1.3. Cluster Architecture
At a high level, a cluster can be viewed as having these parts (which together are often referred to as the cluster stack):
Resources: These are the reason for the cluster’s being — the services that need to be kept highly available.
Resource agents: These are scripts or operating system components that start, stop, and monitor resources, given a set of resource parameters. These provide a uniform interface between Pacemaker and the managed services.
Fence agents: These are scripts that execute node fencing actions, given a target and fence device parameters.
Cluster membership layer: This component provides reliable messaging, membership, and quorum information about the cluster. Currently, Pacemaker supports
Corosync as this layer.
Cluster resource manager: Pacemaker provides the brain that processes and reacts to events that occur in the cluster. These events may include nodes joining or leaving the cluster; resource events caused by failures, maintenance, or scheduled activities; and other administrative actions. To achieve the desired availability, Pacemaker may start and stop resources and fence nodes.
Cluster tools: These provide an interface for users to interact with the cluster. Various command-line and graphical (GUI) interfaces are available.
Most managed services are not, themselves, cluster-aware. However, many popular open-source cluster filesystems make use of a common Distributed Lock Manager (DLM), which makes direct use of Corosync for its messaging and membership capabilities and Pacemaker for the ability to fence nodes.
