So there's a couple of different strategies, and typically any given site is going to use some combination of these, one of them is called Sharding, the idea is that you actually break up the data into separate chunks, so for example, you can imagine for keeping user profiles you could split it up by the user's last name, or by their user name any app server has to be able to talk to any one of these because, you know, when a user with a certain user name comes in you don't know which app server they're going to be assigned to. 