The basic idea of the system was to scan a reverse chronologically ordered list of "user id, tweet id", filtering out any tweet whose user wasn't in the follow set (or sets in the case of scan sharing) until you retrieved enough tweets for the timeline request. There are a bunch of variants in the patent, but that is the basic idea. At the time, I estimated that Twitter was spending 80% of its CPU time in the DC doing thrift/json/html serialization/deserialization and mused about merging all the separate services into a single process. Lot's of opportunity for optimization.