How does the Affero General Public License (AGPL) differ from the GPL?

The Affero General Public License (AGPL) and the GNU General Public License (GPL) are both copyleft licenses designed to ensure software remains open source, but they differ in how they handle distribution and network-based use. The key distinction is that the AGPL extends the GPL’s requirements to cover software accessed over a network, such as web applications or cloud services. While the GPL focuses on traditional distribution (e.g., shipping binaries or source code), the AGPL ensures that even if users interact with modified software remotely, the provider must share the source code of those changes. This makes the AGPL stricter in scenarios where software isn’t directly distributed but is instead offered as a service.

Under the GPL, if you distribute a modified version of a GPL-licensed program (like a compiled binary or source code), you must also provide recipients with the corresponding source code. However, if you host a modified GPL program on a server and let users interact with it via a web interface or API, the GPL does not require you to share your modifications. The AGPL closes this “application service provider loophole” by adding a clause (Section 13) that mandates releasing the source code to anyone using the software over a network. For example, if you modify an AGPL-licensed database like MongoDB and offer it as a cloud service, you must make your changes available to users. The GPL, in contrast, would only require sharing code if you distributed the modified database directly.

The choice between AGPL and GPL depends on the software’s use case. The AGPL is often used for projects where the primary interaction happens over a network, such as SaaS platforms or web apps, to prevent proprietary forks from benefiting without contributing back. For instance, companies building on AGPL code for internal tools might avoid the license due to its stringent requirements. The GPL remains popular for traditional software distributed via downloads or physical media, like the Linux kernel. Developers should consider their goals: AGPL prioritizes enforcing openness in cloud environments, while GPL balances openness with broader adoption in non-network contexts.