I'm using an Application Load Balancer with AWS Elastic Beanstalk
After an AWS automatic server update, I was getting a 503 Service Temporarily Unavailable page. (only this, with near zero server load - single instance)
[PHP 7.3 running on 64bit Amazon Linux/2.9.2 (upgraded from 2.9.1 automatically)]
The Environment itself showed healthy.
I'm using the Application Load Balancer for HTTPS and for future scaling.
It turns out that AWS EB had created a new EC2 instance and terminated the previous instance, and thus there was no instance registered in the Target Group for which the Application Load Balancer was directing to.
How can I configure AWS to have a replacement Elastic Beanstalk EC2 instance be automatically registered in a Target Group for an Application Load Balancer?