Open Source Licenses Explained: A Comparison

published on 14 January 2024

When exploring open source software, most developers would agree that understanding software licenses is crucial yet complex.

By comparing popular open source licenses in this post, you will gain clarity on key license terms, suitability for various projects, and compatibility considerations to confidently select the right open source license.

We will examine permissive and copyleft license models, analyze MIT, Apache, GPL and AGPL licenses in depth, and provide a framework for choosing an open source license aligned with your project's goals and audience.

Understanding Open Source Licenses

An open source license is a legal agreement that specifies the terms under which software source code can be used, modified, and distributed. Open source licenses aim to encourage collaboration and the free exchange of ideas by allowing others to access, modify, enhance, and reuse source code for their own projects.

Selecting the right open source license for a software project is an important decision that impacts how others can interact with the code. Some licenses place minimal restrictions while others require derivative works to remain open source. Understanding the differences can help align the license terms with the project's goals.

What is an Open Source License?

An open source license grants permissions to access, use, modify, and distribute the software's source code. Key aspects include:

  • Access to Source Code: The full source code must be made available and distributed with the software. This enables others to study, enhance, and reuse the code.

  • Modification Rights: The license must allow modifications and derived works from the original software. However, some licenses have copyleft terms requiring derivative works to use the same open source license.

  • Distribution Rights: The software can be freely distributed in original or modified form. Some licenses may have attribution requirements.

  • Limited Warranties: Open source licenses typically disclaim warranties and limit liability as the software is provided "as is". Support may come from the community.

  • Non-Discrimination: No one can be barred from using or accessing the open source software for any field of endeavor.

In summary, open source licenses aim to foster collaboration by granting broad permissions while requiring source code availability and integrity preservation in derivative works.

The Significance of License Selection in Software Development

Choosing an appropriate open source license is important because it determines how others can use, modify, and distribute software going forward. Key considerations include:

  • Commercial Usage: Some licenses prohibit commercial use while others permit it. This impacts business models built around offering services for open source software.

  • Copyleft Effect: Strong copyleft licenses like GPL require derivative works to adopt the same license. Weaker copyleft or permissive licenses like MIT allow proprietary derivatives.

  • Patent Grants: Licenses may differ on whether they provide an explicit patent grant protecting users and contributors from litigation.

  • Compatibility: Mixing code under incompatible licenses can create legal issues. Understanding license compatibility helps avoid problems.

Evaluating project goals, community norms, commercial aspirations, and compatibility requirements helps guide effective license decisions. Striking the right balance enables broadly sharing innovations while protecting end-user and contributor rights.

What are the three major types of open source component licenses?

Open source licenses can be broadly categorized into three major types:

Permissive Licenses

Permissive licenses place minimal restrictions on how the open source software can be used, modified, or distributed. Some popular permissive open source licenses include:

  • MIT License
  • Apache License 2.0
  • BSD 3-Clause License

These allow users to freely use, modify, integrate, and distribute the software without requiring changes to be open sourced. Permissive licenses provide the most flexibility.

Copyleft Licenses

Copyleft licenses require modified works that incorporate open source code also be open sourced under the same terms. Popular strong copyleft licenses include:

  • GNU General Public License (GPL)
  • GNU Affero General Public License (AGPL)

These aim to maximize open access to derivatives of open source works. Weaker copyleft licenses like the Mozilla Public License add slight flexibility.

Public Domain Licenses

Public domain licenses place the software completely into the public domain with no restrictions. For example:

  • Creative Commons CC0
  • The Unlicense

These dedicate works to the public domain, allowing unrestricted usage without formal licensing.

What are the bad open source licenses?

Some open source licenses pose more restrictions and risks compared to others when integrating the open source software into proprietary applications. These licenses are considered high risk.

GNU General Public License (GPL)

The GNU GPL v2.0 and v3.0 are very popular but also highly restrictive open source licenses. Software under GPL must remain under GPL when distributed. This viral copyleft means if you use a GPL component in your software product, the entire product also becomes GPL. This requires providing source code access.

The GPL presents compliance challenges for proprietary software vendors wanting to integrate GPL software. Options like dynamic linking may allow avoiding having the proprietary software become GPL. But legal risks still exist.

GNU Lesser General Public License (LGPL)

The LGPL v3.0 is similar to GPL but applied mainly to software libraries. Derivative works that link to LGPL libraries aren't forced to become LGPL themselves. But significant legal complexities remain when combining open source LGPL and proprietary code.

