In May 2009, Google announced a preview release of AppEngine for Java, making the same scalable data structure on which Google is built (“BigTable”) available to all Java developers. Unlike other cloud computing environments that require you to configure and maintain virtual servers, Google provides the server environment and enables one-click deployment capability for your hosted app via the Google plug-in for Eclipse. In order to achieve this in a scalable and secure manner, Google places many constraints on which tools and frameworks you can use, but if you’re willing to live within them, GAE is an incredibly easy way to host an application that *should* scale. And it’s free to get started.
I’ve written quite a bit on the AppEngine Datastore and JDO in the AppEngine category on this blog.
Google AppEngine Java Persistence (Google engineer’s blog)