Introduction: Clusters

Clusters are groups of servers that are managed together and participate in workload management. A cluster can contain nodes or individual application servers. A node is usually a physical computer system with a distinct host IP address that is running one or more application servers. Clusters can be grouped under the configuration of a cell, which logically associates many servers and clusters with different configurations and applications with one another depending on the discretion of the administrator and what makes sense in their organizational environments.

Clusters are responsible for balancing workload among servers. Servers that are a part of a cluster are called cluster members. When you install an application on a cluster, the application is automatically installed on each cluster member.

Clusters and node groups

Any application you install to a cluster must be able to execute on any application server that is a member of that cluster. Because node groups bound clusters, all of the members of a cluster must be members of the same node group. Therefore, for the application you deploy to run successfully, all of the members of a cluster must be located on nodes that meet the requirements for that application.

In a cell that has many different server configurations, it might be difficult to determine which nodes have the capabilities to host your application. A node group can be used to define groups of nodes that have enough in common to host members of a given cluster. All cluster members in a cluster must be in the same node group.

All nodes are members of at least one node group. When you create a cluster, the first application server you add to the cluster defines the node group that bounds the cluster. All other cluster members you add to the cluster can only be on nodes that are members of this same node group. When you create a new cluster member in the administrative console, you are allowed to create the application server on a node that is a member of the node group for that cluster only.

Nodes can be members of multiple node groups. If the first cluster member you add to a cluster has multiple node groups defined, the system automatically chooses the node group that bounds the cluster. You can change the node group by modifying the cluster settings. Use the Server cluster settings page to change the node group.

Clusters and core groups

In a high availability environment, a group of clusters can be defined as a core group. All of the application servers defined as a member of one of the clusters included in a core group are automatically members of that core group. Individual application servers that are not members of a cluster can also be defined as a member of a core group. The use of core groups enables WebSphere Application Server to provide high availability for applications that must always be available to end users. You can also configure core groups to communicate with each other using the core group bridge. The core groups can communicate within the same cell or across cells.




Related concepts
Clusters and workload management
Core group communications using the core group bridge service
Introduction: Application servers
Related tasks
Balancing workloads with clusters
Setting up a high availability environment
Creating clusters
Managing node groups
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Last updated: Sep 20, 2010 10:03:57 PM CDT
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