Configure heartbeats and reach targets to detect server failures and control when failovers can occur.
Besides the basic criteria of failure detection (the loss of connectivity between active and standby Dispatchers, detected through the heartbeat messages), there is another failure detection mechanism named reachability criteria. When you configure the Dispatcher you can provide a list of hosts that each of the Dispatchers should be able to reach in order to work correctly. The two high availability partners continually communicate with each other through heartbeats, and they update one another on how many reach targets either one of them can ping. If the standby pings more reach targets than the active, a failover occurs.
Heartbeats:Heartbeats are sent by the active Dispatcher and are expected to be received by the standby Dispatcher every half second. If the standby Dispatcher fails to receive a heartbeat within 2 seconds, a failover begins. All heartbeats must break for a takeover from the standby Dispatcher to occur. In other words, when two heartbeat pairs are configured, both heartbeats must break. To stabilize a high availability environment and to avoid failover, add more than one heartbeat pair.