I am searching for the simplest method to store data around the server-side from the GWT application. The applying is normal: includes the customer-side interface and server-side that works all of the information. I simply need to keep data around the server. The applying could be used to google code therefore the solution must obey the application engine restrictions.

Thank you for any hints, Rafal

One choice is the AppEngine DataStore. Technology-not only with the JDO and JPA APIs. If you prefer a simpler API for that AppEngine DataStore, try Objectify.