|
|
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. Each of these run in a separate JVM. See Figure 6-2.
Figure 6-2 Conceptual diagram of a core group
A cell must have at least one core group. The WAS 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. For more information about policies and high availability groups, see 6.2.4, Core group policy and 6.3, High availability group.
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.
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. See Figure 6-3.
Figure 6-3 Conceptual diagram of multiple core groups
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.
Tip: Using core group bridges is a good way to handle intra-cell firewalls.
The core group bridge service can be used when configuring a backup cluster for EJB container failover. We have provided a step-by-step configuration example for the core group bridge service in 2.7, Backup cluster support.
