Caffeine indexing system
Rebuilt indexing infrastructure that indexed pages continuously instead of in batches, producing an index Google described as 50% fresher.
What changed
Caffeine replaced Google's batch-oriented indexing pipeline with an incremental system that could analyse and publish pages as they were crawled. Google previewed it for developer testing in August 2009 and completed the switchover in June 2010. It did not change ranking factors directly, but it made freshness a practical ranking input and greatly shortened the time between publication and indexing.
Timing
Announced on 10 August 2009, with rollout beginning 8 June 2010. No completion date was announced. Rankings typically remain unstable for the whole rollout window, so measurements taken inside it are not a reliable read on the outcome.