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Tomcat Native Clustering
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The tomcat web container provides a native clustering solution that can be configured through geronimo using gbean definitions within config.xml. This document will go through the available gbeans and how to configure them for clustering in a geronimo server with a tomcat web container. Details about each component of the cluster and configuration options can be found in the tomcat documentation here http://tomcat.apache.org/tomcat-6.0-doc/config/cluster.html

Cluster Configuration Elements

Following are parameters or attributes you will need to configure for a Tomcat native clustering.

  • Cluster
    • ClusterListenerChain
    • TomcatValveChain
    • Channel
      • Membership
      • Receiver
      • Sender
      • ChannelInterceptor
        • StaticMember
    • ClusterManager

Sample Clustering Configuration with multicast configuration

Although sufficient for many applications, Tomcat clusters have some limitations.

  • This feature does not replicate stateful session Enterprise JavaBeans (EJBs). You will have to avoid stateful session EJBs in your distributed applications.
  • This feature does not replicate dynamic updates to the Java Naming and Directory Interface (JNDI). You will have to configure all the JNDI names used by your distributed applications in every node of the cluster.
  • This feature does not replicate distributable Web applications to other nodes in the cluster. You will have to deploy your distributable Web applications to every node.

A cluster configuration should be considered when you want to improve the scalability and availability of your Web application. The following sections provide detailed instructions on how to set up your cluster nodes and how to deploy your applications with tomcat clustering enabled.

Setting up a clustering environment

Generally, to set up a small cluster you will need at least 2 nodes and 1 HTTP server. The HTTP server is used to serve requests from clients and ensure well-balanced traffic load among different nodes. Similarly, each node is configured to use the same logical Tomcat engine and enable session affinity.

Planning your cluster

The Tomcat cluster replicates HTTP session data via memory to memory multicast communication.

Every node transmits its session data to every node in the cluster. This algorithm is only efficient when the clusters are small. If the clusters grow too large, the overhead in storage utilization and network traffic becomes excessive. To avoid excessive overhead, consider dividing your nodes into several smaller clusters.

HTTP session data is replicated among the nodes in the cluster using a multicast broadcast. All nodes in the cluster must be on the same physical subnet and multicast broadcast must be supported by that subnet.

Preparing your Web application

To participate in a cluster configuration, your Web application must be implemented correctly.

  • Ensure that every object placed in the HTTP session implements java.io.Serializable. The clustering feature serializes the objects when it distributes them to the other nodes in the cluster.
  • The deployment descriptor for your Web application, that is the web.xml file in the Web archive, must indicate that your Web application is distributable. To do this, insert the distributable element in the deployment descriptor.
    Excerpt from web.xml
    <?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE web-app
    PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
    "http://java.sun.com/dtd/web-app_2_3.dtd">

<web-app>
    <display-name> ... </display-name>
    <description> ... </description>
    <distributable> <distributable/>
    .
    .
    .
</web-app>
  • Do not use stateful session Enterprise JavaBeans (EJBs). The clustering feature does not replicate stateful EJBs among the nodes in the cluster.
  • If your Web application uses a database, make sure the database can be accessed from every node in the cluster. Ensure that the proper JDBC drivers are installed on every node and that the datasource objects are defined correctly on every node.
  • Do not depend on dynamic updates to the Java Naming and Directory Interface (JNDI). You will need to configure all the JNDI names used by your application in every node of the cluster. The clustering feature does not replicate JNDI changes among the nodes in the cluster.

Enabling session affinity

Support for session affinity, also known as sticky session support, allows a load balancing service to route an HTTP request back to the same node that created the HTTP session associated with that request until that node fails. You must use session affinity if you configure an asynchronous type of session replication. With asynchronous replication, the reply is returned before the HTTP session is replicated so there is always a chance that the next request using that session arrives before the replication is complete. In this case, the only way to ensure that the request is processed using the correct session data is to route the request to the node that sent the reply to the last request and originated the replication.

