9.2 Core group
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 JVM.
Figure 9-2 Conceptual diagram of a core group
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 9.2.4, Core group policy and 9.3, 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 singleton is scoped to a WebSphere cluster, such as a Transaction Manager or a messaging engine, then all members of the cluster are part of such a group of JVMs that can host the service. WLM information for clusters is available to all members of the core group in a peer-to-peer fashion.
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 be federated to share WLM information using the core group bridge service. In a large-scale implementation with clusters spanning multiple geographies, use a variety of transport types for different core groups and link them together with the core group bridge service to form flexible topologies. The most important thing is that all JVMs in a core group must be able to open a connection and send heartbeat messages to each other.
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Tip: If your node is installed on a server with IBM HACMP or Tivoli System Automation, you may wish to use the persistent node IP label as the host address for your node because this IP alias is available at boot time whether a HACMP cluster is online or not, and it is also not prone to physical network adapter failures. |