[wildfly-dev] request.getSession() returns a different object for each request - Undertow's session object is the object retrieved by request.getSession()

[Stuart Douglas]

On Sat, Dec 9, 2017 at 3:04 PM, Eric B <ebenzacar at gmail.com> wrote:

> I'm migrating a legacy (circa 2005) n-tier JBoss 4 application to run in
> Wildfly 10.  It was a struggle, but got pretty much everything done.  We've
> been trying as hard as possible not to refactor logic/code/etc except in
> the cases where it is simply no longer compatible.  The goal was to get the
> application working "as-is" in WF, then start tackling the job of breaking
> it apart and refactoring it into proper pieces.
>
> The problem I have run into now is regarding session management.  The
> application was designed to run on multiple nodes, but in standalone
> instances - ie: there is no shared memory, nor any distributed caches.  So
> I'm trying to get my WF10 application to follow the same pattern (even
> though I realize all this exists in WF10 out of the box with
> Infinispan/etc).
>
> One of the requirements is to only have a single session active for a user
> at any time in the "cluster".  The JB4 application accomplished this by
> maintaining a local cache (HashMap) of session objects keyed by username.
> When a user logs into any node, a JMS message is broadcast to all the nodes
> to invalidate any session belonging to that user.  The listener on each
> node then searches in his cache for a matching user/session object and does
> a session.invalidate().  Some extra logic is used for WeakReferences for
> the session objects (in case the session is destroyed by some other flow in
> the container).
>
> As ugly as the solution was, we've tried to follow the same pattern under
> WF10.  But this is failing in many different ways and reinforces my belief
> that it isn't the right approach.  However, I would like to understand
> how/why this is failing.
>
> My server configuration is using a <local-cache> for my "web" cache.  So
> to me this means it is only local to the machine.  But:
>
> 1) on any specific node, request.getSession() returns a different object
> for each request.  The sessionId() remains the same, but the actual object
> ID changes.  This implies that it is a different representation of the
> session object.
>

Undertow returns a different facade for each request. Undertow uses its
internal representation of a session (io.undertow.server.session.Session)
that is stored in the session manager, and wraps a facade around it
(io.undertow.servlet.spec.HttpSessionImpl) that implements Servlet
semantics.


>
> 2) if I persist a local copy of the HttpSession object between requests
> (ex: in a static map) and invalidate the session using the persisted
> object, my request.getSession() object is not updated (ex: the invalid flag
> is still set to false), but the session is dead.  Trying to call
> request.getSession().invalidate() throws IllegalStateException as do
> calls to request.getSession().set/getAttribute()
>

I guess this could be considered a bug in that isNew will not throw an
IllegalStateException, which is the only place that the invalid flag is
used without also consulting the underlying session, but you sound as if
you expect the session to still work after it has been invalidated?

There is no requirement that the same session is represented by a single
java object, and especially it in a distributed environment this is not
possible. Even though we could keep the same facade around for all requests
this can encourage people to write apps that rely on this behaviour, and
also increases the likelihood that the facade will become tenured and
require a full GC to be reclaimed.


>
> 3) over time, my JVM will actually crash with an
> EXCEPTION_ACCESS_VIOLATION in a GC process.  This always seems to correlate
> with a thread that is trying to do some session invalidation via the
> persisted session copy.
>

This is a JVM bug. Which JVM are you using?

To implement this I would remove the WeakReference which will not work with
Undertow's session management strategy, and instead register a listener to
remove the session from the local map when it is invalidated. If you really
want to access the underlying Undertow session you can
call io.undertow.servlet.spec.HttpSessionImpl#getSession() to return the
Underlying Undertow session, although it should not be necessary.

Stuart



>
> Is anyone able to explain this behaviour?  Why is the session object
> always different between requests?  Shouldn't it be the same request?  What
> is Undertow doing with the session objects between requests?  Is the
> Undertow object being passivated in some way and my attempt to invalidate
> if from within my cached version causing this kind of access violation?  Is
> my cached object referencing memory that has been cleared by the GC (ex:
> does the request.getSession() object only a WeakReference to the actual
> Undertow object)?
>
>
> Finally, what would the recommended approach be to doing something like
> this?  Using a distributed web-cache is unfortunately not an option at the
> moment.  So give that, is there some way to access the Undertow session
> manager directly?
>
> Thanks for any insight.  I thought we had a functional solution but in
> production (under real load), the intermittent JVM crashes are telling me
> that our solution is broken.
>
> Eric
>
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