Novell LINUX ENTERPRISE 11 - HIGH AVAILABILITY, Manual, Page: 22 / 298

Messaging and Infrastructure Layer
The primary or first layer is the messaging/infrastructure layer, also known as the
OpenAIS layer. This layer contains components that send out the messages containing
“I'm alive” signals, as well as other information. The program of the High Availability
Extension resides in the messaging/infrastructure layer.
Resource Allocation Layer
The next layer is the resource allocation layer. This layer is the most complex, and
consists of the following components:
Cluster Resource Manager (CRM)
Every action taken in the resource allocation layer passes through the Cluster Re-
source Manager. If other components of the resource allocation layer (or components
which are in a higher layer) need to communicate, they do so through the local
CRM.
On every node, the CRM maintains the Cluster Information Base (CIB) (page 12),
containing definitions of all cluster options, nodes, resources their relationship and
current status. One CRM in the cluster is elected as the Designated Coordinator
(DC), meaning that it has the master CIB. All other CIBs in the cluster are a replicas
of the master CIB. Normal read and write operations on the CIB are serialized
through the master CIB. The DC is the only entity in the cluster that can decide
that a cluster-wide change needs to be performed, such as fencing a node or moving
resources around.
Cluster Information Base (CIB)
The Cluster Information Base is an in-memory XML representation of the entire
cluster configuration and current status. It contains definitions of all cluster options,
nodes, resources, constraints and the relationship to each other. The CIB also syn-
chronizes updates to all cluster nodes. There is one master CIB in the cluster,
maintained by the DC. All other nodes contain a CIB replica.
Policy Engine (PE)
Whenever the Designated Coordinator needs to make a cluster-wide change (react
to a new CIB), the Policy Engine calculates the next state of the cluster based on
the current state and configuration. The PE also produces a transition graph con-
12 High Availability Guide