The Apache Tomcat 5.5 Servlet/JSP Container

Clustering/Session Replication HOW-TO

To run session replication in your Tomcat 5.5 container, the following steps should be completed:

• All your session attributes must implement java.io.Serializable
• Uncomment the Cluster element in server.xml
• Uncomment the Valve(ReplicationValve) element in server.xml
• If your Tomcat instances are running on the same machine, make sure the tcpListenPort attribute is unique for each instance.
• Make sure your web.xml has the <distributable/> element or set at your <Context distributable="true" />
• Make sure that jvmRoute attribute is set at your Engine <Engine name="Catalina" jvmRoute="node1" >
• Make sure that all nodes have the same time and sync with NTP service!

Load balancing can be achieved through many techniques, as seen in the Load Balancing chapter.

Note: Remember that your session state is tracked by a cookie, so your URL must look the same from the out side otherwise, a new session will be created.

Note: Clustering support currently requires the JDK version 1.4 or later.

To enable session replication in Tomcat, three different paths can be followed to achieve the exact same thing:

1. Using session persistence, and saving the session to a shared file system (PersistenceManager + FileStore)
2. Using session persistence, and saving the session to a shared database (PersistenceManager + JDBCStore)
3. Using in-memory-replication, using the SimpleTcpCluster that ships with Tomcat 5 (server/lib/catalina-cluster.jar)

In this release of session replication, Tomcat performs an all-to-all replication of session state. This is an algorithm that is only efficient when the clusters are small. For large clusters, the next release will support a primary-secondary session replication where the session will only be stored at one or maybe two backup servers. Currently you can use the domain worker attribute (mod_jk > 1.2.8) to build cluster partitions with the potential of very scaleable cluster solution. In order to keep the network traffic down in an all-to-all environment, you can split your cluster into smaller groups. This can be easily achieved by using different multicast addresses for the different groups. A very simple setup would look like this:

	DNS Round Robin | Load Balancer / \ Cluster1 Cluster2 / \ / \ Tomcat1 Tomcat2 Tomcat3 Tomcat4

What is important to mention here, is that session replication is only the beginning of clustering. Another popular concept used to implement clusters is farming, ie, you deploy your apps only to one server, and the cluster will distribute the deployments across the entire cluster. This is all capabilities that can go into with the FarmWarDeployer (s. cluster example at server.xml)

In the next section will go deeper into how session replication works and how to configure it.

To make it easy to understand how clustering works, We are gonna take you through a series of scenarios. In the scenario we only plan to use two tomcat instances TomcatA and TomcatB. We will cover the following sequence of events:

1. TomcatA starts up
2. TomcatB starts up (Wait that TomcatA start is complete)
3. TomcatA receives a request, a session S1 is created.
4. TomcatA crashes
5. TomcatB receives a request for session S1
6. TomcatA starts up
7. TomcatA receives a request, invalidate is called on the session (S1)
8. TomcatB receives a request, for a new session (S2)
9. TomcatA The session S2 expires due to inactivity.

Ok, now that we have a good sequence, we will take you through exactly what happens in the session repliction code

1. TomcatA starts up

   Tomcat starts up using the standard start up sequence. When the Host object is created, a cluster object is associated with it. When the contexts are parsed, if the distributable element is in place in web.xml Tomcat asks the Cluster class (in this case SimpleTcpCluster) to create a manager for the replicated context. So with clustering enabled, distributable set in web.xml Tomcat will create a DeltaManager for that context instead of a StandardManager. The cluster class will start up a membership service (multicast) and a replication service (tcp unicast). More on the architecture further down in this document.

2. TomcatB starts up

   When TomcatB starts up, it follows the same sequence as TomcatA did with one exception. The cluster is started and will establish a membership (TomcatA,TomcatB). TomcatB will now request the session state from a server that already exists in the cluster, in this case TomcatA. TomcatA responds to the request, and before TomcatB starts listening for HTTP requests, the state has been transferred from TomcatA to TomcatB. In case TomcatA doesn't respond, TomcatB will time out after 60 seconds, and issue a log entry. The session state gets transferred for each web application that has distributable in its web.xml. Note: To use session replication efficiently, all your tomcat instances should be configured the same.

