Adding members to a cluster

You can use clusters to balance workload in an environment containing multiple application servers.

About this task

If you are migrating from a previous version of the product, you can upgrade a portion of the nodes in a cell, while leaving others at the previous release level. For a time, you might be managing servers that are at a previous release level, and servers that are running at the current release level in the same cell.

When you create a cluster, you specify the node on which the first cluster member resides. In a mixed cell environment, you can use any server from within that node group to create a new cluster member. For example, if the node group to which the cluster belongs consists of a Version 7.0 node, and a Version 6.1 node, you can use a server from either the Version 6.1 or the Version 7.0 node to create a new cluster member.

Avoid trouble: When you add a member to a cluster, the virtual host for the new cluster member is automatically configured if the deployment manager profile and the managed profile (the cluster member) reside on the same LPAR. Therefore, in this situation, the host alias ports are automatically added to the virtual host for the new cluster member. However, if the deployment manager profile and the managed profile reside on different LPARs, you must manually configure the virtual host for the new cluster member.

Use the following procedure to create a new cluster member, view information about existing cluster members, or manage existing cluster members.

1. In the administrative console, click Servers > Clusters > WebSphere application server clusters > cluster_name > Cluster members.
   The Cluster members page lists members of a cluster, and for each member indicates:

   • The node on which the member resides.
   • The version of the application server. This information specifies whether the cluster is a mixed cluster.
   • The configured weight for the member.
   • The runtime weight for the member. This weight indicates the proportionate workload that is currently directed to this cluster member.
   • Whether the member is started, stopped, or encountering problems.
2. Click New to create a new cluster member.

   Clicking New starts the Create a new cluster member wizard. Use this wizard to add new members to an already configured cluster.

   A copy of the first cluster member that you create is stored as part of the cluster data and becomes the template for all additional cluster members that you create. Usually, only one template is available for you to use to create additional cluster members for a cluster. However, if a cluster includes nodes that are at different versions of the product, there is a different template for each version. For example, if a cluster has cluster members that reside on both a Version 6.1 node and a Version 7.0 node, the cluster has two templates. The Version 6.1 template is used when you create an additional cluster member on the Version 6.1 node, and the Version 7.0 template is used when you create an additional cluster member on the Version 7.0 node.

   To view the cluster member templates that are available for creating a new member of a cluster, in the administrative console, click Servers > Clusters > WebSphere application server clusters > cluster_name > Cluster members > Templates.

   1. Specify a name for the application server that you are defining as a cluster member.
      The name must be unique within the node.
   2. Select the node for the cluster member.
   3. Specify a short name for this cluster member.
      The short name is the default job name and identifies the cluster member to the native facilities of the operating system, such as Workload Manager (WLM), Automatic Restart Manager, RACF®, and started task control.
   4. Specify the server weight.

      The weight value you specify controls the number of requests that are directed to the application server. Even though you specify a value of 0 to 20 as the weight of a server, the weight that is given to the server as a member of a cluster is a proportion that is based on the weight assigned to the server, and the sum of the weights of all members of the cluster. In this proportion, the weight that is assigned to the server is the numerator, and the sum of the weights of all members of the cluster is the denominator.

      When you add a new member to a cluster, the number of client, or application requests that are sent to each server in the cluster decreases, assuming the number of requests coming into the cluster stays the same. Similarly when you remove a new member from a cluster, the number of client or application requests that are sent to each server in the cluster increases, assuming the number of requests coming into the cluster stays the same.

      For example, if you have a cluster that consists of members A, B, and C with weights 2, 3, and 4, respectively, then 2/9 of the requests are assigned to member A, 3/9 are assigned to member B, and 4/9 are assigned to member C. If a new member, member D, is added to the cluster and member D has a weight of 5, then member A now gets 2/14 of the requests, member B gets 3/14 of the requests, member C gets 4/14 of the requests, and member D gets 5/14 of the requests.

      On z/OS®, weight is used to balance some of the workload types, but others are balanced by the z/OS system.

      • For HTTP requests, weights are used to distribute HTTP traffic between the web server plug-in and the controller handling the clustered application server. Assign a higher weight value to the application server that should receive the HTTP traffic.
      • For web services calls, information is transferred from a servant in one application server to a controller in another application server. The application server that receives the call has the highest weight value.
      • Weight has no affect on Internet Inter-ORB Protocol (IIOP) requests. IIOP requests are distributed to the correct application server using the sysplex distributor.

      Avoid trouble: For workload balancing with service integration, or with message driven beans in the application server, you require additional configuration. See the topic Configuring high availability and workload sharing of service integration for more information.
   5. Specify whether to generate unique HTTP ports.
   6. Click Add member to finish defining the cluster member.
      The first cluster member for this cluster is used as the template for this cluster member. You can repeat these steps to define other cluster members.
   7. When you finish defining additional cluster members, review the summary information for the new cluster members.
      If you have to change any of the property settings for any of the new members, select that cluster member, and then click Edit. When you finish changing the property settings, click Update member to save your changes.
   8. When you finish defining new cluster members, click Next to view the summary page for the cluster, and then click Finish to create these new cluster members.
3. Click Review, select Synchronize changes with nodes, and then click Save to save your changes.