An open source database is any database application with an open source license and codebase that renders it free to download, view, distribute, modify, and reuse. Developers have more freedom with open source licenses to use existing database technologies along with their own ideas and innovation to build new applications.
The first open-source databases were basic SQL databases, for example, PostgreSQL  and MySQL . Today many NoSQL open source databases also exist, such as document stores like MongoDB  and Couchbase , as well as wide-column databases such as Apache Cassandra , and ScyllaDB  for analysis of large data sets; e.g., those produced by social media, AdTech, or the Internet of Things (IoT).
Why use open source databases? Although the best open source database is largely going to be determined by the specific needs of the project and user, there are some general advantages of open source databases.
Commercial databases can demand major up front and recurring costs for infrastructure, software licenses, and technical expertise before anything is developed or deployed. Commercial licenses generally cost more depending on which features or level of usage attach to them. In contrast, typically open source databases attach no license fees for either the database or its features.
Some companies offer both OSS and enterprise database options with additional features and support. In such “open core” models, the foundational version or core of the product is free, but additional features require commercial licensing. This way, users can start by trialing the OSS version and easily move to the more robust, better-supported version when it’s time to go into production or scale their applications.
Database Extensions and Plugins
There are an impressive range of plugins and extensions available for many open source databases touching on integration, real-time monitoring, and custom data formats. Users can generally add them on to the standard database platform and configure them precisely as needed at no additional cost. However, some open source business models allow for additional commercial licenses — so-called dual licenses or multi-licensing — that may require an additional purchase to use additional features, extensions or plugins.
Community Support and Development
As described above, open source more broadly refers to source code that is available for users to deploy and modify as they choose. Yet it also refers to an ethos of an open source community and culture, with the developer and user community providing mutual support for their peers looking to modify or adopt the software for their own use. This itself can lead to a robust community of new product and feature development and free support.
As new business models emerge, open-source software is likely to exist that can take advantage of related new opportunities. In addition, since users customize their open-source databases in countless ways, the most popular open-source products are very flexible and benefit from massive user communities. These communities generate documentation, YouTube training videos, and Q&A forums with free open source database expert advice.
However, some open source contributors and communities maintain themselves by giving away the open source database code for free use, yet charging other users for application development, cloud hosted services, or technical services and support.
There are several potential disadvantages to open source databases as well—although the most robust, well-used open source databases mitigate against these.
Lack of Vendor Accountability
Commercially-provided software, supported by a vendor, means that the database software comes with explicit warranties and support agreements. However, open source community is often more of an “as-is” offering, with little recourse in case of defects.
While many open source communities are as responsive (or even at times more responsive) than a comparable commercially-provided product, users considering adopting open source databases should objectively research the open source contributors and community backing the project to assess such risks as:
Maintenance and Support
For users that need 24/7 enterprise support with an SLA, open-source products and services may not be the best fit.
Depending on release and platform, documentation breadth and depth for open source databases can vary. Where users deploy additional open source utilities or plugins with the database for increased functionality, this may become an issue, particularly where documentation sources are not robust. Reliance on blog posts, community emails, and the other typical online sources can morph into a time consuming exercise in trial and error.
Some open source databases, especially those with wide adoption, have mature capabilities. However, commercial databases may offer more advanced maturity and functionality; this may be particularly salient for workloads that rely on specific database features.
There are many examples of open source databases on the web to choose from. Examples of popular open source relational databases include: CockroachDB, PostgreSQL, MySQL, and MariaDB. Examples of popular open source NoSQL document databases include MongoDB and CouchDB. Examples of popular open source NoSQL wide column databases include Cassandra and ScyllaDB.. Examples of open source key value databases include Redis, Riak, and ScyllaDB. Examples of open source graph databases include Neo4j and JanusGraph.
When selecting an open source database management system for your production environment, consider the factors outlined in these vendor agnostic expert guides.
Martin Heller is a well-respected industry expert. His database selection guide outlines 12 key questions you should consider when evaluating and selecting an open source database:
His article expands on each of those questions and explores their implications for your database evaluation.
Gigaom offers a number of reports that might be helpful when selecting an open source database. For example:
G2 offers a database selection guide, as well as an extensive library of user reviews on open source SQL and NoSQL databases.
In each review, real users respond to questions such as: