It would have perhaps been better to setup a wildcard DNS/vhost and set it as http://joe.threewords.me. It can still be done but the earlier the better. Time is of essence here.
Regards
Joe
So in your DNS zone you can tell it to route all accounts that start with the letter 'a' to server with IP 1.1.1.1, letter b to IP 2.2.2.2, etc. Server with IP 1.1.1.1 will not only run a headless web app but also route SQL queries to a db shard (all relational data for users starting with the letter a).
This level of "user" partitioning can be deeper. So it's not just the first letter of the user but two or three more letters/numbers. So joe.threewords.me would be on a different server than j4mes.threewords.me (That's j.o.threewords.me vs j.4.threewords.me).
Regards
Joe
That's why they need so many load balancers w/ failover:
twitter.com has address 128.242.240.148 twitter.com has address 128.242.245.212 twitter.com has address 128.121.243.228
Load balancers aren't flexible to work with especially when dealing with sessions. I imagine Twitter's front end nodes are doing nothing but routing URLs to backends, which are doing the real work. So the front end balancer takes http://www.twitter.com/unixy and routes the actual server-side request internally (where scalability is implemented).