Defining Open Source
Open source software is software whose source code is freely available to the public and can be modified and redistributed by anyone without restraint or consequence. This is a very simple, watered-down version of the definition of open source. An official organization called the Open Source Initiative (http://opensource.org), founded in 1998 to organize the open source software movement in an official capacity, has provided a very clear and easy-to-understand definition of open source. During the course of writing this book, I obtained permission from the OSI Board to include it here.
Open source doesn't just mean access to the source code. The distribution terms of open source software must comply with the following criteria:
- Free Redistribution
The license shall not restrict any party from selling or giving away the software as a component of an aggregate software distribution containing programs from several different sources. The license shall not require a royalty or other fee for such sale.
- Source Code
The program must include source code, and must allow distribution in source code as well as compiled form. Where some form of a product is not distributed with source code, there must be a well-publicized means of obtaining the source code for no more than a reasonable reproduction cost preferably, downloading via the Internet without charge. The source code must be the preferred form in which a programmer would modify the program. Deliberately obfuscated source code is not allowed. Intermediate forms such as the output of a preprocessor or translator are not allowed.
- Derived Works
The license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software.
- Integrity of the Author's Source Code
The license may restrict source-code from being distributed in modified form only if the license allows the distribution of “patch files” with the source code for the purpose of modifying the program at build time. The license must explicitly permit distribution of software built from modified source code. The license may require derived works to carry a different name or version number from the original software.
- No Discrimination Against Persons or Groups
The license must not discriminate against any person or group of persons.
- No Discrimination Against Fields of Endeavor
The license must not restrict anyone from making use of the program in a specific field of endeavor. For example, it may not restrict the program from being used in a business, or from being used for genetic research.
- Distribution of License
The rights attached to the program must apply to all to whom the program is redistributed without the need for execution of an additional license by those parties.
- License Must Not Be Specific to a Product
The rights attached to the program must not depend on the program's being part of a particular software distribution. If the program is extracted from that distribution and used or distributed within the terms of the program's license, all parties to whom the program is redistributed should have the same rights as those that are granted in conjunction with the original software distribution.
- License Must Not Restrict Other Software
The license must not place restrictions on other software that is distributed along with the licensed software. For example, the license must not insist that all other programs distributed on the same medium must be open-source software.
- License Must Be Technology-Neutral
No provision of the license may be predicated on any individual technology or style of interface.
The preceding items comprise the definition of open source, as provided by the Open Source Initiative; the definition is found at http://opensource.org/docs/osd, and shown in Figure 2-1.
Open source software source code must be freely available, and any licensing of the open source software must abide by this definition. Based on the OSI definition, WordPress is an open source software project. Its source code is accessible and publicly available for anyone to view, build on, and distribute at no cost anywhere, at anytime, or for any reason.
Several examples of high profile software enterprises, such as the ones in the following list, are also open source. You'll recognize some of these names:
- Mozilla (http://mozilla.org): Projects include the popular Firefox Internet browser and Thunderbird, a popular e-mail client. All projects are open source and considered public resource.
- PHP (http://php.net): An HTML-embedded scripting language. Stands for PHP Hypertext Preprocessor and is a popular software that runs on most Web servers today. Actually, WordPress requires the presence of PHP on your Web server for you to run the WordPress platform successfully on your site.
- MySQL (http://mysql.com): The world's most popular open source database. Used by your Web server to store all the data from your WordPress installation, including your posts, pages, comments, links, plugin options, theme option, widgets, and more.
- Linux (http://www.linux.org): A free and open source operating system used by Web hosting providers, among other organizations.
Figure 2-1: Definition of open source from the Open Source Initiative.
As open source software, WordPress is in some fine company. Open source itself is not a license — I cover licenses in the next section. Rather, open source is a movement — some consider it a philosophy — created and promoted as a way to provide software as a public resource open to community collaboration and peer review. WordPress development is clearly community driven and focused. You can read about the WordPress Community in Book II, Chapter 4.