MediaWiki is a free and open-source wiki application. It was originally developed by the Wikimedia Foundation and runs on many websites, including Wikipedia, Wiktionary and Wikimedia Commons. It is written in the PHP programming language and uses a backend database. The first version of the software was deployed to serve the needs of the Wikipedia encyclopedia in 2002. Wikipedia and other Wikimedia projects continue to define a large part of the requirement set for MediaWiki. The software is optimized to efficiently handle large projects, 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 been a major concern for developers. The software has more than 800 configuration settings and more than 2,000 extensions available for enabling various features to be added or changed.