How can I see if RDS Multi-AZ is working?

Hello,

Take a look at this Knowledge center article

It depends on the type of RDS update. If it is DB Engine update, even if your RDS DB instance uses a Multi-AZ deployment, both the primary and standby DB instances are upgraded at the same time. This causes downtime until the upgrade is complete, and the duration of the downtime varies based on the size of your DB instance. Please refer below url for better understanding of RDS downtime during maintanance https://aws.amazon.com/premiumsupport/knowledge-center/rds-required-maintenance/

DB failover/updates/restarts are recorded under the Events page of you RDS console. You can navigate the Events page and check the details. You can even set the notifications for these events using Amazon SNS. Please refer the below url for SNS setup https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_Events.html

Hello

You can see it through Amazon RDS events

To identify the root cause of an unplanned outage in your instance, view all the Amazon RDS events for the last 24 hours. All the events are registered in the UTC/GMT time by default. To store events a longer time, send the Amazon RDS events to Amazon CloudWatch Events. For more information, see Creating a rule that triggers on an Amazon RDS event. When your instance restarts, you see one of the following messages in RDS event notifications:

The RDS instance was modified by customer: This RDS event message indicates that the failover was initiated by an RDS instance modification.

Applying modification to database instance class: This RDS event message indicates that the DB instance class type is changed.

    - Single-AZ deployments become unavailable for a few minutes during this scaling operation.
    - Multi-AZ deployments are unavailable during the time that it takes for the instance to failover. This duration is usually about 60 seconds. This is because the standby database is upgraded before the newly sized database experiences a failover. Then, your database is restarted, and the engine performs recovery to make sure that your database remains in a consistent state.

The user requested a failover of the DB instance: This message indicates that you initiated a manual reboot of the DB instance using the option Reboot or Reboot with failover.

 The primary host of the RDS Multi-AZ instance is unhealthy: This reason indicates a transient underlying hardware issue that led to the loss of communication to the primary instance. This issue might have rendered the instance unhealthy because the RDS monitoring system couldn't communicate with the RDS instance for performing the health checks.

The primary host of the RDS Multi-AZ instance is unreachable due to loss of network connectivity: This reason indicates that the Multi-AZ failover and database instance restart were caused by a transient network issue that affected the primary host of your Multi-AZ deployment. The internal monitoring system detected this issue and initiated a failover.

The RDS Multi-AZ primary instance is busy and unresponsive, the Multi-AZ instance activation started, or the Multi-AZ instance activation completed: The event log shows these messages under the following situations:

    - The primary DB instance is unresponsive.
    - A memory crunch after an excessive memory consumption in the database prevented the RDS monitoring system from contacting the underlying host. Hence the database restarts by our monitoring system as a proactive measure.
    - The DB instance experienced intermittent network issues with the underlying host.
    - The instance experienced a database load. In this case, you might notice spikes in CloudWatch metrics CPUUtilization, DatabaseConnections, IOPS metrics, and Throughput details. You might also notice depletion of Freeablememory.

Database instance patched: This message indicates that the DB instance underwent a minor version upgrade during a maintenance window because the setting Auto minor version upgrade is enabled on the instance.

Reference - https://aws.amazon.com/premiumsupport/knowledge-center/rds-multi-az-failover-restart/