I want my app to communicate with with server. I want the app to store something like notes and contacts. They can run the app on any system, and get their contacts on any machine by entering their username and password...
As you can see authentication security of user's data is important. I am considering using Google App Engine as the server. Can anyone guide me how I can implement this synchronization mechanism for my application?
I would use java as the language and expose a web service containing the operations required by your app.
Currently java under google app engine it's easiest to use RESTful services. Check out the jersey project. If you can implement their demo using Google App Engine, that's a good start.
Contact me if you get stuck, I've already done this.
http://wikis.sun.com/display/Jersey/Main
The reason for REST over SOAP is that Google App Engine does not have JAX-WS whitelisted, but does have JAX-RS whitelisted now. I believe parts of JAX-WS (can) depend on raw sockets, which are illegal in Google App Engine.