Google and Facebook almost always don't need ACID data. If your Facebook news feed is out of date by a few seconds (or minutes, or more) or you're seeing yesterday's Google search index it's not the end of the world.

For anything that does absolutely need to be ACID, you'd likely want to look into sharding, where specific servers are masters for specific subsets of the database.

How can I achieve ACID on multiple physical front-end servers
I seem to remember reading that there were going to be three trilogies originally in Star Wars, i.e. they would add episodes 7 - 9.

Was that ever the case? What happened to that plan?

Update: looks like that original plan might be back after all!