3. TomcatA receives a request, a session S1 is created.

   The request coming in to TomcatA is treated exactly the same way as without session replication. The action happens when the request is completed, the ReplicationValve will intercept the request before the response is returned to the user. At this point it finds that the session has been modified, and it uses TCP to replicata the session to TomcatB. Once the serialized data has been handed off to the operating systems TCP logic, the request returns to the user, back through the valve pipeline. For each request the entire session is replicated, this allows code that modifies attributes in the session without calling setAttribute or removeAttribute to be replicated. a useDirtyFlag configuration parameter can be used to optimize the number of times a session is replicated.

4. TomcatA crashes

   When TomcatA crashes, TomcatB receives a notification that TomcatA has dropped out of the cluster. TomcatB removes TomcatA from its membership list, and TomcatA will no longer be notified of any changes that occurs in TomcatB. The load balancer will redirect the requests from TomcatA to TomcatB and all the sessions are current.

5. TomcatB receives a request for session S1

   Nothing exciting, TomcatB will process the request as any other request.

6. TomcatA starts up

   Upon start up, before TomcatA starts taking new request and making itself available to it will follow the start up sequence described above 1) 2). It will join the cluster, contact TomcatB for the current state of all the sessions. And once it receives the session state, it finishes loading and opens its HTTP/mod_jk ports. So no requests will make it to TomcatA until it has received the session state from TomcatB.

7. TomcatA receives a request, invalidate is called on the session (S1)

   The invalidate is call is intercepted, and the session is queued with invalidated sessions. When the request is complete, instead of sending out the session that has changed, it sends out an "expire" message to TomcatB and TomcatB will invalidate the session as well.

8. TomcatB receives a request, for a new session (S2)

   Same scenario as in step 3)

9. TomcatA The session S2 expires due to inactivity.

   The invalidate is call is intercepted the same was as when a session is invalidated by the user, and the session is queued with invalidated sessions. At this point, the invalidet session will not be replicated across until another request comes through the system and checks the invalid queue.

Phuuuhh! :)

Membership Clustering membership is established using very simple multicast pings. Each Tomcat instance will periodically send out a multicast ping, in the ping message the instance will broad cast its IP and TCP listen port for replication. If an instance has not received such a ping within a given timeframe, the member is considered dead. Very simple, and very effective! Of course, you need to enable multicasting on your system.

TCP Replication Once a multicast ping has been received, the member is added to the cluster Upon the next replication request, the sending instance will use the host and port info and establish a TCP socket. Using this socket it sends over the serialized data. The reason I choose TCP sockets is because it has built in flow control and guaranteed delivery. So I know, when I send some data, it will make it there :)

Distributed locking and pages using frames Tomcat does not keep session instances in sync across the cluster. The implementation of such logic would be to much overhead and cause all kinds of problems. If your client accesses the same session simultanously using multiple requests, then the last request will override the other sessions in the cluster.

Component Levels:

	Server | Service | Engine | \ | --- Cluster --* | Host | ------ / \ Cluster Context(1-N) | \ | -- Manager | \ | -- DeltaManager | ----------------------------- | | | \ Receiver Sender Membership \ \ -- Valve -- SocketReplicationListener | \ -- ReplicationListener | -- ReplicationValve | -- JvmRouteBinderValve | -- LifecycleListener | -- ClusterListener | \ | -- ClusterSessionListener | -- JvmRouteSessionIDBinderListener | -- Deployer \ -- FarmWarDeployer

	Sender \ -- ReplicationTransmitter | --------- \ IDataSender \ | --- (sync) | \ | -- PooledSocketSender (pooled) | -- SockerSender (synchronous) | --- (async) \ -- AsyncSocketSender (synchronous) -- FastAsyncSocketSender (fastasyncqueue)

The cluster configuration is described in the sample server.xml file. What is worth to mention is that the attributes starting with mcastXXX are for the membership multicast ping, and the attributes starting with tcpXXX are for the actual TCP replication.

The membership is established by all the tomcat instances are sending broadcast messages on the same multicast IP and port. The TCP listen port, is the port where the session replication is received from other members.

The replication valve is used to find out when the request has been completed and initiate the replication.

