An open-source database is a database management system whose source code is released under an open-source license. The Open Source Initiative’s Open Source Definition sets out what such a license must allow: free redistribution, access to the source code, the right to make and distribute modifications and derived works under the same terms, and no discrimination against persons, groups, or fields of use. Applied to databases, this means anyone can run, study, modify, and redistribute the engine without paying license fees.
For decades the relational database market was dominated by expensive proprietary systems. Open-source databases such as MySQL, PostgreSQL, SQLite, and later MariaDB broke that lock, especially on the web, where developers needed a capable database they could deploy freely. MySQL’s own history reflects this origin: it was built because the developers wanted a database fast and flexible enough for their needs and made the result freely available, and it became the “M” in the LAMP stack that ran much of the early dynamic web.
The category now extends well beyond relational engines to include key-value, document, and other stores. It has also become the focus of intense licensing debates. Some companies adopt an “open core” model, keeping advanced features proprietary, and several formerly open-source databases have switched to “source-available” licenses that restrict commercial use as cloud providers and competitors built businesses on the free code. These tensions are a direct continuation of the questions Monty Widenius raised during the MySQL acquisition: who controls an open-source database, and how do you keep it open.