Source-available software is code whose source is published for anyone to read, study, and often modify, but which is distributed under terms that fall short of the Open Source Definition maintained by the Open Source Initiative. The defining gap is freedom of use and redistribution: a true open source license may not discriminate against any field of endeavor, including commercial competition, while source-available licenses typically add exactly that kind of restriction. The result is a middle ground between fully proprietary software, whose source is hidden, and open source, whose source comes with no use restrictions.
The category rose to prominence in the late 2010s as commercial open source companies confronted a recurring business problem. A vendor would build a popular open source product, and a large cloud provider would then offer that same software as a managed service, capturing much of the revenue without a proportional contribution back to the project. To defend their business models, several companies adopted licenses that kept the code readable but restricted the specific activity they feared most: reselling the software as a hosted service.
Three license forms became the touchstones of the movement. The Commons Clause, published in 2018, is a short rider attached to an existing permissive license; its text removes “the right to Sell the Software,” defined to include offering a product whose value derives entirely or substantially from the software’s own functionality, such as a hosted service. MongoDB’s Server Side Public License (SSPL), introduced for MongoDB Community Server versions released on or after October 16, 2018, is based on GPL version 3 but adds a Section 13 that requires anyone offering the software as a public service to release the source of the entire surrounding service stack, including management, monitoring, backup, and hosting software. MariaDB’s Business Source License (BSL) takes a different approach, restricting production use for a defined period before automatically converting to an open source license.
The OSI has declined to approve these licenses as open source, precisely because their use restrictions conflict with the non-discrimination clauses of the Open Source Definition. Critics argued that calling such software “open” blurred a hard-won definition; defenders argued that the alternative was companies abandoning openness entirely and shipping closed binaries. The MongoDB SSPL FAQ frames its restriction narrowly, stressing that ordinary users who run the database or build applications on top of it are unaffected, and that only those offering the software itself as a public service are constrained.
Source-available licensing remains contested terrain. It is a pragmatic response to the economics of cloud-hosted open source, and it preserves the most visible benefit of open source, the ability to read and patch the code, while withholding the freedom that the formal definition treats as essential. Several high-profile relicensings, including Elasticsearch’s move to SSPL and the Elastic License, sit squarely in this category, and the debate over whether such software deserves the open source label continues to shape how the industry talks about licensing.