A key concept in understanding how a Pacemaker cluster functions is a transition. A transition is a set of actions that need to be taken to bring the cluster from its current state to the desired state (as expressed by the configuration).

Whenever a relevant event happens (a node joining or leaving the cluster, a resource failing, etc.), the controller will ask the scheduler to recalculate the status of the cluster, which generates a new transition. The controller then performs the actions in the transition in the proper order.

Each transition can be identified in the logs by a line like:

notice: Calculated transition 19, saving inputs in /var/lib/pacemaker/pengine/pe-input-1463.bz2

The file listed as the "inputs" is a snapshot of the cluster configuration and state at that moment (the CIB). This file can help determine why particular actions were scheduled. The crm_simulate command, described in Section 5.5, “Simulate Cluster Activity with crm_simulate”, can be used to replay the file.