AWS DocumentDB automatic failover when cpu utilization reaches 100%

Hello.

Though we are wondering why doesn't it initiate automatic failover in this case or we need to some additional configuration to enable this.

If you look at the document below, it states that there will be a failover if a database failure occurs.
In other words, even if the CPU usage rate is high, unless AWS judges it as a failure, failover will not occur.
https://docs.aws.amazon.com/documentdb/latest/developerguide/failover.html#:~:text=When%20the%20primary%20instance%20fails,has%20its%20own%20endpoint%20address.

When the primary instance fails, Amazon DocumentDB automatically fails over to an Amazon DocumentDB replica, if one exists.

If it doesn't support automatic failover in this scenario, I could see a solution like triggering a lambda to force failover when CPU utilization alarm breaches threshold. If someone has a more simpler solution, it would be helpful.

Even if you perform a manual failover, unless you identify the cause, the CPU usage will increase in the same way after the failover.
If you're okay with that, I think it's possible to automate failover by using CloudWatch alarms, Amazon SNS, and Lambda.