What is Multi-AZ in RDS?
You can select the Multi-AZ option during the RDS DB instance launch.
RDS Service creates standby instances in a different AZ in the same region and configures "Synchronous Replication" between the Primary and Standby.
You can not read/write to the standby RDS DB instances.
You can not select which AZ in the region will be chosen to create the standby DB instance.
You can however view, which AZ is selected after the standby is created.
Depending on the instance class, it may take 1 to a few minutes to failover to the standby instance
AWS recommends the use of provisioned IOPS instances for Multi-AZ RDS instances.
When Multi-AZ RDS Failover Triggers
In case of failure of the Primary DB instance failure.
In case of AZ failure.
Loss of Primary EC2 instance failure.
EBS failure of primary DB instance.
The primary DB instance is changed.
Patching the OS of the DB instance
Manual Failover (in case of Reboot)
Multi-AZ RDS Failover consequences
During failover, the CNAME (Canonical Name Record) of the RDS DB instance is updated to Map to the Standby IP address.
It is recommended to use the endpoint to reference your DB instances and not its IP address.
The CNAME does not change, because the RDS endpoint does not change.
Few More Points
The RDS endpoint does not change by selecting the Multi-AZ option. However, the Primary and standby instances will have different IP addresses, as they are in different AZ.
It is always recommended that you do not use the IP address to point RDS instances, always use endpoint. By using an endpoint, there will be no change whenever a failover happens.
When do we do Manual Failover?
In case of rebooting.
This is by selecting the "Reboot with Failover" reboot options on the primary RDS DB instances.
A DB instance reboot is required for changes to take effect when you change the DB parameter group or when you change a static DB parameter.