Geronimo Clustering Example - Tomcat web tier clustering

Part 1 - Installing/Running the Clustering Example

Example Overview
This example demonstrates how to use context level clustering with the Tomcat web container in Geronimo. It contains the necessary attachments for setting up cluster members on two separate physical machines. The cluster configuration will allow the two cluster members to replicate httpsession data via memory to memory multicast communication. Also, a load balancer can be used to spray the incoming requests to the available cluster members. In this example, we recommend Apache HTTP server and Apache mod_jk.

Installing the Example
This example contains 4 attachments:

• servlets-examples-cluster-node1.war - web application for Cluster Member 1
• servlets-examples-cluster-node2.war - web application for Cluster Member 2
• servlets-examples-tomcat-cluster-plan-node1.xml - Geronimo deployment plan for node 1
• servlets-examples-tomcat-cluster-plan-node2.xml- Geronimo deployment plan for node 2

Each geronimo cluster member must have a unique jvmRoute designation for both load balancing and session replication.

Unfortunately, the only current way to setup the jvmRoute is by rebuilding the geronimo server.
In the near future, the jvmRoute attribute will be configurable via the var/config/config.xml file. However, for now a new build is required for each cluster member. The jvmRoute should be configured with a unique for each geronimo cluster member as shown below:

Update filename: geronimo/configs/tomcat/src/plan/plan.xml as follows

<gbean name="TomcatEngine" class="org.apache.geronimo.tomcat.EngineGBean">
<attribute name="className">org.apache.geronimo.tomcat.TomcatEngine</attribute>
<attribute name="initParams">
name=Geronimo
+ jvmRoute=nodex
</attribute>

Rebuild geronimo and uncompress the resulting image for each cluster member. Instructions for rebuilding geronimo is beyond the scope of this article. Instructions for building are available on the Geronimo Wiki.

Now that the jvmRoute is set correctly for each cluster member. You must deploy the example on each of the cluster members. For this example the applications are slightly different for each member. The difference is merely to output the current Server number (e.g. Server 1, Server 2) in the output of the application. This will be useful when trying to determine which cluster member is servicing the http request from the browser.

Start the geronimo server on each cluster member and then install the attached applications to the appropriate cluster member assuring that you use the correct deployment plan for each. Note that the deployment plan must be updated with the hostname (or IP address) for each machine. The appropriate spots are identified in the plans with xx.yy.zz.aa.

Memory replication currently requires that all cluster members must reside on the same physical subnet since multicast broadcast is used.

At this point you can test httpsession replication by hitting the application with your browser. Probably something like: http://localhost:8080/servlets-examples-cluster/servlet/SessionExample

The console dialogue (the prompt where you started geronimo) should show that the httpsession data is being transmitted and recieved between the nodes.

Load Balancing and failover
Now you are ready to setup the Load Balancer. You can use Apache HTTP server and mod_jk to load balance between the two cluster members.

Install Apache HTTP server
Install Apache mod_jk

Configuration tips for mod_jk:
worker.list=loadbalancer,status
worker.node1.port=8009
worker.node1.host=your.first.cluster.member.host.name
worker.node1.type=ajp13
worker.node1.lbfactor=1

worker.node2.port=8009
worker.node2.host==your.second.cluster.member.host.name
worker.node2.type=ajp13
worker.node2.lbfactor=1

worker.loadbalancer.type=lb
worker.loadbalancer.balance_workers=node1,node2
worker.loadbalancer.sticky_session=1
worker.status.type=status

/servlets-examples-cluster=loadbalancer
/servlets-examples-cluster/*=loadbalancer

Testing Load Balancing and Failover

Once you get Apache HTPP Server and mod_jk setup correctly.. You can test load balancing and failover by requesting the following urls on port 80.

http://youhost/servlets-examples-cluster - non sticky request (no httpsession request) http://youhost/servlets-examples-cluster/servlet/SessionExample - sticky session (using httpsession and cookies)

You can test failover by stopping one of the geronimo servers and seeing that the next http request fails over into the remaining server. The httpsession data should be recovered and displayed in the refreshed browser window.

Tips
When testing using a web browser, make sure that you erase cookies and cached pages between test cases.
Make sure your application has the distributable attribute defined in web.xml
Make sure all cluster members are on the same physical subnet and that multicast broadcast is supported on the subnet.

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