WebScaleSQL is an open source database management system created as a software branch of MySQL 5.6.
Running MySQL on numerous servers with large amounts of data (at the scale of terabytes and petabytes of data) creates a set of difficulties that in many cases arises the need for implementing specific customized MySQL features, or for introducing changes to MySQL.
WebScaleSQL was announced on 27 March 2014 by Facebook, Google, LinkedIn and Twitter as a joint effort, aiming to provide a centralized development structure for extending MySQL with new features specific to its large-scale deployments, such as building large replicated databases running on server farms. Thus, WebScaleSQL opens a path toward deduplicating the efforts each company had been putting into maintaining its own branch of MySQL, and toward bringing together more developers.
WebScaleSQL is created as a branch of the MySQL's latest production-ready community release, which is version 5.6 as of March 2013. As the project aims to tightly follow new MySQL community releases, branching path has been chosen instead of becoming a software fork. Selection of MySQL community releases for the WebScaleSQL's upstream, instead of selecting some of MySQL forks, was the result of a consensus between the four founding companies; it was concluded that features already existing in version 5.6 of MySQL are adequate for large-scale deployments, with even more such features planned for version 5.7 of MySQL.