Clusters and workload management
Clusters are sets of servers that are managed together and participate in workload management. The servers that are members of a cluster can be on different host machines, as opposed to the servers that are part of the same node and must be located on the same host machine. A cell can have no clusters, one cluster, or multiple clusters.
Servers that belong to a cluster are members of that cluster set and must all have identical application components deployed on them. Other than the applications configured to run on them, cluster members do not have to share any other configuration data. One cluster member might be running on a huge multi-processor enterprise server system, while another member of that same cluster might be running on a smaller system. The server configuration settings for each of these two cluster members are very different, except in the area of application components assigned to them. In that area of configuration, they are identical. This allows client work to be distributed across all the members of a cluster instead of all workload being handled by a single application server.
When you create a cluster, you make copies of an existing application server template. The template is most likely an application server that you have previously configured. You are offered the option of making that server a member of the cluster. However, it is recommended that you keep the server available only as a template, because the only way to remove a cluster member is to delete the application server. When you delete a cluster, you also delete any application servers that were members of that cluster. There is no way to preserve any member of a cluster. Keeping the original template intact allows you to reuse the template to rebuild the configuration.
A vertical cluster has cluster members on the same node, or physical machine. A horizontal cluster has cluster members on multiple nodes across many machines in a cell. One can configure either type of cluster, or have a combination of vertical and horizontal clusters.
See Balancing workloads with clusters to specify the amount of work targeted to each cluster member. We can distribute client tasks according to the capacities of different machines in the enterprise. A Web server plug-in routes application access among cluster members by server weighting, to provide better distribution control.
WebSphere Application Server can respond to increased use of an enterprise application by automatically replicating the application to additional cluster members as needed. This lets you deploy an application on a cluster instead of on a single node, without considering workload.
Multiple servers that can service the same client request is the basis for failover support. If a server fails while processing a client request, the failed request can be rerouted to any of the remaining cluster members. In fact, several servers could fail, and as long as at least one cluster member is running, client requests can continue to be serviced. For further information backing up failed processes, see Replicating data across application servers in a cluster.
See Backup clusters for information on backup cluster support. A backup cluster continues functioning when all cluster members of the primary cluster are not available.
See Also
Introduction: Clusters
Related Tasks
Creating clusters
Replicating data across application servers in a cluster
Balancing workloads with clusters