Detection

A standby server detects a failure of the active server in either of the following ways: Heartbeat Failure or Connection Failure.

Heartbeat Failure—The active server sends heartbeat messages to the standby server to indicate that it is still operating. When a network failure stops the servers from communicating with each other, the standby server detects the interruption in the steady stream of heartbeats. For details, see Heartbeat Parameters.
Connection Failure—The standby server can detect the failure of its TCP connection with the active server. When the active server process terminates unexpectedly, the standby server detects the broken connection.