A cluster resource group (CRG) is
an i5/OS™ system object
that is a set or grouping of cluster resources that are used to manage events
that occur in a clustered environment. The cluster resource group describes
a recovery domain and supplies the name of the cluster resource group exit
program which gets called when certain cluster events occur.
Clusters provides two choices in defining the relationships
between nodes within a cluster: primary-backup model and peer model. Each
of these models can be used together or separately depending on the needs
of your environment.
Primary-backup model
All cluster
resource groups of this category define nodes in the recovery domain with
specific roles: either primary, backup or replicate. The primary and backup
nodes are available access points for cluster resources. However only one
node will be the active access point at a given point in time. This will be
the primary node. Replicate nodes are not available to be an access point.
This can be changed by assigning the replicate node a role of backup. Primary-backup
model cluster resource groups are defined as data resilient, application resilient,
or device resilient. Data resiliency enables multiple copies of data to be
maintained on more than one node in a cluster and enables the point of access
to be changed to a backup node. Application resiliency enables an application
program to be restarted on either the same node or a different node in the
cluster. Device resiliency enables a device resource to be moved (switched)
to a backup node.
Each data and application cluster resource group has a cluster resource group exit program associated with it. The exit program is optional for resilient device cluster resource groups.
In iSeries™ Navigator, cluster resource groups are referred to differently.
- A device CRG is referred to as a switchable
device.
- An application CRG is referred to as a switchable
application.
- A data CRG is referred to as a switchable data
group.
Peer model
All cluster resource groups
of this category define nodes in the recovery domain with a role of peer or
replicate. The peer nodes are available to be the access point for the cluster
resource group. All nodes defined as peer will be the access point when the
cluster resource group is started. Replicate nodes are not available to be
an access point. This can be changed by assigning the replicate node a role
of peer. Within a peer CRG, each node contains replicated data that exists
on each of the nodes. When a node fails in a peer CRG, the failure point is
communicated to other nodes in the cluster, and those nodes continue the operation
from the point of failure.
A cluster administrative domain is represented by a peer CRG with a recovery domain made up of only peer nodes.