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From "Mark Walker" <slic...@hotmail.com>
Subject Cluster Configuration Questions
Date Mon, 18 Jun 2007 00:58:48 GMT
We run a cluster of Tomcat servers with Apache as a front end load balancer 
using mod_jk configured for sticky sessions. Our primary application 
provides users with access to their financial accounts. Fast response times 
are as important as session replication.

We are starting to have problems with response times. The application 
becomes virtually unresponsive. Based on research into how our clustering is 
currently set up, I believe the problem is that the servers are tied up 
replicating session data. We have twelve instances of Tomcat spread across 
three servers (9 in the production cluster, three in the test cluster). Here 
is our current cluster definition (the only values that vary are the 
tcpListenAddress and tcpListenPort):

       <Cluster className="org.apache.catalina.cluster.tcp.SimpleTcpCluster"
                
managerClassName="org.apache.catalina.cluster.session.DeltaManager"
                expireSessionsOnShutdown="false"
                useDirtyFlag="true"
                notifyListenersOnReplication="true">

           <Membership
               className="org.apache.catalina.cluster.mcast.McastService"
               mcastAddr="228.0.0.4"
               mcastPort="45564"
               mcastFrequency="500"
               mcastDropTime="3000"/>

           <Receiver
               
className="org.apache.catalina.cluster.tcp.ReplicationListener"
               tcpListenAddress="10.9.100.2"
               tcpListenPort="4021"
               tcpSelectorTimeout="100"
               tcpThreadCount="6"/>

           <Sender
               
className="org.apache.catalina.cluster.tcp.ReplicationTransmitter"
               replicationMode="pooled"
               ackTimeout="15000"
               waitForAck="true"/>

           <Valve 
className="org.apache.catalina.cluster.tcp.ReplicationValve"
                  
filter=".*\.gif;.*\.js;.*\.jpg;.*\.png;.*\.htm;.*\.html;.*\.css;.*\.txt;"/>

           <ClusterListener 
className="org.apache.catalina.cluster.session.ClusterSessionListener"/>
       </Cluster>

Based on what I have found in my research, It seems we need to either A) 
continue with replicationMode="pooled" and increase the tcpThreadCount 
substantially or B) switch to replicationMode="fastasyncqueue" with a 
tcpTheadCount of "8". I would prefer to continue to use "pooled" to provide 
the best failover if we should need to stop or restart a cluster instance. 
However, I cannot afford to have the application "disappear" from the end 
user's perspective, due to session replication demands.


Questions for a pooled cluster:
	How high should the tcpThreadCount be set? Should the value be related to 
the average number of sessions?
	Should the ackTimeout be altered to help prevent the application from 
getting stuck doing replication?


Questions for a fastasyncqueue cluster:
	The javadocs for FastAsyncSocketSender say to "Limit the queue lock 
contention under high load!" How?
	They also say "after one minute idle time, or number of request (100) the 
connection is reconnected with next request. Change this for production 
use!" Change it higher or lower? Should the value be related to the average 
number of sessions?
	Another concern is the comment about the ackTimeout default of 15 seconds 
is "very low for big all session replication messages after restart a node". 
That description seems to accurately describe our servers. I was concerned 
that this value was might be too high for our servers in a "pooled"

Any other helpful suggestions will be greatly appreciated!


Thanks!


Mark

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