What happens after failover in RDS

Yes, upon RDS Multi-AZ failover the failed primary instance is brought back up as the new standby instance to reinstate the high-availability of your database.

To elucidate the above mentioned:

The failover process is typically completed within 60-120 seconds where the standby instance is promoted as the new primary instance allowing you to resume your DB activities in the shortest amount of time.

While in the background, the failed primary instance is diagnosed by the RDS internal health monitoring system and remediation actions are taken based on the detected fault. The remediation action may involve simply rebooting the faulty instance to even replacement of the hardware depending on the detected fault. Once the old primary node is recovered, it is brough back up as the new Standby instance ensuring your DBs high-availability.

The recovery time of the failed node may vary upon the type of fault and the recovery process applied. Also, recovery time largely depends on the DB workload at the time of crash as RDS would perform data recovery and roll-back any uncommitted transactions eliminating any data inconsistencies across the nodes while providing you a single-box experience.

References:

Amazon RDS Under the hood – Multi-AZ: https://aws.amazon.com/blogs/database/amazon-rds-under-the-hood-multi-az/

Failover process in RDS: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Concepts.MultiAZSingleStandby.html#Concepts.MultiAZ.Failover