[jboss-user] [Clustering/JBoss] - Re: ConcurrentModificationException during session serializa

Tue Jun 23 17:40:27 EDT 2009

I agree, that running with only one node in the cluster shouldn't require "sticky sessions=true".  

When we do run with more than one cluster node, we need to enable "sticky sessions" to avoid multiple http session attribute changes (to the same http session attribute) coming from different nodes (at the same time).

Thank you for explaining this part of the problem.  Of course, it goes deeper than not configuring the http load-balancer (please always enable "sticky http sessions" :-)

anonymous wrote : The problem as it seems to be is the web session gets modified while it is been serialized. Specifically the serialization of an org.ajax4jsf.util.LRUMap instance holding some JSF(?) state is failing with ConcurrentModificationException, so I've first thought it could be a RichFaces library problem.

>From your description of the problem, it sounds like the following events have occurred:

An HTTP request is received for user Sharon, during the processing of this request (R1), an HTTP session attribute change is made.  As part of the HTTP processing after request processing, we attempt to replicate the attribute change.  Meanwhile, another HTTP request (R2) for user Sharon was also processed that generated an HTTP session attribute change.  As part of R2 processing, the same attribute change was transmitted (from a different HTTP client service thread) and applied to the collection (while the R1 client request handling thread is iterating through the collection).  

The java.util.ConcurrentModificationException occurs when iterating through the collection, while another thread modifies the collection.  

Before we discuss possible solutions, lets see if we agree on the above.  If I am not stating things correctly above, please explain your understanding of what is happening.  

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