Every secondary node has a heartbeat connection, a dedicated connection to the coordinator node. This connection is established at the time of secondary node startup and remains active as long as the secondary node is active.
Both the coordinator and secondary node monitor the heartbeat connection. If this connection is broken, the node is declared offline. If the heartbeat is not reestablished within a given time period, the coordinator can automatically exclude the secondary server depending on setting of the option MPX_AUTOEXCLUDE_TIMEOUT.