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I want to use CMS distributed under GPL-v3 licenses. I am building a web application with closed code and my app invokes via REST API backend under GPL-v3. Question: is it legal to use GPL-v3 along with a client app with closed code and charge money for the app. I am not changing the code of CMS just using it as is.

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  • I don't understand what you mean when you say you want to use GPL-v3 (the license) with a client app with closed code. If you are using GPL-v3 for your code, then it will be open source by definition.
    – Brandin
    Aug 31, 2021 at 8:32
  • I think the OP wants to distribute a GPLv3 CMS along with some of his/her own closed-source code that interacts with the CMS via a REST API.
    – MadHatter
    Aug 31, 2021 at 8:48
  • Just to clarify guys. My solution consist of 2 parts: CMS - GPL-3 license. My web application (SAAS) - closed code. I don't want to distribute CMS , I am using it for internal needs to speed up the process of developing my Web App. I am not changing CMS code I just request it via REST API to get the content GET/POST operations, etc ... The question is: I want to charge money (subscription) for my App. is it legal to use a closed code app with GPL-3 CMS and charge money? Aug 31, 2021 at 12:48
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    So you are just "using" the GPL software, not distributing it. That is generally allowed by the GPL without any special obligation on your part. This related QA probably answers your question: opensource.stackexchange.com/questions/10610/…
    – Brandin
    Aug 31, 2021 at 15:13
  • Ok, suppose In my case My Web App (closed code ) is for online learning , I store content in CMS. Deployment on cloud 2 docker containers: 1 is for CMS ( GPL-v3) 2nd container is for Web App (closed code ). I am not distributing CMS, my product is a course but the content of the course is stored in CMS and my Web App reads/updates it via Rest API. Just to confirm , in this case I can preserve my code close and can charge for the service? Sep 1, 2021 at 6:51

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I am not a lawyer. Seek professional legal advice.

GPL-3 code must remain open. The FSF states that,

If a library is released under the GPL (not the LGPL), does that mean that any software which uses it has to be under the GPL or a GPL-compatible license? (#IfLibraryIsGPL)

Yes, because the program actually links to the library. As such, the terms of the GPL apply to the entire combination. The software modules that link with the library may be under various GPL compatible licenses, but the work as a whole must be licensed under the GPL.

It therefore depends on whether you are distributing the GPL software with your product or creating an entirely distinct product. As the intention of any API is to allow interaction from distinct systems, I think that if your product interacts with a REST API, as opposed to directly calling library functions, it could be seen as a distinct product. As I say, you can either,

  • seek professional legal advice.
  • avoid the issue entirely by releasing your source under a GPL-compatible licence.

TLDR; It depends. Ask an actual, IRL, human lawyer.


Side note (a); most people put too much weight in their source code. Code is not where the value lies, the value is in the data. Even the UK GDS has decided that open is best. Unless you are working on some sort of Secret Squirrel stuff, open source your code.


Side note (b); if you are using an open source project in your business and making money from it, contributing to that project either financially or by making your code open source too

  • "feels" like the right thing to do
  • ensures the work you rely on continues to be maintained

This is why you see so many large companies sponsoring those projects. AmEx, for example, sponsors Django.

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  • "Your closed code can reference it and still remain closed" I'm not sure what you mean by reference, but the FSF thinks that closed code can't use GPL code via eg library calls, and remain closed when distributed.
    – MadHatter
    Sep 1, 2021 at 8:26
  • I totally agree about the support of Open Source. I already see writing some extensions and plugins to the CMS (GPL-v3) and it will be open source. For me, it is important to be sure that I am doing the right thing in terms of licensing. Sep 1, 2021 at 9:26
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    @MadHatter It would appear that I just did exactly what my parents did in my childhood - me: "Dad, can I [x]?" Dad: "Ask your mother." me: "Mum, can I [x]?" Mum: "Ask your father" LOL Sep 1, 2021 at 10:48
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    @JamesGeddes But you can license your code with an MIT (expat) license only if you are not "extending" AGPL-licensed software. I made the comment because many people assume people want their addon/plugin or companion software to be a different license because of greed. Some of us want our software to be free without restrictions at all and allow it to be used with commercial software. The problem with GPL and AGPL is that it is viral and doesn't work well with commercial software. It is really hard to work with a GPL or AGPL program and not have your code fall under those licenses. Jun 28, 2022 at 0:32
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    There are so many examples of commercial software that is also libre (as opposed to £0) and open source. As I mentioned, the vast majority of people do not care about your code, they care about how you help them solve their problems. Jun 28, 2022 at 8:39

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