New user, question for requirements, about datastores interoperability

[Nicolo Zanetti]

Hello, I'm new and I wanted to create some applications using GAE and
GWT (Java).
However I was not able to understand from the documentation that I was
able to find if GAE can satisfy certain my requirements, or if I
should move to something else instead.

Actually I'm planning to make a game. (it is not in real time)
However, in future I'm also planning to do more games, and I would
like the user to not have to register again.
So if the user registered for the "game 1", that is hosted in "GAE
sandbox 1", there is a way for the "game 2", that is hosted in "GAE
sandbox 2", to recognize the user from his username and password?
There should be a "database 0" with the duty to host the users login
and password, so all the games can query the same destination, this
would allow to not have separate account for each game. (This
"database 0" would hold also the virtual currency not specific to the
game of the users.)

So:
Can this "database 0" be hosted in GAE datastore? Or should it be
something external and different?

I have also an harder question,
The games I'm planning to make require the games to interact each
other.
For example in a game about "mining" you can find some metals that you
can decide to send to another game that is about "engineering", is it
possible for a GAE enviroment tell to another enviroment to add
resources for the user? Or the GAE enviroments are totally secluded
from each other?
May this be solved by something external as above? Or I should move to
another hosting platform?

[Gerald Tan]

One application cannot directly access the datastore of another application.
You can however allow one app to send queries to another app using urlfetch.

[Max]

you can't send messages among different apps directly. But you can write your own protocol and communicate through url fetch / xmpp etc.

With regards to user account, an easy way it to use google account directly.