Other Highly Restrictive Licenses

Other copyleft licenses like the Mozilla Public License 2.0 and Affero GPL v3.0 also pose high risks for proprietary vendors. They require providing source code access while restricting commercialization.

When leveraging these licenses, proprietary software vendors must implement strict open source management procedures. This includes license compliance reviews, archive requirements, and access provisioning for source code.

To mitigate risks, permissive open source licenses like MIT, Apache 2.0, or BSD are better options for integrating with proprietary software.

What is the best open source license?

The GNU General Public License (GNU GPL) is considered one of the best open source licenses for developers seeking maximum freedom and collaboration.

The key features that make the GNU GPL a top choice include:

  • Copyleft protections: The GNU GPL is a strong copyleft license, meaning any projects that use GPL code must also release their source code under the same GPL license. This helps ensure the software remains free and open source long-term.

  • Viral protections: The copyleft provisions have a "viral" effect, where any project linking to or including GPL code must adopt the GPL. This spreads software freedom across projects.

  • Freedom to modify and share: The GNU GPL grants users full rights to use, study, modify, and share the software for any purpose. Modified versions must also use the GPL license.

  • Prevents proprietary forks: Companies can't take GPL code, modify it, and release a proprietary version. The viral protections require them to keep their software open source as well.

The GNU GPL does have some downsides, including potential license compatibility issues with other open source licenses. Projects that want to maximize business adoption sometimes prefer permissive licenses like MIT or Apache.

But for developers focused on software freedom and open collaboration, the GNU GPL remains a gold standard. It helped drive the free software movement and is used by over 65% of open source projects on GitHub.


What is free license vs open source license?

Open source licenses focus on the availability of source code and the ability to modify and share it freely. Free software licenses focus more on protecting the user's freedom to use, modify, and share the software.

Key differences:

  • Open source licenses allow commercial use, proprietary modifications, and closed-source derivatives. Free software licenses restrict these to protect user freedoms.

  • Open source focuses on the practical benefits of open development. Free software focuses on the ethical imperative to respect user freedoms.

For example, the MIT license is a popular open source license that allows almost unlimited commercial use and proprietary derivatives. By contrast, the GNU GPL is a free software license focused on preventing proprietary derivatives that don't share source code.

So while open source and free software share similarities, their core focus and restrictions differ regarding commercial use and proprietary code. Ultimately, open source prioritizes the open availability and collaboration enabled by shared source code. Free software makes protecting user freedoms a moral imperative above all else. Understanding these philosophical differences can help guide your own licensing decisions.

Exploring the 5 Types of Open Source Licenses

Open source licenses govern how open source software can be used, modified, and distributed. They aim to maximize openness and collaboration while protecting the rights of creators. Broadly, open source licenses fall into two main categories:

Permissive Open Source License Types

Permissive licenses place minimal restrictions on how software can be used, modified and redistributed. Some popular permissive licenses include:

  • MIT License - Allows reuse of software freely, even in proprietary products. Requires license and copyright notice be included with software.

  • BSD License - Similar to MIT license. Lets people reuse code freely. Requires redistributions retain notice of copyright and disclaimer.

  • Apache License 2.0 - Permissive license that allows software to be reused freely. Requires license and notice be provided with software. Suits commercial software.

The minimal restrictions of permissive licenses make them popular for commercial products.

Understanding Copyleft Licenses

Copyleft licenses require modified versions of the software code to remain open source and reusable. Popular strong copyleft licenses include:

  • GNU GPLv3 - Requires modified software be licensed under GPLv3 terms. Suits community/open source projects.

  • GNU AGPLv3 - Variant of GPLv3 but requires network services using the software also distribute source code.

Weaker copyleft licenses like GNU LGPL allow linking to proprietary code. Copyleft licenses aim to maximize openness.

Business Source License and Other Hybrid Models

Other open source license models are emerging, like the Business Source License adopted by MariaDB. It starts off permissive but later adopts a copyleft approach. Some projects dual license under both permissive and copyleft terms. Companies should carefully evaluate license models based on use cases.

In summary, permissive and copyleft licenses take different approaches but share the goal of enabling open collaboration. Companies should evaluate license terms based on distribution and commercialization plans.

In-depth Comparison of Permissive and Copyleft Licenses

Provides a detailed feature comparison of the most widely used open source licenses like GPL, MIT, BSD, highlighting key similarities and differences.

