Google App Engine: one app, multiple domains

Question

I would like to build a web application based on App Engine and use Google accounts to authenticate users. And I need that users from multiple domains could login. From what I have read it seems that it is impossible to do this just using Google Accounts API. Why is that so? I really like the simplicity of it.

The second option of authentication in Application Settings is 'federated login' but it is marked as experimental and only thing I was able to find about it, is that is has something to do with OpenID. Could it solve my problem?

I have also found a tip (actually on this server) which recommended using Apps Marketplace. Firstly, I was really excited about it because I thought that Apps Marketplace would create separated instance of my application for each paying domain. But I tried one application also based on App Engine and it seems to me that all that Marketplace does is simple redirection and I have to care about authenticating myself (probably mentioned 'federated login'). So it is again impossible to use the simplicity of Google Accounts API. Or am I wrong?

Answer 1

Well since nobody has answered my question, I have to do it myself :)

I spent many hours reading Google documentation and I came to the conclusion that Apps Marketplace would be the best solution for me. Applications from Marketplace use SSO to authenticate users which is the mentioned 'federated login'. I found some examples of use in the documentation and it is actually pretty straightforward. I just don't understand why it is marked as experimental. The downside is that users with standard Google account won't be able to login but it is not a priority. A good thing on the other hand is that App Engine provides Namespaces API. Using this API it is actually possible to create a separate instance for each domain which is what I originally hoped for.