This has been the missing piece of the puzzle for many developers creating scalable web-apps running within Amazon EC2 and S3. Until now the only way to utilise a persistent data store was to run a DB on a volatile EC2 instance, spooling the data off to a persistent store at regular intervals.
Whilst Amazon SimpleDB isn’t a true RDBMS, it looks like it should be capable of fulfilling a large number of potential use-cases with a bit of creative thinking. There’s a also a hint at true support for RDBMS (probably by providing some form of persistent file store within an EC2 instance utilising S3) …
...Over time, we plan to continue to add features that make it as easy as possible for developers to pursue whichever option they prefer for obtaining database functionality.
UPDATE: The blogosphere has been alive with opinions about Amazon SimpleDB. Good summary of the pros and cons on High Scalability
