Open Source Licenses: A Comprehensive Overview
Open source software constitutes a significant part of the modern technological framework, contributing to as much as 90% of the software stack through various frameworks, libraries, databases, operating systems, and standalone applications. The appeal of open source software lies in its promise of enhanced control and transparency, yet it remains a battleground between open source and proprietary models, often leading companies to revert from open source to protect commercial interests. At the core of this tension is the issue of licensing.
Open source licenses are broadly categorized into two types based on the Open Source Initiative (OSI) definition: "permissive" and "copyleft." Permissive licenses, favored by businesses for commercial use, impose few restrictions on how software can be modified and distributed. In contrast, copyleft licenses, while offering similar freedoms, require that any modifications remain under the same license, a stipulation less appealing to companies protecting proprietary work.
Permissive
MIT
The MIT license, originating from the Massachusetts Institute of Technology in the 1980s, is a leading open source license, particularly popular in the GitHub community. It is used by prominent projects such as React and Ruby, allowing developers significant freedom in using and modifying the software. However, it lacks explicit patent rights, which can pose legal uncertainties if the software interacts with patented technology.
Apache License 2.0
Published by the Apache Software Foundation in 2004, the Apache License 2.0 includes a patent grant to protect users from litigation, hence preferred by projects involving patented technologies. This license is notably used by the Android Open Source Project (AOSP), helping Google expand Android's reach against competitors like Apple in mobile operating systems. The Apache License 2.0 is more verbose than MIT due to the included patent language.
Other Permissive Licenses
The BSD licenses also fall under the permissive category, with the 2-Clause and 3-Clause versions offering variations such as the "no endorsement" provision. Additionally, the MIT No Attribution License (MIT-0) simplifies usage by eliminating the need for attribution in derivative works, closely resembling public domain status while retaining the author's rights.
Copyleft
GNU General Public License (GPL) v. 2.0 and 3.0
The GPL licenses, introduced by the Free Software Foundation, emphasize community collaboration by mandating modifications remain open source. Notable projects like WordPress and Linux utilize these licenses. However, Linux remains on GPL 2.0 due to disagreements with GPL 3.0's provisions, such as the anti-Tivoization clause.
GNU Affero General Public License (AGPL) 3.0
The AGPL 3.0, also by the FSF, extends copyleft protections to web-based software run from servers, requiring source code release even when the software is networked. This has gained traction alongside the rise of cloud computing.
GNU Lesser General Public License (LGPL)
The LGPL offers a "weaker" form of copyleft, aimed at software libraries, permitting linkage with proprietary code without obligating open sourcing of the entire proprietary software.
Mozilla Public License 2.0
The MPL 2.0, from the Mozilla Foundation, is a weak copyleft license applying to individual files rather than entire projects, offering flexibility for proprietary integration.
Public Domain and Creative Commons
Some developers seek to release software into the public domain, which traditional licenses do not facilitate directly. Tools like the Unlicense and Creative Commons' CC0-1.0 provide mechanisms for such releases, though with varying legal recognition and implications.
"Faux-pen" Source
Alternative licensing models exist, such as dual licensing and "open core," which blend open source availability with commercial restrictions. These options challenge the open source definition, as seen with MongoDB's Server Side Public License (SSPL) and MariaDB's Business Source License (BUSL). Ethical concerns also arise with certain licenses like the Hippocratic License, which incorporate usage morals.