Google moves to transactional processing

Google in order to update its web index in a continuous fashion has developed a highly scalable transactional infrastructure to enable the online updating of its index. Before, the index was generated from scratch periodically using map reduce. With the current approach new crawled web pages are indexed in minutes.

The paper on the underlying percolator system at SOSP 2010:

http://www.google.com/research/pubs/pub36726.html