Modifications of server software under GPL, with web/CLI interface

Question

Let's say we have a piece of software which is licensed under the GPL. There are no binaries; it's a framework of groups of services and libraries of Python code running on a server.

As far as I can understand, private persons can modify GPL code and run it privately without having to disclose anything. They're not distributing anything.

But if a corporation/entity is using this software in question, or other libraries/software that simply runs on a server or in the "cloud" as a backend without distributing anything, do they need to disclose their modified source code?

The software in question here does let users interact with it through a web interface. You can also use a binary to communicate with the backend software through a CLI client program. Apart from that, everything takes place on a server.

The users of this software are not the public per se, but let's say for example students/employees at a college/university or similar.

The main question here is: Can the original authors of the software demand to see the modifications? Can the end users? If neither is true, what license other than the GPL would be appropriate? Would Affero GPL cover only the latter group, and not the former? Can the former group even be covered here?

Answer 1

If a corporation/entity is using this software in question ... without distributing anything, do they need to disclose their modified source code?

As I discuss in more length here, the obligations to free your modified software are generally triggered when you convey a copy. At the risk of oversimplifying, for an individual, this means copying it to another person (legal or otherwise); for an organisation, it generally means copying it outside the organisation. In short: no, they do not.

Can the original authors of the software demand to see the modifications? Can the end users?

Once copies are conveyed, the recipients of those copies have the right to get GPL-licensed source along with anything else they get. They may then distribute further copies to whomsoever they wish, but they are not obliged to do so. In short: neither group can (well, they can, but their demands can be ignored).

If neither is true, what license other than the GPL would be appropriate?

The Affero GPL is a licence which grants to remote interactive users the right to get freely-licensed copies of the source. That would certainly be a candidate.

Would Affero GPL cover only the latter group, and not the former? Can the former group even be covered here?

The AGPL covers "all users interacting with it remotely through a computer network". That certainly includes your latter group (the end-users). As I read it, it doesn't immediately include your former group (the original authors), but they can join the latter group by interacting with the software over a network. Again, as I read it, this can be as simple an interaction as, say, loading a front page in a web browser; the AGPL doesn't specify any particular depth or complexity to that interaction. At that point, they too are entitled to freely-licensed copies of the source.