See the final link below for more on James Hamilton’s take on Google Megastore, an intriguing RDBMS/NoSQL hybrid
Megastore is the data engine supporting the Google Application Engine. It’s a scalable structured data store providing full ACID semantics within partitions but lower consistency guarantees across partitions.
I wrote up some notes on it back in 2008 Under the Covers of the App Engine Datastore and posted Phil Bernstein’s excellent notes from a 2008 SIGMOD talk: Google Megastore. But there has been remarkably little written about this datastore over the intervening couple of years until this year’s CIDR conference papers were posted. CIDR 2011 includes Megastore: Providing Scalable, Highly Available Storage for Interactive Services.
An excerpt from a Google article (p. 233) referenced in the Perspectives post (sorry for the brute-force/screen clip approach; Adobe Reader returned an error when I tried to copy the paragraph):