[smanek]

Searches are likely done off slaves - I suspect that is not presented properly because of the oversimplification of the diagram.

You can just use a few bits from an MD5 hash to decide server as long as you know how many servers you're going to have up front. The problem is that if you later wanted to add or remove a server, you would need to come up with a new scheme and move every piece of data around so it's on the right server (which would take days/weeks).

The more scalable/flexible solution is to use a consistent hashing algorithm (check out some of the papers on Chord) so that adding or removing a server doesn't require you to move as much data around.