One of the most important performance considerations is the synchronous (pooled or not pooled) versus asynchronous replication mode. In a synchronous replication mode the request doesn't return until the replicated session has been sent over the wire and reinstantiated on all the other cluster nodes. There are two settings for synchronous replication. Pooled or not pooled. Not pooled (ie replicationMode="fastasnycqueue" or "synchronous") means that all the replication request are sent over a single socket. Using synchronous mode can potentially becomes a bottleneck when a lot of messages generated, You can overcome this bottleneck by setting replicationMode="pooled" but then you worker threads blocks with replication . What is recommended here is to increase the number of threads that handle incoming replication request. This is the tcpThreadCount property in the cluster section of server.xml. The pooled setting means that we are using multiple sockets, hence increases the performance. Asynchronous replication, should be used if you have sticky sessions until fail over, then your replicated data is not time crucial, but the request time is, at this time leave the tcpThreadCount to be number-of-nodes-1. During async replication, the request is returned before the data has been replicated. async replication yields shorter request times, and synchronous replication guarantees the session to be replicated before the request returns.

The parameter "replicationMode" has four different settings: "pooled", "synchronous", "asynchronous" and "fastasyncqueue"

Simple Cluster Configuration

Simple one line configuration <Server port="8011" shutdown="SHUTDOWN" > <GlobalNamingResources> <Resource name="UserDatabase" auth="Container" type="org.apache.catalina.UserDatabase" description="User database that can be updated and saved" factory="org.apache.catalina.users.MemoryUserDatabaseFactory" pathname="conf/tomcat-users.xml" /> </GlobalNamingResources> <Service name="Catalina"> <Connector port="9012" protocol="AJP/1.3" <Connector port="9013" maxThreads="10" minSpareThreads="4" maxSpareThreads="4" /> <Engine name="Catalina" defaultHost="localhost" jvmRoute="node1"> <Realm className="org.apache.catalina.realm.UserDatabaseRealm" resourceName="UserDatabase" /> <Host name="localhost" appBase="webapps"> <Cluster className="org.apache.catalina.cluster.tcp.SimpleTcpCluster"/> </Host> </Engine> </Service> </Server> The default mode configuration setup a fastasyncmode cluster configuration with following parameters: Open Membership receiver at 228.0.0.4 and send to multicast udp port 8012 Send membership every 1 sec and drop member after 30sec. Open message receiver at default ip interface at first free port between 8015 and 8019. Receiver message with SocketReplicationListener Configure a ReplicationTransmitter with fastasyncmode sender mode. Receiver message with SocketReplicationListener. Add ClusterSessionListener and ReplicationValve. NOTE: Use this configuration when you need very quick a test cluster with at your developer machine. You can change the default attributes from cluster sub elements. Use following cluster attribute prefixes sender., receiver., service., manager., valve. and listener.. Example configure cluster at windows laptop with network connection and change receiver port range <Cluster className="org.apache.catalina.cluster.tcp.SimpleTcpCluster" service.mcastBindAddress="127.0.0.1" receiver.tcpListenPort="9070" receiver.tcpListenMaxPort="9075" /> WARNING: When you add you sub elements, there overwrite the defaults complete. Example configure cluster with cluster failover jsessionid support. In this case you need also the defaultmode Cluster listener ClusterSessionListener and ReplicationValve. <Cluster className="org.apache.catalina.cluster.tcp.SimpleTcpCluster" service.mcastBindAddress="127.0.0.1" receiver.tcpListenPort="9070" receiver.tcpListenMaxPort="9075" > <ClusterListener className="org.apache.catalina.cluster.session.ClusterSessionListener" /> <ClusterListener className="org.apache.catalina.cluster.session.JvmRouteSessionIDBinderListener" /> <Valve className="org.apache.catalina.cluster.tcp.ReplicationValve" filter=".*\.gif;.*\.js;.*\.css;.*\.png;.*\.jpeg;.*\.jpg;.*\.htm;.*\.html;.*\.txt;" primaryIndicator="true" /> <Valve className="org.apache.catalina.cluster.session.JvmRouteBinderValve" enabled="true" /> <Cluster/>

