MediaWiki is a free web-based wiki software application. Developed by the Wikimedia Foundation, it is used to run all of its projects, including Wikipedia, Wiktionary and Wikinews. Numerous other wikis around the world also use it to power their websites. It is written in the PHP programming language and uses a backend database. The software is optimized to correctly and efficiently handle projects of all sizes, including the largest wikis, which can have terabytes of content and hundreds of thousands of hits per second. Because Wikipedia is one of the world's largest websites, achieving scalability through multiple layers of caching and database replication has also been a major concern for developers. Wikipedia and other Wikimedia projects continue to define a large part of the requirement set for MediaWiki. The software is highly customizable, with more than 700 configuration settings and more than 1,800 extensions available for enabling various features to be added or changed. More than 600 automated and semi-automated bots and other tools have been developed to assist in editing MediaWiki sites. MediaWiki is free and open source software and is distributed under the terms of the GNU General Public License version 2 or any later version while its documentation is released under the Creative Commons BY-SA 3.0 license and partly in the public domain. MediaWiki development has generally favored the use of open-source media formats.