The Tomcat Web container provides a native clustering solution that can be configured through Geronimo using gbean definitions within config.xml or your deployment descriptors . This document will go through the available GBeans and how to configure them for clustering in a Geronimo server with a tomcat web container.
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.
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.
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.
To participate in a cluster configuration, your Web application must be implemented correctly.
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.
For every node in the cluster, update config.xml as follows after the server is stopped
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.