MongoDB is released under the GNU Affero General Public License. This Free Software Foundation license is fairly new, and thus we wanted to talk about how this license differs from GPL.
Our goal with using AGPL is to preserve the concept of copyleft with MongoDB. With traditional GPL, copyleft was associated with the concept of distribution of software. The problem is that nowadays, distribution of software is rare: things tend to run in the cloud. AGPL fixes this “loophole” in GPL by saying that if you use the software over a network, you are bound by the copyleft. Other than that, the license is virtually the same as GPL v3.
To say this another way: if you modify the core database source code, the goal is that you have to contribute those modifications back to the community.
Note however that it is NOT required that applications using mongo be published. The copyleft applies only to the mongod and mongos database programs. This is why Mongo DB drivers are all licensed under an Apache license. You application, even though it talks to the database, is a separate program and “work”.
As always, we want your feedback (and participation) — comments welcome both here and in the forums.