Twitter runs on MySQL on Red Hat Enterprise Linux, on a managed hosting service NTT Managed Hosting Platform, which obviously was not designed to handle the current load. Twitter is one of a number of popular Web sites that have been built on the LAMP architecture. LAMP is a stack of simple, yet powerful technologies that to this day is behind a lot of popular Web sites: Linux, Apache, MySQL, and Perl. (In Twitters' case, the scripting engine is Ruby on Rails rather than Perl, but Twitter's architecture is still basically LAMP). The main problem with Twitter is the performance bottleneck caused by the MySQL relational database. Query-based relational database system just don't scale very well, especially for social-messaging apps like Twitter where the data is not that suitable to partitioning into multiple databases. For example, a popular user like Google Search Engine Optimization guru Matt Cutts can have more than 2,000 Twitter followers, which means each tweet to or from him must be written and rewritten thousands of times. In spite of the fact that Twitter limits messages to 140 characters, you're still talking about a huge number of SQL queries that have the potential to bottleneck in a RDBMS.
Cloud computing offers one solution to the RDBMS bottleneck problem, but just because you can have multiple points of failure doesn't mean that your system won't fail, as we've seen with the case of Amazon's S3 online storage service. Derided at the moment as "could computing" or "fog computing," a cloud computing solution may be premature -- at least for the next few years -- as a workable solution for websites that require high-availability backup. A more workable solution might be a distributed, fault-tolerant and schema-free document-oriented database like CouchDB, currently being incubated by the open-source Apache project. Neither a relational or object-oriented database, a CouchDB database is a flat collection of uniquely named documents. CouchDB also provides a RESTful HTTP API for reading and updating (add, edit, delete) these database documents in addition to supporting incremental replication with bi-directional conflict detection and resolution. A distributed DBMS like CouchDB might just be the ticket to meet the backup needs of high-demand, high-availability social-messaging websites like Twitter, at least until a workable cloud computing solution appears on the horizon.