Simple Engine Cluster Configuration for all hosts

Simple one line engine configuration <Server port="8011" shutdown="SHUTDOWN" > <GlobalNamingResources> <Resource name="UserDatabase" auth="Container" type="org.apache.catalina.UserDatabase" description="User database that can be updated and saved" factory="org.apache.catalina.users.MemoryUserDatabaseFactory" pathname="conf/tomcat-users.xml" /> </GlobalNamingResources> <Service name="Catalina"> <Connector port="9012" protocol="AJP/1.3" <Connector port="9013" maxThreads="10" minSpareThreads="4" maxSpareThreads="4" /> <Engine name="Catalina" defaultHost="localhost" jvmRoute="node1"> <Realm className="org.apache.catalina.realm.UserDatabaseRealm" resourceName="UserDatabase" /> <Cluster className="org.apache.catalina.cluster.tcp.SimpleTcpCluster"/> <Host name="localhost" appBase="webapps"/> </Engine> </Service> </Server> See default mode configuration description as simple host cluster example before.

Complex Cluster Configuration

Example Configure cluster with complete sub elements. Activate this node as master farm delopyer. Message receiver is NIO based ReplicationListener with six parallel worker threads. <Server port="8011" shutdown="SHUTDOWN" > <GlobalNamingResources> <Resource name="UserDatabase" auth="Container" type="org.apache.catalina.UserDatabase" description="User database that can be updated and saved" factory="org.apache.catalina.users.MemoryUserDatabaseFactory" pathname="conf/tomcat-users.xml" /> </GlobalNamingResources> <Service name="Catalina"> <Connector port="9012" protocol="AJP/1.3" <Connector port="9013" maxThreads="10" minSpareThreads="4" maxSpareThreads="4" /> <Engine name="Catalina" defaultHost="localhost" jvmRoute="node1"> <Realm className="org.apache.catalina.realm.UserDatabaseRealm" resourceName="UserDatabase" /> <Host name="localhost" appBase="webapps"> <Cluster className="org.apache.catalina.cluster.tcp.SimpleTcpCluster" doClusterLog="true" clusterLogName="clusterlog" manager.className="org.apache.catalina.cluster.session.DeltaManager" manager.expireSessionsOnShutdown="false" manager.notifyListenersOnReplication="false" manager.notifySessionListenersOnReplication="false" manager.sendAllSessions="false" manager.sendAllSessionsSize="500" manager.sendAllSessionsWaitTime="20"> <Membership className="org.apache.catalina.cluster.mcast.McastService" mcastAddr="228.0.0.4" mcastBindAddress="127.0.0.1" mcastClusterDomain="d10" mcastPort="45564" mcastFrequency="1000" mcastDropTime="30000"/> <Receiver className="org.apache.catalina.cluster.tcp.ReplicationListener" tcpListenAddress="auto" tcpListenPort="9015" tcpSelectorTimeout="100" tcpThreadCount="6" <Sender className="org.apache.catalina.cluster.tcp.ReplicationTransmitter" replicationMode="fastasyncqueue" doTransmitterProcessingStats="true" doProcessingStats="true" doWaitAckStats="true" queueTimeWait="true" queueDoStats="true" queueCheckLock="true" ackTimeout="15000" waitForAck="true" keepAliveTimeout="80000" keepAliveMaxRequestCount="-1"/> <Valve className="org.apache.catalina.cluster.tcp.ReplicationValve" filter=".*\.gif;.*\.js;.*\.css;.*\.png;.*\.jpeg;.*\.jpg;.*\.htm;.*\.html;.*\.txt;" primaryIndicator="true" /> <Valve className="org.apache.catalina.cluster.session.JvmRouteBinderValve" enabled="true" /> <ClusterListener className="org.apache.catalina.cluster.session.ClusterSessionListener" /> <ClusterListener className="org.apache.catalina.cluster.session.JvmRouteSessionIDBinderListener" /> <Deployer className="org.apache.catalina.cluster.deploy.FarmWarDeployer" tempDir="${catalina.base}/war-temp" deployDir="${catalina.base}/war-deploy/" watchDir="${catalina.base}/war-listen/" watchEnabled="true"/> </Cluster> </Host> </Engine> </Service> </Server>

