What is dual licensing?
Dual licensing is a software distribution strategy under which the same codebase is offered under two distinct licences: typically a copyleft open-source licence (e.g., GPL, AGPL) for non-commercial or community use, and a separate commercial licence for proprietary or enterprise use. Dual licensing lets vendors monetise software while preserving open-source community engagement; it requires the vendor to hold copyright (or have CLA-equivalent rights) to all contributions so it can grant both licences.
Mechanics of dual licensing
- Copyright concentration: vendor must own (or have broad CLA-assignment of) all contributions to grant proprietary licences.
- Open-source side: typically strong copyleft (GPL, AGPL) so that proprietary users cannot avoid the commercial licence by using the OSS terms in closed-source products.
- Commercial side: bespoke EULA: no copyleft obligations, optional support/SLA, indemnity, professional services.
- Trademark separation: distinct trademark policy reserves brand rights for paid version.
Famous dual-licensing examples
- MySQL (Oracle, ex-Sun/MySQL AB): GPL v2 + commercial: the canonical dual-licensing case.
- Qt (Qt Group): LGPL + commercial for proprietary integration.
- MongoDB (until SSPL pivot 2018): AGPL + commercial; later abandoned dual-licence for Server Side Public License.
- Elastic (until license change 2021): AGPL + commercial; later moved to Elastic License + SSPL.
- Sentry, Plausible, Cal.com: modern AGPL + commercial dual-licence open-core companies.
Dual licensing as a business model
Dual licensing works when the open license is restrictive enough to make commercial licensing rational: the classic engine pairs GPL/AGPL-family copyleft (network use triggers source obligations) with a paid proprietary licence for companies that cannot or will not comply. The legal load-bearing wall is ownership: the licensor must hold rights to relicense every contribution, which is why serious dual-licensing projects run CLAs (contributor licence agreements) from day one; a popular project without CLAs has given its veto to every contributor. Diligence on open-core companies reads exactly this: licence inbound (CLA coverage), licence outbound (what the community edition permits), trademark control, and whether “open” marketing matches the licence reality after any relicensing events.
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