A good post on part of Twitter architecture
Capacity planning was also one of the more important reasons why the site hasn’t gone down. Twitter has two data centers running that can handle the entire site being failed into it. Every important service that runs can be run out of one data center. The total capacity available at anytime is actually 200%. This is only for disaster scenarios, most of the time both data centers are serving traffic. Data centers are at most 50% utilized. Even this would be busy in practice. When people calculate their capacity needs, they figure out what is needed for one data center serving all traffic, then normally add headroom on top of that! There is a ton of server headroom available for extra traffic as long nothing needs to be failed over. An entire data center failing is pretty rare, it only happened once in my five years there.
- Software Engineering