WebSphere Application Server V7.0: Concepts ... - IBM Redbooks

A cell must contain at least one core group, although multiple core groups are
supported. Each core group contains a core group coordinator to manage its
high availability relationships, and a set of high availability policies that are used
to manage the highly available components within that core group.
WebSphere Application Server provides one standard core group: the
DefaultCoreGroup that is created during installation. New server instances are
added to the default core group as they are created.
In most cases one core group is sufficient for establishing a high availability
environment. However, certain topologies require the use of multiple core
groups. A basic rule is that all members of a core group require full IP visibility.
Therefore, you have to create multiple core groups if you spread the application
servers of your cell across different firewall zones.
Note: A large number of application servers in a cell increases the overhead
of core group services and server startup times. You might consider creating
additional core groups when you have more than 50 servers in a cell.
If you are using a DMZ secure proxy server with dynamic routing, the routing
information is exchanged using core groups. In this case you need to create a
tunnel access point group to establish a core group bridge tunnel between the
core groups that are running on both sides of the firewall.
The core group contains a bridge service that supports cluster services that span
multiple core groups. Core groups are connected by access point groups. A core
group access point defines a set of bridge interfaces that resolve IP addresses
and ports. It is through this set of bridge interfaces that the core group bridge
provides access to a core group.
When moving core group members to new core groups, remember the following
information:
► Each server process within a cell can only be a member of one core group.
► If a cluster is defined for the cell, all cluster members must belong to the same
core group.
Network communication between all members of a core group is essential. The
network environment must consist of a fast local area network with full IP visibility
and bidirectional communication between all core group members. IP visibility
means that each member is entirely receptive to the communications of any
other core group member.
Chapter 7. Performance, scalability, and high availability 253