What is heartbeat protocol?

Answer 1

A heartbeat is a message sent between machines at a regular interval of the order of seconds. If a heartbeat isn't received for a time -- usually a few heartbeat intervals -- the machine that should have sent the heartbeat is assumed to have failed. A heartbeat protocol is generally used to negotiate and monitor the availability of a resource, such as a floating IP address. Typically when a heartbeat starts on a machine it will perform an election process with other machines on the heartbeat network to determine which, if any machine owns the resource. On heartbeat networks of more than two machines it is important to take into account partitioning, where two halves of the network could be functioning but not able to communicate with each other. In a situation such as this it is important that the resource is only owned by one machine, not one machine in each partition.

As a heartbeat is intended to be used to indicate the health of a machine it is important that the heartbeat protocol and the transport that it runs on is as reliable as possible. Effecting a fail-over because of a false alarm may, depending on the resource, be highly undesirable. It is also important to react quickly to an actual failure, so again it is important that the heartbeat is reliable. For this reason it is often desirable to have heartbeat running over more than one transport, for instance an ethernet segment using UDP/IP, and a serial link.