Copyleft on Your License

Copyleft

Mon, 01 Jan 0001 00:00:00 +0000

Copyleft is a licensing requirement that derivative works of a licensed work must be distributed under the same (or a compatible) licence. The term is a play on “copyright”: rather than reserving rights, copyleft uses copyright law to compel downstream sharing.

Licenses fall on a spectrum:

None (permissive)

No share-back requirement. MIT, Apache-2.0, BSD, ISC. Derivative works can be relicensed as anything, including proprietary. The trade-off: a proprietary fork can absorb your work without giving anything back.

Creative Commons Attribution-ShareAlike 4.0

Mon, 01 Jan 0001 00:00:00 +0000

CC BY-SA 4.0 is the copyleft Creative Commons licence. Anyone may share, remix, adapt, and build on the material for any purpose, including commercial, provided they give appropriate credit and license their adaptations under the same licence or a CC-approved compatible licence.

It is the right choice for content that should stay in the commons even as it is remixed. Wikipedia uses CC BY-SA 4.0 in combination with the GNU Free Documentation License.

GNU General Public License v3.0

Mon, 01 Jan 0001 00:00:00 +0000

GPL-3.0 is a strong copyleft license. Anyone who distributes the software, modified or not, must make the complete corresponding source available under GPL-3.0. It includes an explicit patent grant and patent-retaliation clause.

It also includes anti-tivoization provisions — you cannot distribute GPL-3.0 software on hardware that prevents users from installing modified versions.

Pick GPL-3.0 when you want to make sure derivative works stay open. Pick AGPL-3.0 if you also want to close the loophole where software offered as a network service is not considered “distributed”.

GNU Lesser General Public License v3.0

Mon, 01 Jan 0001 00:00:00 +0000

LGPL-3.0 is a “library” variant of GPL-3.0. If you modify the LGPL-3.0 library itself, your modifications must be released under LGPL-3.0. If you just use the library — link to it, call its APIs — your own code can stay under any license, even proprietary.

LGPL-3.0 is a good fit for reusable libraries where you want modifications to the library itself to stay open, but you are fine with the library being called from closed-source code.

Mozilla Public License 2.0

Mon, 01 Jan 0001 00:00:00 +0000

MPL-2.0 applies copyleft at the file level: if you modify an MPL-2.0 file, that file stays under MPL-2.0, but new files can be under a different license. This makes it easier to combine MPL-2.0 code with proprietary code than GPL or LGPL.

It includes an explicit patent grant and retaliation clause. It is compatible with both GPL and Apache-style licenses, which makes MPL-2.0 a common choice for projects that want some copyleft protection without scaring off commercial users.

Server Side Public License v1.0

Mon, 01 Jan 0001 00:00:00 +0000

SSPL is MongoDB’s response to the AGPL “loophole”: if you offer the software as a managed service, you must also release under SSPL the source code of every program you use to offer that service — provisioning, monitoring, orchestration, backup. In effect, it is strong copyleft extended to your service stack.

The OSI rejected SSPL as not meeting the Open Source Definition, specifically on Criterion 6 (no discrimination against fields of endeavour). It is therefore fair-code / source-available, not open source.