With Jack's really great book I'm getting more and more familiar with writing little ajax-based xmpp applications :) Now I'd like to build a more complex one with storing data into a database (e.g couchdb ).

Does anybody have some experience with xmpp applications and persistence layers? How could an architecture look like? How do I manage the access model? Which xmpp-user are allowed to read/write which parts of the database? Does It makes sense to implement something like a "database agent"?

I'd like to discuss some possible and existing solutions here.

Chesspark did authentication based on JID. Most of our users were local to our own server, so this worked very well. It's possible for a bad server admin to abuse it for external users, but this is also true with email.

The database just checked credentials against the JID, and since we were always speaking XMPP, we knew the server had authenticated them.

The rest of the application state was stored just as with any other kind of application.

One other note: we also stored some per user configuration for clients and such in Private XML storage on the server.

You can find a bit more detail in my chesspark architecture posts over at my blog. Just search for "chesspark" on http://metajack.im

Yes, that's the most obvious way :)

And here comes the technical question: How can I do that?

At the moment I use the same username and password for both xmpp and couchdb. It works but It's awful to keep the login data in sync.
As I could see, you have some experience with couchdb, right? Do you know how to tell couchdb to use the JID as access model?

We wrote an authentication plugin for ejabberd that uses CouchDB to authorize requests. That is one way to link one to the other.

At Chesspark we modified the built in ODBC integration to use our normal table structure instead of the standard ejabberd one. It just needs a JID and a password, which were already stored in our database for the web application. Instead of copying the data to another table for ejabberd, we just patched ejabberd to use our existing table.

Authentication plugins in both ejabberd and CouchDB are surprisingly easy to write.