Cluster Configuration for ReplicationTransmitter

List of Attributes Attribute Description Default value replicationMode replication mode (synchronous, pooled, asynchronous or fastasyncqueue) pooled processSenderFrequency Control the sender keepalive status and drop sender socket connection after timeout is reached. Check every processSenderFrequency value engine background ticks. 2 compress compress bytes before sending (consume memory, but reduce network traffic - GZIP) false ackTimeout acknowledge timeout 15000 waitForAck Wait for ack after data send false autoConnect is sender disabled, fork a new socket false doTransmitterProcessingStats create processing time stats false Example to get statistic information, wait for ack at every message send and transfer at compressed mode <Sender className="org.apache.catalina.cluster.tcp.ReplicationTransmitter" replicationMode="fastasyncqueue" compress="true" doTransmitterProcessingStats="true" ackTimeout="15000" waitForAck="true" autoConnect="false"/>

Cluster Configuration for ReplicationTransmitter (fastayncqueue - mode)

List of Attributes Attribute Description Default value keepAliveTimeout active socket keep alive timeout 60000 keepAliveMaxRequestCount max request over this socket -1 doProcessingStats create Processing time stats false doWaitAckStats create waitAck time stats false resend resend message after failure, can overwrite at message false queueDoStats activated queue stats false queueCheckLock check to lost locks false queueAddWaitTimeout queue add wait time (tomcat connector thread waits) 10000 queueRemoveWaitTimeout queue remove wait time (queue thread waits) 30000 maxQueueLength max queue length (default without limit) -1 threadPriority change queue thread priority (1-10 ; 5 is normal) 5 Example to get a lot of statistic information and not wait for ACK <Sender className="org.apache.catalina.cluster.tcp.ReplicationTransmitter" replicationMode="fastasyncqueue" doTransmitterProcessingStats="true" doProcessingStats="true" doWaitAckStats="true" queueTimeWait="true" queueDoStats="true" queueCheckLock="true" waitForAck="false" autoConnect="false" keepAliveTimeout="320000" keepAliveMaxRequestCount="-1"/>

Cluster Configuration for ReplicationTransmitter ( asynchronous - mode)

List of Attributes Attribute Description Default value keepAliveTimeout active socket keep alive timeout 60000 keepAliveMaxRequestCount max request over this socket -1 doProcessingStats create Processing time stats false doWaitAckStats create waitAck time stats false resend resend message after failure, can overwrite at message false Example to get a processing statistic information, resend after failure and wait for ACK <Sender className="org.apache.catalina.cluster.tcp.ReplicationTransmitter" replicationMode="asynchronous" doProcessingStats="true" doWaitAckStats="true" resend="true" keepAliveTimeout="320000" keepAliveMaxRequestCount="-1"/>

Cluster Configuration for ReplicationTransmitter ( synchronous - mode)

List of Attributes Attribute Description Default value keepAliveTimeout active socket keep alive timeout 60000 keepAliveMaxRequestCount max request over this socket -1 doProcessingStats create Processing time stats false doWaitAckStats create waitAck time stats true resend resend message after failure, can overwrite at message false Example to get a no processing statistic information, no wait for ACK, after 10000 request renew socket and autoconnect before first request is send. <Sender className="org.apache.catalina.cluster.tcp.ReplicationTransmitter" replicationMode="synchronous" autoConnect="true" keepAliveTimeout="-1" keepAliveMaxRequestCount="10000"/>

Cluster Configuration for ReplicationTransmitter ( pooled - mode)

List of Attributes Attribute Description Default value keepAliveTimeout active socket keep alive timeout 60000 keepAliveMaxRequestCount max request over this socket -1 maxPoolSocketLimit max pooled sockets (Sender Sockets) 25 resend resend message after failure, can overwrite at message false Example to get a no processing statistic information, wait for ACK, after 10000 request renew socket, only 10 SockerSender available and autoconnect before first request is send. <Sender className="org.apache.catalina.cluster.tcp.ReplicationTransmitter" replicationMode="pooled" autoConnect="true" maxPoolSocketLimit="10" keepAliveTimeout="-1" keepAliveMaxRequestCount="10000"/>

Cluster Configuration for ReplicationTransmitter ( DeltaManager Attribute)

