Key component of facebook’s chat scalability is their channel server. They choose to use erlang a language that is fast getting the much needed attention for building real time infrastructure.

The channel servers are the most intricate piece of the backend. They’re responsible for queuing a given user’s messages and pushing them to their web browser via HTTP. We made an early decision to write the channel servers in Erlang. The language itself has many pros and cons, but we chose Erlang to power Chat because its model lends itself well to concurrent, distributed, and robust programming. It’s easy to model our millions of concurrent users with a few lightweight processes each, where the same tactic in, say, C++ would have been more daunting. Programming languages are always a tradeoff; Erlang makes some hard things easy but, unfortunately, some easy things hard.

Here are links to some of the facebook engineering notes.

Facebook chat and challenges from May 13, 2008 – http://www.facebook.com/note.php?note_id=14218138919&id=9445547199&index=9

Chat stability and scalability (chris piro) - http://www.facebook.com/note.php?note_id=51412338919

Engineering tech talk videos on chat – http://www.facebook.com/video/video.php?v=631726942083

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