[stickfigure]

See, rename a field and "all your data" is lost, unless you migrate the data from the old field to the new one

This is not true.

I wrote Objectify, a popular third-party java API to App Engine's datastore. The data migration primitives worked out building Objectify are what ScottH built into Morphia, the Java "ORM" system for MongoDB. With a small number of primitives (mostly @AlsoLoad and lifecycle callbacks) it's possible to make significant structure changes on-the-fly with zero downtime.

This is, IMHO, the best thing about schemaless datastores. There's no longer any compelling reason (at least, in the datastore) to take down a system for "scheduled maintenance".

For more information, here is the relevant section of Objectify's documentation:

http://code.google.com/p/objectify-appengine/wiki/Introducti...