Each second, the primary instance writes to a system database as a heartbeat signal. If multiple heartbeats aren't detected, failover is initiated. This occurs if the primary instance is unresponsive for approximately 60 seconds or the zone containing the primary instance experiences an outage.
The standby instance now serves data upon reconnection.
Through a shared static IP address with the primary instance, the standby instance now serves data from the secondary zone.
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There is no difference in working with non-HA and HA instances, so your application does not need to be configured in any particular way. When failover occurs, any existing connections to the primary instance and read replicas are closed, and it will take approximately 2-3 minutes for connections to be reestablished. Your application reconnects using the same connection string or IP address, so you do not need to update your application after failover.
Why is this important? https://cloud.google.com/sql/docs/postgres/high-availability#failover-overview
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