1.4. Pacemaker Architecture

At the highest level, the cluster is made up of three pieces:

Core cluster infrastructure providing messaging and membership functionality (illustrated in red)
Non-cluster aware components (illustrated in green).
In a Pacemaker cluster, these pieces include not only the scripts that knows how to start, stop and monitor resources, but also a local daemon that masks the differences between the different standards these scripts implement.
A brain (illustrated in blue)
This component processes and reacts to events from the cluster (nodes leaving or joining) and resources (eg. monitor failures) as well as configuration changes from the administrator. In response to all of these events, Pacemaker will compute the ideal state of the cluster and plot a path to achieve it. This may include moving resources, stopping nodes and even forcing nodes offline with remote power switches.

1.4.1. Conceptual Stack Overview

Figure 1.4. Conceptual overview of the cluster stack

When combined with Corosync, Pacemaker also supports popular open source cluster filesystems. footnote:[ Even though Pacemaker also supports Heartbeat, the filesystems need to use the stack for messaging and membership and Corosync seems to be what they’re standardizing on.

Technically it would be possible for them to support Heartbeat as well, however there seems little interest in this. ]

Due to recent standardization within the cluster filesystem community, they make use of a common distributed lock manager which makes use of Corosync for its messaging capabilities and Pacemaker for its membership (which nodes are up/down) and fencing services.

Figure 1.5. The Pacemaker stack when running on Corosync