Bugs item #863113, was opened at 2003-12-19 20:33
Message generated for change (Comment added) made by slaboure
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Category: Clustering
Group: v3.2
Status: Closed
Resolution: Invalid
Priority: 5
Submitted By: Jason Tetrault (airsquig)
Assigned to: Thomas Peuss (tpeuss)
Summary: Session Replication Inconsistent
Initial Comment:
This Bug report is a result of discussions on the JBoss
Forums:
http://www.jboss.org/index.html?
module=bb&op=viewtopic&t=43602
Versions Replicated on:
JBoss 3.2.3, JBoss 3.2.2, JBoss 3.2.4 RC, JBoss 4.0
RC
Version researched:
JBoss 3.2.4 nightly
Overview:
It was noticed that HTTP Session Replication appeared
inconsistent in a clustered environment. Further
research showed that this was happening under round
robin load balancing.
Basically, it was found that a Session would replicate to
another node ONCE and only Once, after that, the
sessions are inconsistent.
This shows itself in a apache round robin, NON Sticky
load balanced configuration. On the third hit is when
this bug shows itself.
Now, after adding some trace, what I found is the
following:
It seem that the
org.jboss.web.tomcat.session.ClusterManager has a
local session container(sessions). If and only if the
session is not in the local container, it will access the
HTTPSessionMBean to get the session, which calls the
org.jboss.ha.httpsession.beanimpl.ejb.ClusteredHTTPSes
sionBeanImpl EJB. All works well. If node A is hit first,
then Node B, Node B will get the session from the EJB
that was set from node A.
Now, Once both nodes have the session in their cache,
the ClusterManager does not appear to go back to the
MBean to re-get their session on get session requests, it
just uses the version in local sessions container. This
means that a session will replicate ONCE, and after
that, tomcat uses its local version. This causes an
inconsistent session state.
It does look like each Servlet Container is updating
there version in the EJB but, it is off because it is based
off the inconsistent session it its local cache.
Now, to test this, I made a quick code change to
ClusterManager.findSession()method to get the session
from the MBean and not use the one from sessions.
This appeared to fix the problem (You have to call
sessions.sessions.get(id) or it will break. Again, I did
not spend much time on fixing it). Exactly how you
want to fix this is up to you, I can see a few ways:
1. Somewhat like mentioned above.
2. Making the backend call the invalidate on
the MBean to get rid of the sessions session.
3. others.
Reproduction Information:
This can be replicated by 1 JSP in a clustered
environment in a session replicated web application.
The attached JSP will replicate this problem. Basically,
on each cluster, change the string being appended to
the session key to the node name.
In an apache round robin environment, one would
expect that the string in the session would look like the
following:
Hit1:
NodeA
Hit2:
NodeA,NodeB
Hit3:
NodeA,NodeB,NodeA
Hit4:
NodeA,NodeB,NodeA,NodeB
Hit5:
NodeA,NodeB,NodeA,NodeB,NodeA
Right now, this is what happens:
Hit1:
NodeA
Hit2:
NodeA,NodeB
Hit3:
NodeA,NodeA
Hit4:
NodeA,NodeB,NodeB
Hit5:
NodeA,NodeA,NodeA
Hit6:
NodeA,NodeB,NodeB,NodeB
Now, you do not necessarily need the apache round
robin configuration but, it helps. Email me if you have
any questions.
Jason
----------------------------------------------------------------------
>Comment By: Sacha Labourey (slaboure)
Date: 2003-12-21 16:41
Message:
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Hello Thomas,
No, you shouldn't redirect back, but a second failure could
make it go back to the first node, and it wouldn't use the last
known state, this is what is described in the first scenario.
If you hashmap is just holding *my* session, then simply drop
it as the clustering code keeps both a serialized and non-
serialized representation of the session, so it won't cost much.
Why do you think that the tomcat clustering code was never
designed to work without sticky sessions, I mean, which
cases wouldn't work? As we can enabled synchronous
replication, it should be ok IMHO (except in concurrent
update), but in that case the spec is dump anyway.
Cheers,
sacha
----------------------------------------------------------------------
Comment By: Thomas Peuss (tpeuss)
Date: 2003-12-21 16:01
Message:
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Sacha,
I see the problem of short network hangs leading to a
failover to another node. But why should I redirect a client
back to his "old" node after it comes up again? The session
on the old node is dead and will be removed after some time.
The local session cache is there because the Tomcat session
manager I derived the clustering session manager from has a
HashMap where it holds its sessions. My understanding of the
clustering code is that it holds the serialized sessions and
I have to deserialize them on access (which is costly - but
what do I tell you ;-) ).
If this is no longer the case we can remove the local
session cache and use the cluster cache on every access. I
think this is more straight forward anyway.
But this is still no bug because the Tomcat clustering code
was never designed to work without sticky session.
CU
Thomas
----------------------------------------------------------------------
Comment By: Sacha Labourey (slaboure)
Date: 2003-12-21 08:44
Message:
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I don't agree Thomas: while it is not really the best thing to
do not to use sticky sessions (what if you have concurrent
(frame) requests to the same session?), the described
problem can occur even in sticky-session situations if we
have a set of minor network outages between the LB and the
JBoss boxes.
What is the purpose of this local session cache anyway? We
already have the cache at the EJB level for http sessions,
why do we need another one? Is it because its creation is
costly?
----------------------------------------------------------------------
Comment By: Thomas Peuss (tpeuss)
Date: 2003-12-20 12:03
Message:
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The JBoss HTTP-Clustering ONLY works with sticky session for
performance reasons. Maybe we can introduce a configuration
option that allows a use in a round-robin fashion (I am sill
wondering why someone wants to do this).
So this is not a bug. You can add this as a feature request.
CU
Thomas
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