In Geronimo v2.2, users can have the same experiences as in Tomcat Web container to configure session affinity. To enable session affinity, you must modify servel.xml under <GERONIMO_HOME>/var/catalina dirctory and configure the <Engine> with a jvmRoute attribute value that is unique for each node in the cluster. The load balancer will return this value in the session cookie or the encoded URL returned to the browser. When a related request arrives it can use the value to route the request to the correct node. See

  1. For every node in the cluster, update server.xml as followed after the server is stopped:
    Excerpt from server.xml
     ...
 <Engine name="Catalina" defaultHost="${ServerHostname}" jvmRoute="nodeId">
 ...

    where

    • nodeId is a node identifier that is unique among all the nodes in the cluster. If you are using mod_jk, make sure that the jvmRoute attribute value matches your worker name in workers.properties.
  2. Restart the server to enable the new configuration.

Engaging load balancing and failover

Initially, the server configuration includes an AJP connector suitable for exchanging messages with a load balancing service. See Configuring a remote Apache HTTP server for more information about the HTTP server configuration.

Deploying your application to a cluster

To deploy your application to a cluster and make sure HTTP sessions are replicated successfully, cluster configuration elements of Tomcat Web containers must be configured using gbean definitions within deployment plans.
If you want to deploy your Web application to a cluster, install your WAR files to the appropriate cluster member, assuring that you use the correct deployment plan for each member. Here is the template for your Web application deployment plan. See Creating deployment plans for Web applications for more information about the parameters.

geronimo-web.xml
<web-app xmlns="http://geronimo.apache.org/xml/ns/j2ee/web/tomcat-1.2"
         xmlns:dep="http://geronimo.apache.org/xml/ns/deployment-1.2">

    <dep:environment>
      <dep:moduleId>
        <dep:groupId>org.mygroup</dep:groupId>
        <dep:artifactId>web-cluster-server1</dep:artifactId>
        <dep:version>2.2</dep:version>
        <dep:type>war</dep:type>
      </dep:moduleId>
      <dep:dependencies>
      	<dep:dependency>
      	<dep:groupId>console.realm</dep:groupId>
        <dep:artifactId>geronimo-properties-realm</dep:artifactId>
        <dep:version>1.0</dep:version>
        <dep:type>car</dep:type>
        </dep:dependency>
      </dep:dependencies>
      <dep:hidden-classes/>
      <dep:non-overridable-classes/>
    </dep:environment>

    <context-root>/servlet-examples-cluster</context-root>

    <security-realm-name>geronimo-properties-realm</security-realm-name>
    <security>
      <default-principal>
        <principal name="anonymous" class="org.apache.geronimo.security.realm.providers.GeronimoUserPrincipal"/>
      </default-principal>
      <role-mappings>
        <role role-name="tomcat">
          <principal name="admin" class="org.apache.geronimo.security.realm.providers.GeronimoGroupPrincipal"/>
        </role>
      </role-mappings>
    </security>

