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Cluster WebSphere Servers
WebSphere Application Server's built-in clustering allows enterprises to run efficiently by preventing degradation and downtime
by Kulvir Singh Bhogal and Javid Jamae

Posted April 14, 2004

Two major problems that can occur when running applications on an application server are performance degradation (because of high load) and downtime (because of hardware or software failure). Both problems can be remedied by using the clustering features provided by WebSphere Application Server (WAS) 5.X. Clustering provides scalability through load balancing and high availability through failover, which combat high-load performance degradation and downtime, respectively. (For more detailed information on these terms, see the sidebar, "Clustering Terminology.")

A WAS cluster is a group of application servers that appear to a client as a single application server. From a client perspective, the existence of multiple servers is transparent—the client need not know that it is dealing with a cluster rather than a single application server.

To perform clustering, you need to obtain both WAS and WebSphere Network Deployment (WND). These two products are installed separately and have no out-of-the-box dependencies on one another. If so desired, each could be installed on a separate computer.

In a nonclustered development environment, or for simple deployments, most people will install just WAS. Developers can also use the WebSphere Test Environment built into IBM WebSphere Application Developer (IBM's tooling product based on the Eclipse framework) to build and test applications. WAS includes an administrative console, which is a Web application that allows you to manage the application server. A single instance of WAS is sometimes referred to as a WAS instance, a base server, a managed application server, or a managed server. For consistency, we will use the term WAS instance. A WAS instance runs inside what is known as a node, a logical group of WAS instances, on the same physical computer. You can install multiple nodes containing multiple WAS instances on the same physical computer.

WND allows you to manage multiple WAS instances through an administrative Web application called the deployment manager (sometimes called the cell manager). The deployment manager runs within a node inside a WAS instance, so when you install WND you are in fact installing an entire WAS instance as well (see Figure 1). From within a node you can issue a command to the deployment manager that will incorporate the node's WAS instances in a cell, or a logical group of nodes. WebSphere refers to the process of adding nodes to a cell as federation. After a node has been federated, its administrative console is literally uninstalled and replaced with what is known as a node agent (see Figure 2). The node agent is an administrative process that allows the deployment manager to administer the node. Only one deployment manager can run per cell. In production, we recommend installing WND on a machine that does not run any of your other production WAS instances.