Heartbeat enables the highly available operation of services. In case of an error, a backup system takes over the task performed by the malfunctioning computer and the service remains available. This is made possible with the help of a heartbeat signal, which the computer systems exchange via a network connection. If the notification is not sent by a computer, the functioning computer assumes that its counterpart is no longer available (e.g. due to a defect) and subsequently takes over its task. The program is part of the Linux-HA (High-Availability Linux) project. Heartbeat is free software and runs on all popular unixoid operating systems.
You can find more information on the project page.
keepalived is software for managing computer clusters. keepalived can be used to monitor the proper functioning of a cluster based on a Linux virtual server for the purpose of failover.
Pacemaker is an Open Source resource manager for cluster systems operated on Linux.
Pacemaker is offered as a standard module for many popular Linux distributions, otherwise it is available as a free extension.
Pacemaker makes a considerable contribution to achieving high availability with the management and monitoring of server services started via script, even on large systems. Pacemaker is generally used in conjunction with Corosync or Heartbeat. Corosync organizes the communication between the cluster nodes.