[jboss-user] [Clustering/JBoss] - Re: Session Replication - Concurrency Problems?

[bstansberry@jboss.com]

Thanks.  Please clarify a bit:

1) You mention that sticky sessions are working correctly, but your #2 indicates they aren't.  Are you finding your #1 problem even when you know sticky sessions are working?

2) In either case, when you see issues with requests failing over and the expected state not being there, do you have any feeling as to the interval between the requests and the load the servers are under?

You can get the "Possible Concurrency" message with this scenario:

1) Request 1 goes to server A and replicates to server B.
2) Request 2 goes to server A but replication to B hasn't completed.
3) Request 3 goes to server B.  Replication of Request 2 is not completed, so Request 3 sees out of date session state, which could lead to NPE or might not cause any noticeable problem.  When the session replicates back to server B, B will log the "Possible Concurrency" message.

With REPL_ASYNC, there is a higher risk of this, as Request 2 is allowed to complete before replication is completed.  The greater the load the system is under, the longer it might take the replication to complete.  The shorter the interval between request 2 and 3, the greater the chance the request hasn't completed replication.

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