List of Attributes Attribute Description Default value expireSessionsOnShutdown When server stopped, expire all sessions also at backup nodes (only for testing) false maxActiveSessions Number of active sessions. (Default is no limit) -1 notifyListenersOnReplication Notify application session listener to session creation and expiring events at backup nodes true notifySessionListenersOnReplication Notify application session listener to attribute changes at backup nodes true stateTransferTimeout Timeout that session state transfer is complete. Is attribute stateTransferTimeout == -1 then application wait that other node send the complete session state 60 sendAllSessions Flag to send sessions as splited blocks true sendAllSessionsSize Number of serialize sessions inside a send block session message. Only useful when sendAllSessions==false 1000 sendAllSessionsWaitTime wait time between two session send blocks. 2000 sendClusterDomainOnly Send all session messages only to member inside same cluster domain (value od Membership attribute mcastClusterDomain). Also don't handle session messages from other domains. true stateTimestampDrop DeltaManager queued Sessions messages when send GET_ALL_SESSION to other node. with stateTimestampDrop all messages before state transfer message creation date (find session) are dropped. Only other GET_ALL_SESSION events are handle with date before state transfer message. true Example send all sessions at blocks. Serialize and send 100 session inside on block. <Sender className="org.apache.catalina.cluster.tcp.ReplicationTransmitter" replicationMode="fastasyncqueue" sendAllSessions="false" sendAllSessionsSize="100" keepAliveTimeout="-1" keepAliveMaxRequestCount="-1"/> Note: As Cluster.defaultMode=true you can configure the manager attributes with prefix manager.. Note: With Cluster.setProperty(<String>,<String>) you can modify attributes for all register managers. The method exists as MBeans operation.

Monitoring is a very important question when you use a cluster. Some of the cluster objects are JMX MBeans

Add the following parameter to your startup script with Java 5:

	set CATALINA_OPTS=\ -Dcom.sun.management.jmxremote \ -Dcom.sun.management.jmxremote.port=%my.jmx.port% \ -Dcom.sun.management.jmxremote.ssl=false \ -Dcom.sun.management.jmxremote.authenticate=false

Activate JMX with JDK 1.4:

1. Install the compat package
2. Install the mx4j-tools.jar at common/lib (use the same mx4j version as your tomcat release)
3. Configure a MX4J JMX HTTP Adaptor at your AJP Connector

   	<Connector port="${AJP.PORT}" handler.list="mx" mx.enabled="true" mx.httpHost="${JMX.HOST}" mx.httpPort="${JMX.PORT}" protocol="AJP/1.3" />

4. Start your tomcat and look with your browser to http://${JMX.HOST}:${JMX.PORT}
5. With the connector parameter mx.authMode="basic" mx.authUser="tomcat" mx.authPassword="strange" you can control the access!

List of Cluster Mbeans

Name	Description	MBean ObjectName - Engine	MBean ObjectName - Host
Cluster	The complete cluster element	type=Cluster	type=Cluster,host=${HOST}
ClusterSender	Configuration and stats of the sender infrastructure	type=ClusterSender	type=ClusterSender,host=${HOST}
ClusterReceiver	Configuration and stats of the recevier infrastructure	type=ClusterReceiver	type=ClusterReceiver,host=${HOST}
ClusterMembership	Configuration and stats of the membership infrastructure	type=ClusterMembership	type=ClusterMembership,host=${HOST}
IDataSender	For every cluster member it exist a sender mbeans. It exists speziall MBeans to all replication modes	type=IDataSender, senderAddress=${MEMBER.SENDER.IP}, senderPort=${MEMBER.SENDER.PORT}	type=IDataSender,host=${HOST}, senderAddress=${MEMBER.SENDER.IP}, senderPort=${MEMBER.SENDER.PORT}
DeltaManager	This manager control the sessions and handle session replication	type=Manager,path=${APP.CONTEXT.PATH},host=${HOST}	type=Manager,path=${APP.CONTEXT.PATH},host=${HOST}
ReplicationValve	This valve control the replication to the backup nodes	type=Valve,name=ReplicationValve	type=Valve,name=ReplicationValve,host=${HOST}
JvmRouteBinderValve	This is a cluster fallback valve to change the Session ID to the current tomcat jvmroute.	type=Valve,name=JvmRouteBinderValve, path=${APP.CONTEXT.PATH}	type=Valve,name=JvmRouteBinderValve,host=${HOST}, path=${APP.CONTEXT.PATH}

Please see the clustering section of the FAQ.