What Sentry Did and What It Called It

In 2023, Sentry migrated its self-hosted product from the Business Source Licence to the Functional Source Licence (FSL), a source-available instrument written to allow broad use while prohibiting competing commercial services for a two-year exclusion window, after which the code converts to Apache 2.0. The mechanics sit close to HashiCorp's Business Source Licence — delayed open source, with a commercial carve-out during the restriction period.

What distinguished Sentry's move was the label it applied to the category. The company began promoting "fair source" as a third lane between open source and proprietary software: code that is readable, forkable, and eventually free, but not OSI-approved today. The FSL itself is published at fsl.software, and Sentry's engineering blog framed the position explicitly: the company was not claiming OSI approval and did not intend to seek it.

An open sticker-covered laptop on a wooden table, the screen showing a terminal prompt, an adult's hands on the keyboard partially visible at the bottom of the frame
PLATE 02Most of this infrastructure is maintained from a desk like this one, in hours nobody is billing for.Photo: Rafael Minguet Delgado / Pexels

That candour has not resolved the naming dispute.

The OSI Problem and the Imitation Pressure

The Open Source Initiative defines open source by a specific list of criteria — redistribution must be free, derived works must be permitted, no discrimination against fields of endeavour. The FSL fails on the commercial-use restriction during its exclusion period. Sentry has acknowledged this. The issue is not deception; it is category capture. When a well-funded company with a well-staffed communications team promotes a new label, the label gains traction regardless of its technical precision. "Fair source" risks occupying the sympathetic middle ground that "open source" once did, making it harder for users and downstream integrators to distinguish between software that carries enduring redistribution rights and software that merely publishes its source.

Critics from the OSI and the broader open-source governance community have made versions of this argument. Bruce Perens, co-creator of the Open Source Definition, has consistently opposed source-available licences being positioned as proximate to open source. The counter-argument — articulated by several companies that adopted FSL after Sentry, including some SaaS infrastructure vendors — is that the AGPL and other strong copyleft licences have already made the "open source" umbrella incoherent for commercial use.

A printed Software Bill of Materials document on a desk beside a keyboard, the document's header and table structure legible, a pen resting across the top corner
PLATE 03A bill of materials only helps if somebody downstream is obliged to read it.
Photo: Kindel Media / Pexels

Neither side is wrong about everything. The funding pressure that drives licence changes is real: the pattern of hyperscaler cloud providers building managed services on MIT or Apache-licensed code without contributing upstream is documented and ongoing. What remains unresolved is whether a new category name solves the structural problem or simply creates a new one — a proliferation of sympathetic-sounding labels each calibrated to a different commercial tolerance, with the OSI's approved list as the only fixed reference point that actually means something specific.