There are lots of Backend-as-a-Server (BaaS) that are open source like:
However I have not seen any that is made with GAE/J. I tried doing a quick port of Usergrid as it is the most mature open source BaaS I see. however it is very dependent on Cassandra and Hazelcast as far as my porting is concerned.
Are there any BaaS made for the GAE platform?
there is one in python which comes closer but not as a full fledged BaaS, appengine-rest-server