    <cluster>TomcatCluster</cluster>
    <gbean class="org.apache.geronimo.tomcat.cluster.CatalinaClusterGBean" name="TomcatCluster">
        <attribute name="className">org.apache.catalina.ha.tcp.SimpleTcpCluster</attribute>
        <attribute name="initParams"/>
        <reference name="ClusterManager">          
            <name>TomcatClusterManager</name>                
        </reference>
        <reference name="TomcatValveChain">
            <name>ReplicationValve</name>
        </reference>
        <reference name="ClusterListenerChain">
            <name>ClusterSessionListener</name>
        </reference>
        <reference name="Channel">
            <name>TomcatGroupChannel</name>
        </reference>
    </gbean>
  <!-- Cluster Manager -->
    <gbean name="TomcatClusterManager" class="org.apache.geronimo.tomcat.cluster.ClusterManagerGBean">
        <attribute name="className">org.apache.catalina.ha.session.DeltaManager</attribute>
        <attribute name="initParams">name=${clusterName}
           channelSendOptions=6
           expireSessionsOnShutdown=false
           notifyListenersOnReplication=true
           mapSendOptions=6</attribute>
    </gbean>
  <!-- Cluster Channel -->
    <gbean class="org.apache.geronimo.tomcat.cluster.ChannelGBean" name="TomcatGroupChannel">
        <attribute name="className">org.apache.catalina.tribes.group.GroupChannel</attribute>
        <attribute name="initParams"/>
        <reference name="Membership">
            <name>TomcatMembership</name>
        </reference>
        <reference name="Receiver">
            <name>TomcatReceiver</name>
        </reference>
        <reference name="Sender">
            <name>TomcatSender</name>
        </reference>
        <reference name="ChannelInterceptor">
            <name>TomcatChannelInterceptor</name>
        </reference>
    </gbean>
  <!-- Cluster Membership -->
    <gbean class="org.apache.geronimo.tomcat.cluster.MembershipServiceGBean" name="TomcatMembership">
        <attribute name="className">org.apache.catalina.tribes.membership.McastService</attribute>
        <attribute name="initParams">
            mcastAddr=228.0.0.4
            mcastPort=45564
            mcastFrequency=500
            mcastDropTime=3000
        </attribute>
    </gbean>
  <!-- Cluster Receiver -->
    <gbean class="org.apache.geronimo.tomcat.cluster.ReceiverGBean" name="TomcatReceiver">
        <attribute name="className">org.apache.catalina.tribes.transport.nio.NioReceiver</attribute>
        <attribute name="initParams">
            tcpListenAddress=auto
            tcpListenPort=4001
            tcpSelectorTimeout=100
            tcpThreadCount=6
        </attribute>
    </gbean>
  <!-- Cluster Sender -->
    <gbean class="org.apache.geronimo.tomcat.cluster.SenderGBean" name="TomcatSender">
        <attribute name="className">org.apache.catalina.tribes.transport.ReplicationTransmitter</attribute>
        <attribute name="initParams">
            replicationMode=pooled
            waitForAck=true
        </attribute>
    </gbean>
  <!-- Cluster Valves-->
    <gbean class="org.apache.geronimo.tomcat.ValveGBean" name="ReplicationValve">
        <attribute name="className">org.apache.catalina.ha.tcp.ReplicationValve</attribute>
        <attribute name="initParams">filter=.*\.gif;.*\.js;.*\.css;.*\.png;.*\.jpeg;.*\.jpg;.*\.htm;.*\.html;.*\.txt;</attribute>
        <reference name="NextValve">
        	<name>JvmRouteBinderValve</name>
        </reference>
    </gbean>
  <!-- Cluster Route Binder -->
    <gbean class="org.apache.geronimo.tomcat.ValveGBean" name="JvmRouteBinderValve">
        <attribute name="className">org.apache.catalina.ha.session.JvmRouteBinderValve</attribute>
        <attribute name="initParams">enabled=true</attribute>
    </gbean>
  <!-- Cluster Listener -->
    <gbean class="org.apache.geronimo.tomcat.cluster.ClusterListenerGBean" name="ClusterSessionListener">
        <attribute name="className">org.apache.catalina.ha.session.ClusterSessionListener</attribute>
        <reference name="NextListener">
            <name>JvmRouteSessionIDBinderListener</name>
        </reference>
    </gbean>
  <!-- Cluster Binder Listener-->
    <gbean class="org.apache.geronimo.tomcat.cluster.ClusterListenerGBean" name="JvmRouteSessionIDBinderListener">
        <attribute name="className">org.apache.catalina.ha.session.JvmRouteSessionIDBinderListener</attribute>
    </gbean>
  <!-- Cluster Channel Interceptor -->
    <gbean class="org.apache.geronimo.tomcat.cluster.ChannelInterceptorGBean" name="TomcatChannelInterceptor">
        <attribute name="className">org.apache.catalina.tribes.group.interceptors.TcpFailureDetector</attribute>
    </gbean>
</web-app>

where

  • web-cluster-server1 should match the WAR file name. It can be different for each node in the cluster.

