How does Twitter search work?

Question

I know how Google search works. A web-crawler crawls the web following links and downloading every page as it goes along. Then various algorithms index the page based on its content.

How does Twitter search work? Does it index every word?

I have seen Twitter say "Older tweets are temporarily unavailable.". Does this mean that they only hold a subset of the tweets (recent tweets) relating to a query

Answer 1

This is not the answer, but could shed some light on what you're looking for:

http://highscalability.com/scaling-twitter-making-twitter-10000-percent-faster

Answer 2

Yes, they currently only hold 7 days old tweets according to this explanation: http://searchengineland.com/where-have-all-the-old-tweets-gone-33579