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    <p>When you talk about scaling at the size of Facebook, is a whole different ball park. Latest estimates put Facebook datacenter at about <a href="http://www.datacenterknowledge.com/archives/2010/06/28/facebook-server-count-60000-or-more/" rel="nofollow noreferrer">60000 servers</a> (sixty thousand). Only the cache is estimated to be at about <a href="http://perspectives.mvdirona.com/2010/06/20/FacebookSoftwareUse.aspx" rel="nofollow noreferrer">30 TB</a> (terabytes) ina a masive <a href="http://www.facebook.com/note.php?note_id=39391378919" rel="nofollow noreferrer">Memcached cluster</a>. Although their back end is stil MySQL, is used as a pure key-value store, according to publicly <a href="http://royal.pingdom.com/2010/06/18/the-software-behind-facebook/" rel="nofollow noreferrer">available information</a>:</p> <blockquote> <ul> <li>Facebook uses MySQL, but primarily as a key-value persistent storage, moving joins and logic onto the web servers since optimizations are easier to perform there (on the “other side” of the Memcached layer).</li> </ul> </blockquote> <p>There are various other technologies in use there:</p> <ul> <li><a href="http://wiki.github.com/facebook/hiphop-php/" rel="nofollow noreferrer">HipHop</a> to compile PHP into native code</li> <li><a href="http://www.facebook.com/note.php?note_id=76191543919" rel="nofollow noreferrer">Haystack</a> for media (photo) storage</li> <li><a href="http://www.facebook.com/notes/facebook-engineering/bigpipe-pipelining-web-pages-for-high-performance/389414033919" rel="nofollow noreferrer">BigPipe</a> for HTTP delivery</li> <li><a href="http://cassandra.apache.org/" rel="nofollow noreferrer">Cassandra</a> for Inbox search</li> </ul> <p>You can also watch this year SIGMOD 2010 key address <a href="http://hosted.mediasite.com/mediasite/Viewer/?peid=459a9f2cb92d4830b40d3e64003a50e61d" rel="nofollow noreferrer">Building Facebook: Performance at big scale</a>. They even present their basic internal API:</p> <pre><code>cache_get ($ids, 'cache_function', $cache_params, 'db_function', $db_params); </code></pre> <p>So if you connect the dots you'll see that at such scale you no longer talk about a 'big database'. You talk about huge clusters of services, key-value storage partitioned across thousands of servers, many technologies used together and so on and so forth.</p> <p>As a side note, you can also see a pretty good presentation of <a href="http://www.sdsqlug.org/presentations/May2009/MySpace_DB_Overview.pptx" rel="nofollow noreferrer">MySpace internals</a>. Although the technology stack is completely different (Microsoft .Net and SQL Server based, with a huge emphasis on message passing via <a href="http://rusanu.com/2009/07/26/myspace-uses-sql-server-service-broker-to-protect-integrity-of-1-petabyte-of-data/" rel="nofollow noreferrer">Service Broker</a>) there are similar points in how they approach storage. To sum up: <strong>application layer partitioning</strong>. </p>
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