how to implement JPA
a quick search in google gave me this:


Using JPA with App Engine
Quotas


Also, in the datastore you can store only entities, this means that when you save some user data, it will be stored in the Users entity with all the other users data, what you have to do is implement relationships between entities like in a regular database (MySQL, SQL Server, etc.), check it out here:


Using the Datastore with JDO