A core group is a collection of deployment managers (dmgr), node agents, and application servers in a cell. A default core group, most appropriate named DefaultCoreGroup, is created during the installation of the dmgr. A core group is one of WebSphere high availability features.
In the left panel of the dmgr admin console, select Servers > Core Groups > Core group settings, and the core groups will be displayed. In this example, the only core group is the DefaultCoreGroup.
Selecting a core group and then selecting Core group servers will display the components that are members of the core group. In this example, the core group contains the dmgr, two node agents, and two application servers.
Each cell will contain at least one core group. For example, if you have 3 cells, each cell will have it's own core group.
- Cell01 = DefaultCoreGroup
- Cell02 = DefaultCoreGroup
- Cell03 = DefaultCoreGroup
A node agent and the application servers in the node agent can only be a member of one core group. In other words, a node agent cannot reside in two or more different core groups. When you federate a node into the dmgr, the node agent and the application servers in the node will automatically be added to the DefaultCoreGroup.
Singleton Services / High availability
The members of a core group communicate with each other, and this communication helps to maintain high availability. By default, DCS is used for communication between the core group members.
Core Group Coordinator
One of the members of the core group will be assigned as the core group coordinator. The core group coordinator keeps tabs on each member, to know if they are still available. The core group coordinator also selects the JVM that will run a singleton service. A singleton service is a service that is run by only one JVM in a cluster.
Instead of letting one of the member be magically assigned as the coordinator, at Servers > Core Groups > Core group settings > DefaultCoreGroup > Preferred coordinator servers, you can select the member that will be the coordinator.