Clustering has emerged as the solution of choice for scaling up server components, and also to make such components highly available. As open-source tools increasingly find their way into the enterprise infrastructure, clustering has become an important focus of open-source enterprise projects.

Apache's Geronimo is an open-source J2EE implementation built around a light-weight kernel and plug-in components that provide various server functionalities.

Matthew Jording has written a tutorial for IBM DeveloperWorks on clustering Geronimo servers, Use Apache Geronimo to Build a Cluster. The first part of his five-part series, Exploring Manageability (free registration required), is devoted to an explanation of J2EE clustering, and shows why Geromino is well-suited to be deployed in a clustered fashion:

The components that implement the various Java EE tiers surrounding the Geronimo kernel have been chosen for their decoupled and clustering strengths... Geronimo contains several projects that provide some component of clustering..

• ActiveCluster is a generic clustering API used by ActiveMQ and WADI Application Distribution Infrastructure (WADI). It simplifies operations that are typical for clustering applications.
• ActiveIO is built on top of ActiveCluster and is used by ActiveMQ for clustering across a network of brokers.
• ActiveSpace is used by ActiveMQ to support Staged Event-Driven Architecture (SEDA)-style load-balancing data across consumers.
• WADI is built on top of the ActiveCluster API to provide clustering for HttpSessions.

The second installment of the article, Developing the Cluster Nodes, focuses on setting up a Geronimo cluster. Geromino uses Tomcat or Jetty as the components providing the Web-tier functionality, and this segment of Jordin's article describes how to cluster Tomcat with JMXBeans:

Tomcat has some base clustering built into it. So as not to reinvent the wheel, you can leverage the hard work done by Tomcat's developers as your Geronimo Web-tier clustering.

Traditionally, Tomcat cluster management and monitoring has relied on the cluster supporting Java management objects (MBeans) within Tomcat itself. JMX support was available in the Tomcat 4.1 release, but has been enhanced in Tomcat 5 to provide comprehensive server monitoring and administration using Java Management Extensions (JMX). JMX is now a core part of Catalina, Tomcat's servlet implementation. Tomcat 5 uses JMX MBeans for implementing the manageability of Tomcat. You can now manage all of Tomcat's internals, such as service, engine, and host, using JMX technology.

Subsequent parts of the article promise to describe how to deploy and monitor a Geronimo cluster.

As open-source tools take on enterprise-class features, such as clustering, they become increasingly viable for deployment of high-availability and highly scalable enterprise applications. However, as the article illustrates, some assembly may be required to create a manageable solution in an actual deployment scenario.

With increasingly choices—free or otherwise—what clustering solution do you prefer for your J2EE apps?