On each node, deploy your Web application, either via admin console or deploy command, following this syntax:

deploy --user name --password word deploy archive plan

where

  • name is replaced with a user name authorized to manage the server. If you omit this option, you will be prompted to enter a user name.
  • word is replaced with the password used to authenticate the user. If you omit this option, you will be prompted to enter a password.
  • archive is replaced with a file specification to your Web application WAR file.
  • plan is replaced with a file specification to your deployment plan.

Note: After installation, the user name is initially system, and the password initially manager.

Sample Tomcat Clustering with Unicast Configuration

The following code snippet is part of a clustering example that uses unicast configuration. Static members are defined using org.apache.geronimo.tomcat.cluster.StaticMemberGBean. You have to define each static member within your deployment plan to make sure your application is clustered successfully, and make sure that TCP ports for session replication are defined consistently on each node.

excerpt from geronimo-web.xml
...
    <cluster>TomcatCluster</cluster>
        <gbean name="TomcatCluster" class="org.apache.geronimo.tomcat.cluster.CatalinaClusterGBean">
            <attribute name="className">org.apache.catalina.ha.tcp.SimpleTcpCluster</attribute>
            <attribute name="initParams"/>
            <reference name="ClusterListenerChain"> 
                    <name>TomcatClusterListenerChain</name>
            </reference>
            <reference name="TomcatValveChain">
                    <name>ReplicationValve</name>  
            </reference>
            <reference name="Channel">
                    <name>TomcatChannel</name>
            </reference>
            <reference name="ClusterManager"> 
                    <name>TomcatClusterManager</name>
            </reference>
        </gbean>

        <gbean name="TomcatClusterListenerChain" class="org.apache.geronimo.tomcat.cluster.ClusterListenerGBean">
            <attribute name="className">org.apache.catalina.ha.session.ClusterSessionListener</attribute>
            <attribute name="initParams"/>
        </gbean>

        <gbean name="ReplicationValve" class="org.apache.geronimo.tomcat.ValveGBean">
            <attribute name="className">org.apache.catalina.ha.tcp.ReplicationValve</attribute>
            <attribute name="initParams">.*\.gif;.*\.js;.*\.jpg;.*\.png;.*\.htm;.*\.html;.*\.css;.*\.txt;</attribute>
            <reference name="NextValve">
        			<type>TomcatValve</type>
            	                <name>JvmRouteBinderValve</name>
            </reference>
        </gbean>

        <gbean name="JvmRouteBinderValve" class="org.apache.geronimo.tomcat.ValveGBean">
            <attribute name="className">org.apache.catalina.ha.session.JvmRouteBinderValve</attribute>
            <attribute name="initParams">enabled=true</attribute>
        </gbean>

        <gbean name="TomcatClusterManager" class="org.apache.geronimo.tomcat.cluster.ClusterManagerGBean">
            <attribute name="className">org.apache.catalina.ha.session.DeltaManager</attribute>
            <attribute name="initParams">name=${clusterName}
                                         channelSendOptions=6
                                         expireSessionsOnShutdown=false
                                         notifyListenersOnReplication=true
                                         mapSendOptions=6
            </attribute>
        </gbean>

        <gbean name="TomcatChannel" class="org.apache.geronimo.tomcat.cluster.ChannelGBean">
            <attribute name="className">org.apache.catalina.tribes.group.GroupChannel</attribute>
            <attribute name="initParams"/>
            <reference name="Membership"> 
                    <name>ClusterMembership</name>
            </reference>
            <reference name="Receiver">
                    <name>ClusterReceiver</name>                
            </reference>
            <reference name="Sender">                
                    <name>ClusterSender</name>
            </reference>
            <reference name="ChannelInterceptor">
                    <name>DisableMcastInterceptor</name>                
            </reference>
        </gbean>

        <gbean name="ClusterMembership" class="org.apache.geronimo.tomcat.cluster.MembershipServiceGBean">
            <attribute name="className">org.apache.catalina.tribes.membership.McastService</attribute>
            <attribute name="initParams">address=228.0.0.4
                                         port=45564
                                         frequency=500
                                         dropTime=3000
            </attribute>
        </gbean>

