Why my open-source code uses the Mozilla Public License

I believe an open-source software license should ensure that code can always be used everywhere. The Mozilla Public License does a great job of meeting that goal.

What is an open-source software license?

By default, you are the only person allowed to use any code you write. That’s because you automatically own a copyright for your source code as soon as you type it.

For anyone else to use your code, you have to give them permission in the form of a legal document called a license.

Writing your own software license is complex, so multiple organizations have written generic open-source software licenses which you can use to safely grant other people permission to use your code.

What types of open-source software licenses are there?

Most open-source software licenses allow anyone to use the code freely. The differences arise from the additional rules each license requires people to obey.

Permissive Licenses

Licenses with few extra rules are called permissive licenses. The MIT license, Apache license, and BSD licenses are commonly used permissive licenses.

Benefits of Permissive Licenses

Permissive licenses are great because they allow code to easily be used by anyone, for any project (open-source or not), with few limitations. For example, my Expressive Resume project is licensed permissively so that anyone can use it to create a resume while keeping the corresponding formatting code private.

Problems with Permissive Licenses

The problem with permissive licenses is that their lenient permissions make it difficult/impossible to ensure that improvements to the code remain available to everyone. For example, some organization could legally copy the code for Expressive Resume, make a few private improvements, then start sharing the result with a more restrictive license (e.g. selling the improved code). As long as that organization gives me credit for the original version, there’s no legal mechanism to force them to share their improvements.

Copyleft Licenses

Licenses with strict conditions are called copyleft licenses. The GPL family of licenses [1] [2] [3] are the most famous examples of copyleft licenses.

Benefits of Copyleft Licenses

Copyleft licenses are great because they ensure that all usages of the code remain available for everyone. For example, the Linux operating system uses a copyleft license which requires anyone who shares an improved version of Linux to also share their source-code. Additionally, if someone copied an algorithm from the Linux code to build a video game, all the code for the video game would have to be open-sourced even though a video game has nothing to do with operating systems.

Problems with Copyleft Licenses

The problem with copyleft licenses is that their strict code-sharing requirements often discourage people and companies from using copyleft-licensed code. Most companies, small or large, want to keep portions of their code secret to maintain a competitive advantage, which is usually impossible if they use any GPL licensed code.

Sometimes other circumstances make it impossible to use copyleft-licensed code. For example, apps published on the Apple App Store cannot legally contain any code licensed under the copyleft GPL license because some of Apple’s Terms of Service conflict with the GPL.

What type of license is the Mozilla Public License?

The Mozilla Public License (abbreviated as the MPL) is a middle ground between permissive and copyleft licenses.

Like permissive licenses, code shared under the MPL can be used in any project, public or private.

Like copyleft licenses, the MPL ensures that improvements to the original code are available for everyone.

Unlike permissive licenses, modifications to the licensed code must be open-sourced.

Unlike copyleft licenses, new software which only uses the licensed code can stay private.

For example, if you use my multicounter library in a Python project you don’t need to open-source anything. If you fix a bug or add a new feature to multicounter, then only that bug fix/feature needs to be open-sourced. If you copy an algorithm from multicounter into your code, then only the file containing the copied algorithm needs to be open-sourced while everything else can stay private.

As a result, the Mozilla Public License preserves most of the benefits of permissive and copyleft licenses, while simultaneously mitigating their problems.

Why do you prefer the Mozilla Public License for your open-source projects?

I put a lot of time into building useful, high-quality software. I want people to use my work to build amazing new things I never would have imagined. By using the Mozilla Public License for my code, you and everyone else in the world can freely use my code for any of your projects, open-source or not.

However, when I provide a strong foundation to everyone for free, I think it’s only fair to expect people to share any enhancements they make to that foundation in the same way. By using the Mozilla Public License, I ensure that improvements to my code are neither locked away behind a proprietary paywall nor burdened with the extra restrictions of a strict copyleft license like the GPL.

How do I use the Mozilla Public License for my open-source projects?

It’s easy!

Create a file called LICENSE in the root folder of your project and copy the license text from Mozilla’s website. You don’t need to change the license text at all.

You should also add a short comment to the top of each source code file indicating its MPL license. Mozilla also makes it easy to copy/paste these comments from their website.

To make everything even easier, I like to use the following VS Code Plugins:

  • Choose a License: Create the LICENSE file without ever leaving VS Code.
  • licenser: Automatically adds license headers to each source code file.

Where are good places to learn more?

The best place to learn more about the Mozilla Public License is the official FAQ.

If you want to learn more about open-source software licensing in general, I recommend the following sites:

  • Choose a License: Helps developers understand which software license(s) best match their values/goals for a project.
  • Fossa Blog: A blog with detailed explanations of all common open-source software licenses using everyday language.
    • Produced by a company which specializes in building automated tools to help companies follow the rules of software licenses.

Of course, you should always seek any legal advice from a lawyer who specializes in copyright law.

While I am knowledgeable about open-source software licensing, I am not an attorney. Thus, my comments cannot be considered legal advice under any circumstances.

This post is licensed under CC BY 4.0 by the author.