martin Rezk, Mariano Rodriguez-Muro, and Guohui Xiao. Ontop: Answering SPARQL Queries over Relational Databases (opens new window). In: Semantic Web Journal 8.3 (2017), pp. 471–487.

• If you want to cite the techniques behind Ontop, check our publications.