Analyzing Permissive Licenses: MIT and Apache Licenses

Permissive licenses like the MIT and Apache licenses place minimal restrictions on reuse and distribution of open source software.

Key features of permissive licenses:

  • Allow software to be used for any purpose, including commercial use.
  • Allow software to be modified and distributed without requiring source code access.
  • Require license and copyright notice be included with software.
  • Allow software to be sublicensed and integrated with other software licenses.

The MIT license is short and simple. The Apache license provides more detail on patent and trademark rights. Both allow great freedom in using and distributing software.

Examining Copyleft Licenses: GNU GPL and Affero GPL

Copyleft licenses like GNU GPL aim to ensure open access to source code for derivative works.

Key features of copyleft licenses:

  • Allow commercial and non-commercial use.
  • Require modified versions to use the same license terms.
  • Require source code access for derivative works.
  • May require attribution for authors.

The GNU GPL focuses on protecting open source code. The AGPL license extends requirements to network-based software services. Copyleft licenses enforce openness of modifications.

License Compatibility and Integration Challenges

Permissive licenses like MIT have high compatibility with other licenses. Copyleft licenses like GNU GPL have more restricted compatibility:

  • MIT code can be reused easily in other projects.
  • Integrating MIT with GPL requires licensing derivative works under GPL.
  • GPL cannot be easily mixed with closed-source software.
  • AGPL has additional restrictions for network-based services.

Understanding compatibility helps avoid license conflicts when reusing open source software. Legal review may be required to ensure compliance.

How to Choose the Best Open Source License for Your Project

Project Goals and the Role of Licensing

When starting an open source project, one of the most important decisions is choosing the right license. The license sets the terms under which others can use, modify, and distribute your project. As such, it should align with your goals for collaboration, community growth, and potential commercialization.

Consider whether you want your project to:

  • Allow modifications and derivative works
  • Require derivative works to remain open source
  • Allow integration with proprietary software
  • Allow patent litigation against users of your project

For example, if your priority is building an active open source community, you may prefer a permissive license like MIT or Apache 2.0 to minimize barriers to contribution. However, if you want to ensure commercial entities cannot close-source modifications, a copyleft license like GPLv3 may be more suitable.

Audience Considerations and License Perception

The license can also impact your project's perception amongst developers. More permissive licenses tend to be preferred by commercial entities as they allow proprietary licensing of modifications. However, some open source purists favor copyleft terms to uphold software freedom.

Consider your target contributors and whether the community values align with licenses like GPL vs Apache/MIT. Also factor in their level of legal/licensing expertise. More complex licenses can deter less sophisticated developers, even though terms may technically suit their needs.

Before settling on a license, evaluate compatibility with other projects you may integrate or depend on. For example, combining code under GPL and Apache 2.0 requires special care to avoid license conflicts.

Likewise, pay attention to potential dependencies' licenses. Using a project that has an incompatible open source license can impact the licensing choices available for your own source code. Tools like FOSSA can automatically check for and resolve license issues in dependencies.

Carefully weighing project intentions, audience perceptions, and license interoperability from the outset can ensure you pick the best open source license for your goals. The choice directly impacts who can use your project and under what terms, so align it with your vision for the project's future.

Conclusion: Mastering Open Source License Selection

Recap of Open Source License Types

Open source licenses can be broadly categorized into permissive and copyleft licenses. Permissive licenses like MIT and BSD allow reuse of code without requiring changes to be open sourced. Copyleft licenses like GPL require derivative works to be licensed under the same terms.

There are also many variants of these licenses with specific conditions around commercial use, attribution, patent grants etc. It's important to understand the key differences in order to select the right open source license for your project.

Final Thoughts on Choosing an Appropriate License

When choosing an open source license, consider your project goals and community you want to build around it. Permissive licenses encourage more contributions from companies by allowing commercialization. Copyleft licenses ensure code remains open even downstream.

Also evaluate compatibility with other licenses if you use external libraries. Some licenses like GPL are incompatible with permissive ones. Carefully review all terms and conditions before selection.

The Importance of Comprehending Licensing Terms

Taking the time to fully understand open source license terms is critical before applying them to avoid legal issues down the line. Violating terms unknowingly could mean litigation. Make sure all contributors agree to licensing terms as well.

Having clarity on usage conditions, distribution, derivatives, patents etc. will prevent headaches. Consult a lawyer if unsure. Comprehending license implications is an important part of open source development.

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