Related: The director of Twitter's Application Services group, Raffi Krikorian, gave an interesting talk about the architecture of Twitter's real-time delivery service at QCon in October. In the talk he discusses in pretty good detail the ways in which Twitter uses Redis to cache copies of users' timelines as tweets are made.

Here's a link to a video of the talk: http://www.infoq.com/presentations/Real-Time-Delivery-Twitte...