Pacemaker keeps your applications running when they or the machines they’re running on fail. However it can’t do this without connectivity to the other machines in the cluster – a significant problem in its own right.

Rather than re-implement the wheel, Pacemaker supports existing implimentations such as Heartbeat. Heartbeat provides:

• a mechansigm to reliably send messages between nodes,
• notifications when machines appear and disappear
• a list of machines that are up that is consistent throughout the cluster

Heartbeat was also the first stack supported by the Pacemaker codebase.

via FAQ – ClusterLabs.