Atomic deploys at Etsy
Rasmus Lerdorf explains the logic behind atomic deploys at Etsy:
A key part of Continuous Integration is being able to deploy quickly, safely and with minimal impact to production traffic. Sites use various deploy automation tools like Capistrano, Fabric and a large number of homegrown rsync-based ones. At Etsy we use a tool we built and open-sourced called Deployinator.