The tomcat cluster implementation provides session replication, context attribute replication and cluster wide WAR file deployment. While the Cluster configuration is fairly complex, the default configuration will work for most people out of the box.

The Tomcat Cluster implementation is very extensible, and hence we have exposed a myriad of options, making the configuration seem like a lot, but don't lose faith, instead you have a tremendous control over what is going on.

You can place the <Cluster> element inside either the <Engine> container or the <Host> container.
Placing it in the engine, means that you will support clustering in all virtual hosts of Tomcat, and share the messaging component. When you place the <Cluster> inside the <Engine> element, the cluster will append the host name of each session manager to the managers name so that two contexts with the same name but sitting inside two different hosts will be distinguishable.

To configure context attribute replication, simply do this by swapping out the context implementation used for your application context. <Context className="org.apache.catalina.ha.context.ReplicatedContext"/> This context extends the Tomcat StandardContext so all the options from the base implementation are valid.

Manager:
The session manager element identifies what kind of session manager is used in this cluster implementation. This manager configuration is identical to the one you would use in a regular <Context> configuration.
The default value is the org.apache.catalina.ha.session.DeltaManager that is closely coupled with the SimpleTcpCluster implementation. Other managers like the org.apache.catalina.ha.session.BackupManager are/could be loosely coupled and don't rely on the SimpleTcpCluster for its data replication.

Channel:
The Channel and its sub components are all part of the IO layer for the cluster group, and is a module in it's own that we have nick named "Tribes"
Any configuring and tuning of the network layer, the messaging and the membership logic will be done in the channel and its nested components. You can always find out more about Apache Tribes

Valve:
The Tomcat Cluster implementation uses Tomcat Valves to track when requests enter and exit the servlet container. It uses these valves to be able to make intelligent decisions on when to replicate data, which is always at the end of a request.

Deployer:
The Deployer component is the Tomcat Farm Deployer. It allows you to deploy and undeploy applications cluster wide.

ClusterListener:
ClusterListener's are used to track messages sent and received using the SimpleTcpCluster. If you wish to track messages, you can add a listener here, or you can add a valve to the channel object.

Deprecated settings: In the previous version of Tomcat you were able to control session manager settings using manager.<property>=value. This has been discontinued, as the way it was written interfers with the ability to support multiple different manager classes under one cluster implementation, as the same properties might have the different effect on different managers.