Martin Fowler explains how things are different when server volume reaches epic scale:

A couple of years ago I was talking to a couple of friends of mine who were doing some work at eBay. It's always interesting to hear about the techniques people use on high volume sites, but perhaps one of the most interesting tidbits was that eBay does not use database transactions.

It's a fascinating look at how common conventions have to be re-examined when traffic reaches certain extremely high levels.