User Management

What I am trying to do is to develop a dhtml based chat system.
So far everything is working as planned, took me over 6 hours to get it
working, but I am pleased with the results, it is comparable to java
chat. With no page refresh.
My problem is
How do we manage which users are online on chat and which are not.
I do not wish to use any application variables nor do i wish to use
session variables. i am trying to avoid the 2
Either the user may simply close the chat window, or he might just go
and browse another site. Either way I do not get to write the even to
the database.