        <gbean name="ClusterReceiver" class="org.apache.geronimo.tomcat.cluster.ReceiverGBean">
            <attribute name="className">org.apache.catalina.tribes.transport.nio.NioReceiver</attribute>
            <attribute name="initParams">address=IPAddress1
                                         port=TCP_port1
                                         selectorTimeout=100
                                         maxThreads=6
            </attribute>
        </gbean>

        <gbean name="ClusterSender" class="org.apache.geronimo.tomcat.cluster.SenderGBean">
            <attribute name="className">org.apache.catalina.tribes.transport.ReplicationTransmitter</attribute>
        </gbean>

        <gbean name="DisableMcastInterceptor" class="org.apache.geronimo.tomcat.cluster.ChannelInterceptorGBean">
            <attribute name="className">org.apache.geronimo.tomcat.interceptor.DisableMcastInterceptor</attribute>
            <attribute name="initParams"/>
            <reference name="NextInterceptor">
                    <name>TcpPingInterceptor</name>                
            </reference>
        </gbean>

        <gbean name="TcpPingInterceptor" class="org.apache.geronimo.tomcat.cluster.ChannelInterceptorGBean">
            <attribute name="className">org.apache.catalina.tribes.group.interceptors.TcpPingInterceptor</attribute>
            <attribute name="initParams">Interval=60</attribute>
            <reference name="NextInterceptor">
                    <name>TcpFailureDetector</name>                
            </reference>
        </gbean>

        <gbean name="TcpFailureDetector" class="org.apache.geronimo.tomcat.cluster.ChannelInterceptorGBean">
            <attribute name="className">org.apache.catalina.tribes.group.interceptors.TcpFailureDetector</attribute>
            <attribute name="initParams"/>
            <reference name="NextInterceptor">                
                    <name>StaticMember1Interceptor</name>                
            </reference>
        </gbean>

        <gbean name="StaticMember1Interceptor" class="org.apache.geronimo.tomcat.cluster.ChannelInterceptorGBean">
            <attribute name="className">org.apache.catalina.tribes.group.interceptors.StaticMembershipInterceptor</attribute>
            <attribute name="initParams"/>
            <reference name="StaticMember">
                    <name>StaticMember2</name>                
            </reference>
            <reference name="NextInterceptor">                
                    <name>MessageDispatch15Interceptor</name> 
            </reference>
        </gbean>

        <gbean name="MessageDispatch15Interceptor" class="org.apache.geronimo.tomcat.cluster.ChannelInterceptorGBean">
            <attribute name="className">org.apache.catalina.tribes.group.interceptors.MessageDispatch15Interceptor</attribute>
            <attribute name="initParams"/>
            <reference name="NextInterceptor">
                    <name>ThroughputInterceptor</name>                
            </reference>
        </gbean>

        <gbean name="ThroughputInterceptor" class="org.apache.geronimo.tomcat.cluster.ChannelInterceptorGBean">
            <attribute name="className">org.apache.catalina.tribes.group.interceptors.ThroughputInterceptor</attribute>
            <attribute name="initParams"/>
        </gbean>

        <gbean name="StaticMember2" class="org.apache.geronimo.tomcat.cluster.StaticMemberGBean">
            <attribute name="className">org.apache.catalina.tribes.membership.StaticMember</attribute>
            <attribute name="initParams">port=TCP_port2
                                         securePort=-1
                                         host=IPAddress2
                                         domain=test-domain
                                         UniqueId={2,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0}
            </attribute>
        </gbean>
...

Where

  • IPAddress1 is the IP address or host name of current static member.
  • TCP_port1 is the TCP port on the current node to listen for session replication data from other static members.
  • IPAddress2 is the IP address or host name of the second static member.
  • TCP_port2 is the TCP port on the second static member to listen for session replication data from other static members

To convert this example to a multicast configuration, the DisableMCastInterceptor, StaticMemberInterceptor, and StaticMember definitions need to be removed. Also, the value for the "address" attribute for the TomcatReceiver definition should be changed to "auto".

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