Unity's operation is controlled by the Sequencer process, which works in an Active-Standby pair.
The default Failover mode is automatic. The Failover mode is controlled by the HAFailoverMode configuration value. Changing the HAFailoverMode value requires a restart all Unity services.If the Sequencer process fails unexpectedly or is stopped, the operating system automatically closes the socket connection. The Watchdog processes on both Unity servers receive notification of the event. If the Failover mode is set to automatic, either Watchdog process can initiate a Failover. If configured for Manual Failover, you must change the Sequencer process state on a Failover event.