Well, I kept encountering technical issues with GAE.
1) When working with the GAE plugins in eclipse, it seems that I cannot include any third party external jars into the project. It will compile corretly, but throws a classnotfoundexception in runtime, when the third party jar is going to be used. My alternative is to include all the source files for the 3rd party jar into the project.
2) No thread creation on the server side. It looks like GAE does not allow thread creation inside the server side servlet. Because my application needs to monitor a website on an interval base, I planned to spawn a thread to achieve that, although I would do it differently if I have total control on the backend. It gives me an access exception (if I remember it correctly) when I tried to do so.
3) Because of #2, I have to move the polling part inside the same thread which receives the async request on the server side. After maybe 30 seconds (the thread sleeps for 20 seconds), a DeadlineExceededException is throw. I think GAE does not allow a servlet thread to be executed more than 30 seconds.
With that, I decided not to build this application on GAE. I am not saying GAE is bad, but it just does not meet the technical requirement for this specific type of application.