What Is an Open Source Database?
The Detailed Answer
The term "open source" has a specific, formal meaning defined by the Open Source Initiative (OSI), the organization that has maintained the Open Source Definition since 1998. A database qualifies as open source when its license satisfies all ten criteria in the OSD, including free redistribution, access to source code, permission to create derived works, no discrimination against persons or fields of use, and technology neutrality. This definition ensures that open source software provides genuine freedom to its users, not just access to the code.
Open source databases differ from proprietary databases like Oracle Database, Microsoft SQL Server, and IBM Db2 in several fundamental ways. There are no per-server, per-core, per-socket, or per-user licensing fees. You can deploy the database on as many servers as you need without negotiating license agreements or tracking compliance. You can read the source code to understand how the database works, verify its security, and diagnose bugs at the deepest level. You can modify the code to fix issues, add features, or optimize performance for your specific workload. And you can do all of this without asking anyone for permission.
Why Open Source Licensing Matters for Databases
The license on your database affects more than legal compliance. It shapes your long-term flexibility in ways that may not be obvious when you first choose a database for a new project.
Vendor lock-in is the most practical concern. With a genuinely open source database, no single company can change the terms under which you use the software. PostgreSQL's permissive license and community governance mean that even if every company currently contributing to PostgreSQL disappeared, the code would remain freely available and the community could continue development. With a source-available database under SSPL or BSL, the company controlling the license can change terms at any time, as Redis demonstrated in 2024 when it moved from BSD to a restrictive license.
Cloud portability is another factor. Open source databases can be deployed on any cloud provider, on-premises, or in hybrid configurations without licensing complications. Source-available databases may restrict how cloud providers can offer them, which affects the availability and pricing of managed services. Valkey, as a BSD-licensed Redis fork, is now offered as a managed service by multiple cloud providers, giving users more competitive options than were available when Redis was the only choice.
Contribution and customization matter for organizations that need to modify the database. Open source licenses guarantee your right to fork, patch, and redistribute modifications. Source-available licenses may permit modifications for internal use but restrict redistribution, limiting your ability to share improvements with the community or contribute upstream.
The Most Important Open Source Databases
The open source database ecosystem is mature and covers every major workload category. PostgreSQL is the most feature-rich relational database, released under the ultra-permissive PostgreSQL License. MySQL and MariaDB are the most widely deployed relational databases, both under the GPL. SQLite is the most deployed database engine of any kind, released into the public domain. Apache Cassandra provides distributed wide-column storage under Apache 2.0. Valkey provides Redis-compatible in-memory caching under BSD. ClickHouse provides columnar analytics under Apache 2.0. Neo4j Community provides graph database capabilities under the GPL. Apache CouchDB provides document storage under Apache 2.0.
Together, these databases cover relational OLTP, document storage, key-value caching, wide-column distributed writes, graph traversals, columnar analytics, time-series data, and embedded/serverless use cases. There is no workload category where you must use a proprietary or source-available database, which is the strongest argument for the health and maturity of the open source database ecosystem.
How Open Source Databases Are Funded
Open source databases sustain development through several business models. Some are backed by companies that sell managed cloud services (Supabase and Neon for PostgreSQL, Aiven for multiple databases, Instaclustr for Cassandra). Others are supported by companies that sell enterprise features, support contracts, and consulting (MariaDB Corporation, Percona for MySQL/PostgreSQL, Neo4j Inc). PostgreSQL is funded by a broad ecosystem of contributing companies rather than a single commercial entity, which is one reason its governance is considered the most resilient.
Community contributions from individual developers, universities, and organizations that use the database also play a significant role. PostgreSQL's core development includes contributors from dozens of companies and independent developers across multiple continents. This diversity of funding and contribution sources reduces the risk that any single business decision (like a license change or acquisition) could disrupt the project.
An open source database is one whose license is approved by the Open Source Initiative, guaranteeing your freedom to use, modify, and distribute it without restriction. PostgreSQL, MySQL, MariaDB, SQLite, Cassandra, Valkey, and ClickHouse all qualify. MongoDB and Redis do not, despite publishing their source code.