A core group is a high availability domain within a cell. It serves as a physical grouping of JVMs in a cell that are candidates to host singleton services. It can contain stand-alone servers, cluster members, Node Agents, or the Deployment Manager.
A cell must have at least one core group. The WebSphere Application Server creates a default core group, called DefaultCoreGroup, for each cell. Each JVM process can only be a member of one core group. Naturally, cluster members must belong to the same core group. At runtime, the core group and policy configurations are matched together to form high availability groups
A set of JVMs can work together as a group to host a highly available service. All JVMs with the potential to host the service join the group when they start. If the scope of the singleton (such as a Transaction Manager or a messaging engine) is a WebSphere cluster then all members of the cluster are part of such a group of JVMs that can host the service.
In a large-scale implementation with clusters spanning multiple geographies, you
can create multiple core groups in the cell and link them together with the core
group bridge to form flexible topologies. The most important thing is that every
JVM in a core group must be able to open a connection to all other members of
the core group.
A core group cannot extend beyond a cell, or overlap with other core groups. Core groups in the same cell or from different cells, however, can share workload management routing information using the core group bridge service