Mark asks a question which I think touches on a basic misunderstanding many people have with ColdFusion and sessions:
I am developing an AJAX RSS feed reader for a portal. I've read several proposed methods of handling sessions, but I don't any address my specific need. In order for the client script to retrieve external feeds I have setup a proxy which simply checks whether it is given a valid URL, it grabs the feed and caches it for a minute so that common feeds don't spawn external GETs more than once a minute. Also, If users leave their portal page open, the client will update the feeds every few minutes.
What I don't want is the portal session timeout to be affected by the feeds being retrieved via the proxy, so I put it in its own subfolder with its own Application.cfm. For some measure of security, I want to require that the client has a cookie with a valid, unexpired session. In order to check this cross-application (proxy->portal) I'm using this in Application.cfm:
<cfset a = ArrayNew(1) />
<cfset sessionClass = a.getClass().forName('coldfusion.runtime.SessionScope') />
<cfset sess = StructNew() />
<cfset sess.expired = sessionClass.getMethod('expired', a) />
<cfset tracker = createObject("java","coldfusion.runtime.SessionTracker")>
<cfset sessions = tracker.getSessionCollection('PORTAL')>
<cfset expired = sess.expired.invoke(sessions['PORTAL_'&PORTAL_Session], a)>
Now I should be able to check whether expired is true/false, right? ..Nope. Expired only seems to change when the timeout has been reached, apparently it is not set to true when the user clicks logout.
So first off - it is critical you understand that - out of the box - there is no way for you end a user's session. Just because a user clicked "Logout", it does not end his session. I assume your logout code does something like this:
<cfset structDelete(session, "loggedin")>
While this removes data from the session, and may remove the only key you ever set, this did nothing to end the session. The user still has a session and will continue to have one until timed out by the server.
You may be able to find an API in ColdFusion to truly kill the session, but it isn't officially supported. I like to play around with the undocumented APIs too, but I always recommend against this. You have no guarantee it will work.
So obviously there are multiple other ways you can solve this. I think the cookie approach is best. In your main application, I'd simply set a cookie called "LASTHIT". On every request, set it to now.
In your sub application, see if LASTHIT was set more than 20 minutes ago. If so, consider the request invalid.