How to cluster WebSphere application servers

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There are two types of clusters:

  • Horizontal
  • Vertical

Usually, a horizontal cluster is preferred. In a horizontal cluster, JVMs that reside on different physical machine are clustered. The obvious advantage to a horizontal cluster is that if one physical server goes down, the other physical server can continue to produce the applications in the JVM, resulting in high-availability. Also, from a systems engineer perspective, a horizontal cluster lets the engineer do work on one cluster member without bringing down the applications being produced by the JVMs in the cluster. For example, the engineer could bring down the server01 JVM, patch the server, restart the JVM, and bring it back up, without causing an outage, as the server02 JVM can still produce the application.

In a vertical cluster, JVMs that reside on the same physical machine are clustered. Similar to a horizontal cluster, a vertical cluster lets the engineer do work on one cluster member without bringing down every JVM. However, if the physical server goes down, the entire cluster also goes down.

Create new cluster with new JVMs.

  1. In the left panel of the WebSphere web console, expand Servers > Clusters, and select WebSphere application server clusters.
  2. Select New.
  3. At Step 1, enter the name of the cluster (such as cluster1) and select Next.
  4. At Step 2, give the member a name (such as server03), select the first node, and select Next.

Note: You cannot add a JVM that already exists in the node. The creation of a member will create a new JVM.

Note: If a node is not listed, add the node.

  1. At Step 3, enter the name for the second member (such as server04), select the second node, select Add Member, and select Next. Almost always, you will want to keep Generate unique HTTP ports selected, to ensure that each member has a unique port number, to avoid a port conflict situation. Refer to virtual hosts for more information on ports.
  2. Select Finish
  3. Select Save.

Synchronize the node. You can then select the cluster and select start to start both JVMs (server03 and server04) in the cluster.


At Servers > Server types > WebSphere application servers, the servers in the cluster will be listed. In this example, server01 is not part of a cluster, and server03 and server04 are members of cluster1. This view also can be used to know if the JVMs are in a vertical or horizontal cluster. In this example, server03 and server04 are in a horizontal cluster, as the servers are on different servers (was1 / was2).


Selecting the cluster and then expanding Cluster members allows you to see the members that are in the cluster.


Since web service applications do not have a UI and are not fronted by a web server, there isn’t a need for session and context replication between web services JVMs.  These apps should be stateless, meaning an invocation of the app shouldn’t be dependent on data created/updated in session or context from a previous call. It may be reasonable to not cluster web services JVMs.

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