What is dual licensing in open-source projects?

Dual licensing in open-source projects refers to offering the same software under two distinct licenses, allowing users to choose the terms that best fit their needs. Typically, one license is a free, open-source license like the GNU General Public License (GPL), which requires derivative works to also be open-source. The second is a commercial license, often paid, that removes these restrictions. This approach enables developers to monetize their work while still contributing to the open-source community. For example, a company might release a project under the GPL to encourage community contributions but offer a proprietary license for enterprises that want to integrate the software without releasing their own code as open-source.

A common example is MySQL, which uses dual licensing to serve both open-source developers and commercial users. Under the GPL, anyone can use, modify, and distribute MySQL, but if they embed it into a proprietary product, they must open-source their entire application. To avoid this, companies can purchase a commercial license, which grants them the freedom to keep their code private. Another example is Qt, a framework developed by the Qt Company, which offers a GPL version alongside a commercial license. This model allows hobbyists and open-source projects to use Qt freely, while enterprises pay for flexibility and support.

Dual licensing has trade-offs. On the positive side, it provides a revenue stream to sustain development and offers users flexibility. However, managing two licenses can create complexity, such as ensuring code contributions comply with both licenses. Some developers also criticize the model for potentially fragmenting the community, as paid users might prioritize features that benefit their commercial needs over the broader open-source user base. Projects considering dual licensing must weigh these factors carefully and communicate licensing terms transparently to avoid confusion. Ghostscript, a PDF interpreter, initially used dual licensing but later shifted to the AGPL to simplify its model, illustrating how strategies can evolve based on practical challenges.