Study & Write T6 (Copyleft)

In this post I would like to evaluate how different copyleft licenses can have an impact on projects and what are the advantages of each type of licenses. Open source licenses can be divided in two main categories: copyleft and permissive licenses. The difference between these two types is that copyleft licenses are persistent (all derivative works must be licensed and distributed under the same license as the original project), but permissive licenses are not. Also, copyleft licenses have a viral effect and permissive licenses do not. Viral effect means that combining copyleft licensed work with a work licensed under a different license leads to the resulting work falling under the copyleft license. Though, not all copyleft licenses have a viral effect. Copyleft licenses can be divided into two subcategories: weak copyleft and strong copyleft licenses. Both strong and weak copyleft licenses are persistent, but only strong copyleft licenses have a viral effect, which means that works licensed under weak copyleft licenses can be included in an aggregate work that is licensed using a different license. Permissive licenses on the other hand are not persistent and do not have a viral effect, so derivative works can be published under a different license. In addition, permissive licenses don’t require the source code to be published, but copyleft licenses require the source code of the project to be open for everybody. 

As we can see, these types of licenses are quite different and here comes the question: "Which license to choose when distributing your project?". To answer this question, all benefits of each license types should be taken into account.

 

Why companies may choose a permissive license?

Some companies want the option to distribute a proprietary version and don't like the idea of having to use the GPL for derivative works. Additionally, the GPL doesn't always mix well with other open source licenses. Finally, companies that do not want to deal with or do not want their project to be open source may choose a permissive license.


Why companies may choose a copyleft license? 

Sometimes, choosing a copyleft license may be a question of morality. If you have the rights to use other project freely, these rights should also be passed to your users. If the goal in producing software is to provide free software to users, then copyleft is really only way to do that. Another reason to choose a copyleft license is that this ensures that any competitors that want to make use of the software also have to contribute changes back, so companies could protect their investment in software.

 

 

Examples of projects that use a strong copyleft license.

  • MySQL — Uses a combination of GPL and commercial license
  • MariaDB — Uses GPL v2
  • Drupal — Uses GPL v2
 

Examples of projects that use a weak copyleft license.

  • WineHQ — Uses LGPL v2.1
  • GNU Aspell — Uses LGPL v2.1
  • OpenOffice.org 3 — Uses LGPL v3
 

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