# Functional Source License (FSL) The Functional Source License (FSL) is a "Fair Source" license created by [[Sentry]] for SaaS companies that want to ship source-available code without permanently giving up commercial protection. Each version released under FSL automatically converts to a fully permissive [[MIT License]] or [[Apache 2.0 License]] after **two years** — making it OSI-aligned in the long run, while protecting against short-term competitive free-riding. It is used by [[Warden]] (`getsentry/warden`) and other Sentry projects. ## The Core Mechanic Code released under FSL today is: - Source-available immediately — anyone can read it, modify it, contribute, and use it for almost any purpose - Restricted only against "competing uses" that undermine the producer - Automatically relicensed to Apache 2.0 (or MIT, depending on the project's choice) **2 years from release** The two-year clock applies per-version, so a steady release cadence keeps a continuously-rolling permissive open source tail behind the latest version. ## Why It Exists FSL is the answer to a specific failure mode the Sentry team observed: traditional [[MIT License|MIT]] / [[Apache 2.0 License|Apache 2.0]] / [[BSD License|BSD]] licenses offer no protection against well-funded competitors taking the code and reselling it as a hosted service without contributing back. Meanwhile, the alternatives have their own problems: | License family | Problem FSL solves | |---|---| | MIT / [[Apache 2.0 License|Apache 2.0]] / BSD | No protection against harmful free-riding | | [[Affero General Public License (AGPL)|AGPLv3]] | Too restrictive for many SaaS contexts; copyleft creates legal anxiety for commercial users | | Open core | Permanently restricts features; opaque | | BSL (Business Source License) | Too much variability in the "Additional Use Grant"; 4-year period seen as too long | FSL pins the conversion target (Apache 2.0 or MIT) and shortens the term to 2 years — making it more predictable than BSL and more sustainable than pure permissive licensing. ## What "Fair Source" Means "Fair Source" is a movement, not a license. It promotes licenses that are: - **Source-available** (you can read, modify, redistribute) - **Time-limited proprietary** (with an automatic transition to true OSS) - **Free-riding-resistant** (block direct competitors from reselling the same product) FSL is the canonical Fair Source license. Other examples include the Fair Core License (FCL) for derivative works. ## Caveats - **Not OSI-approved** during the proprietary window — strict open-source-only policies will reject FSL code at adoption time - Definition of "competing use" requires legal interpretation; large enterprises sometimes hesitate - Contributors typically sign a CLA so the producer can relicense the project later if needed ## References - https://fsl.software/ - https://github.com/getsentry/fsl.software ## Related - [[MIT License]] - [[Apache 2.0 License]] - [[BSD License]] - [[Affero General Public License (AGPL)]] - [[GNU General Public License (GPL)]] - [[LGPL License]] - [[Sentry]] - [[Warden]]