The Affero General Public License, often abbreviated as Affero GPL and AGPL (and sometimes informally called the Affero license) refers to two distinct, though historically related, free software licenses: (1) the Affero General Public License, version 1 (published by Affero, Inc. in March 2002, and based closely on the GNU General Public License, version 2 (GPLv2)), and (2) the GNU Affero General Public License, version 3 (published by the Free Software Foundation in November 2007, and closely resembling the GNU General Public License, version 3 (GPLv3)).
Both versions of the AGPL were designed to close a perceived Application service provider loophole in the ordinary GPL. Each version differs from the version of the GNU GPL on which it is based in having an additional provision addressing use of software over a computer network. The additional provision requires that the complete source code be made available to any network user of the AGPL-licensed work, typically a web application.
The Free Software Foundation has recommended that the GNU AGPLv3 be considered for any software that will commonly be run over a network.
Around late February 2002, Kuhn suggested, based on the idea of a program that prints it own source code, that GPLv2 be supplemented with a section 2(d) that would require derivative works to maintain a download source feature that would provide complete and corresponding source code. Kuhn argued that there was precedent for such a requirement in GPLv2 section 2(c), which required preservation of certain features by downstream distributors and modifiers.
Moglen and Kuhn wrote the text of the proposed new section 2(d), and provided it to Poole, who then requested and received permission from the FSF to publish a derivative of GPLv2 for this purpose. In March 2002, Affero, Inc. published the original Affero General Public License (AGPLv1) for use with the Affero project and made the new license available for use by other software-as-a-service developers.
The FSF contemplated including the special provision of AGPLv1 into GPLv3 but ultimately decided to publish a separate license, nearly identical to GPLv3 but containing a provision similar in purpose and effect to section 2(d) of AGPLv1. The new license was dubbed the GNU Affero General Public License, the retention of the Affero name indicating its close historical relationship with AGPLv1. The GNU AGPL was given version number 3 for parity with the GPL, and the current GNU Affero General Public License is often abbreviated AGPLv3.
The finalized version of AGPLv3 was published by the FSF on November 19, 2007.
Stet is the first software system known to be released under AGPLv3 (on November 21, 2007)., and is the only known program to be used primarily for the production of its own license.
Widely used AGPLv3-licensed projects include the UK Prime Minister's petitions website.
By contrast, GPLv3 and AGPLv3 each include clauses (in section 13 of each license) that together achieve a form of mutual compatibility for the two licenses. These clauses explicitly allow the "conveying" of a work formed by linking code licensed under the one license against code licensed under the other license. In this way, the copyleft of each license is relaxed to allow distribution of such combinations.
To establish an upgrade path from AGPLv1 to the FSF's AGPLv3, Affero, Inc. published the Affero General Public License version 2, which is merely a transitional license that allows recipients of software licensed under "AGPLv1 or any later version as published by Affero, Inc." to distribute the software, or derivative works, under AGPLv3.