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InfoQ Homepage News Sentry Introduces Non-Open-Source Functional Source License

Sentry Introduces Non-Open-Source Functional Source License

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Sentry has recently announced the creation and adoption of the Functional Source License (FSL), a non-compete license that converts to Apache 2.0 or MIT after two years. Similar to the Business Source License (BSL) but with a shorter non-compete period and less variability, the new license received mixed feelings from the community.

Promoted as a license for "freedom without free-riding", the Functional Source License has been introduced as Sentry felt that the additional use grant of the BSL license resulted in different meanings for different implementations such as MariaDB, Redis, and HashiCorp. Sentry claims the FSL enables developers to have more freedom while preventing so-called free-riding.

Explaining what drives the relicensing of both Sentry and Codecov under FSL, Chad Whitacre, head of open source at Sentry, explains what Sentry believes are the two major flaws of the BSL license:

First, the default non-compete time period is four years, which is a really long time in the software world. This can make it feel like the eventual change to Open Source is only a token effort. It almost might as well be 100 years. (...) The more serious flaw is that BSL has too many parameters: the change date, the change license, and the additional use grant.

The change date is now set at two years, half the BSL default, converting to either Apache (FSL-1.0-Apache-2.0) or MIT (FSL-1.0-MIT) after that. The company behind the error tracking and performance monitoring platform believes that the short period provides protection against competition, but also acts as an incentive to continue innovating. Peter Zaitsev, founder at Percona and open source advocate, is not convinced and comments:

Two years or Three Years - does not matter. What you tend to get is useless unsupported security bug-ridden code, having very little practical value.

This is not the first time Sentry has adopted a new license, moving to BSD-3 in 2009 and switching to SBL later on. The 2022 acquisition of Codecov was followed by a further controversial license change earlier this year. Dotan Horovits, technology evangelist at and Cloud Native Ambassador, summarizes on X (formerly Twitter):

Sentry relicensed again. After 11 years as open source (BSD) they moved to non OSS Business Source License (BSL/BUSL) in 2019. Now they came up with their own invention: Functional Source License (FSL). ultimately another "source-available".

The new license is not recognized as Free and Open Source Software (FOSS) but, according to the authors, allows SaaS companies to embrace open-source principles while protecting commercial interests. Whitacre clarifies:

In plain language, you can do anything with FSL software except economically undermine its producer through harmful free-riding. (...) We value user freedom and developer sustainability. Free and Open Source Software (FOSS) values user freedom exclusively. That is the source of its success and the source of the free-rider problem.

Zaitsev again disagrees:

The key practical value of real open source is having a choice to do things on your own and a choice of vendors. Non-compete source available licenses are fundamentally at odds with it.

As part of the company FOSS Funders commitment, Sentry recently donated 500K USD to open-source maintainers.

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