Can I use GPL libraries temporarily for prototyping and make the future code closed source?

Question

I am working on a prototype for a software system, which (at least at the start) will be closed source.

In order to save time, I'm thinking of using (that is, statically linking) a library that is licensed under GPLv3, so I can test my design quickly. If I distributed the software at this stage, I'd have to distribute source code along with it.

What if I don't, but satisfy myself that my system works, and then replace the GPL library with my own code before distributing? Would the result be "contaminated" by the GPL?

I have a feeling that retaining the GPL library in my Git history or not may make a difference.

Answer 1

The GPL writes:

You may convey a work based on the Program, or the modifications to produce it from the Program, in the form of source code under the terms of section 4, provided that you also meet all of these conditions:

So this condition only applies if your work is "based on" the library, which the licence defines as follows:

To “modify” a work means to copy from or adapt all or part of the work in a fashion requiring copyright permission, other than the making of an exact copy. The resulting work is called a “modified version” of the earlier work or a work “based on” the earlier work.

That is, your program is "based on" the library if and only if it is a derivative work according to copyright law. The legal definition of that term varies somewhat among jurisdictions, and usually does not directly address software. For instance, the US Copyright Act writes:

A “derivative work” is a work based upon one or more preexisting works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications which, as a whole, represent an original work of authorship, is a “derivative work”.

What this means for software must be interpreted by the courts, based on earlier similar rulings. I am not sufficiently familiar with the relevant case law in your jurisdiction to say with certainty how a court would decide your case. One could argue that "replacing the GPL library with own code" is an act of translation, particularly if your code is strongly inspired by the GPL implementation. Even reusing the API of the GPL library could land you in hot water (see Oracle vs. Google).

If the answer matters to you, I recommend seeking competent legal advice rather than asking strangers on the internet.

Answer 2

As long as you don't release the software to anyone while you are linking to GPL'd libraries, you are safe. The viral aspect of GPL only kicks in if you distribute your software.

It would be better if you could find a library with a more permissive licence, of course, like LGPL or APL2 or MIT.

Answer 3

I don't think your question is actually about the GPL. It's about the prototype and whether it will be used in the future as the basis for the deliverable software system.

If you're making a throwaway prototype and you are not going to reuse any of the code in your deliverable system, then go ahead and use a GPL library.

Three Approaches You Can Take

However, if you're going to evolve the prototype (something lots of managers push for!) you have three approaches you can take:

1. Move the non-core parts into separate applications that communicate with your core over JSON or a REST API or some other Inter-Process Communication language/library. Your non-core parts thus can be GPL as well and you can use any GPL libraries in them.
2. Design your code so that you can swap the libraries out. This means creating a facade that hides the implementation details. When you're ready to switch to a proprietary library or MIT/BSD library.
3. Don't use the GPL'd code at all.

I suggest you go with the first approach because then you have open source work that you can use in the future as part of your professional portfolio.

The second approach is also good because that's how you should be designing the system anyway, creating the exact functions/classes you need and stubbing them out until you have the library or custom code that fills in that functionality.

Answer 4

I can think of two aspects to consider with your approach. The first is straightforward, by not distributing your project or (or as it's GPLv3, making it available for public use) whilst you are using code that is released under the GPL, it is hard to see how you would be required to distribute your code under the GPL license too under the redistribution terms.

The second aspect is possibly more significant to you. When you create your own implementation to replace the GPL library, you need to be careful not to create an derivative work. Whilst I am sure you have good intentions, on not directly copying source code - you are more likely than not going to copy significant portions of the library API.

If this is a commercial product, this risk needs to be considered and assessed by reading through the GPLv3 license carefully, and where there is doubt, asking for a professional legal opinion.

Answer 5

If you are planning to write your own code to replace the GPL code you will have a potential problem because you are not writing the code in a clean room environment. You would really want to have someone who has never looked at the GPL code write any replacement library. On the other hand if you want to simply swap out a GPL library for an already published library that is under another license then this is not a problem, the other library was presumably already written in a clean room environment.

Answer 6

If you provide access to the revisions using the GPL'd code, they will be completely GPL'd. But you don't want to, as that wouldn't be closed-source...

For any later state which no longer uses any GPL'd code, that you used GPL-code sometime earlier is simply irrelevant.

Answer 7

The GPL is only triggered on distribution... you can do whatever you want if you don't release a modified version or derivative work.

I have a feeling that retaining the GPL library in my Git history or not may make a difference.

If you mean publishing your source to a public repository like GitHub, then yes, you may have an issue. Just using git is irrelevant if it's private.