A real-world lesson from a production-like AWS lab challenge
The Scenario That Should Scare You
Imagine this: your AWS environment has two Availability Zones, public and private subnets, an Application Load Balancer, Auto Scaling. Your architecture diagram looks solid. Then one Availability Zone goes down, your ALB fails over instantly, your EC2 instances in AZ-B are running fine. But your application is still broken.
Because every private subnet instance, including those in AZ-B, is routing outbound traffic through one NAT Gateway sitting in AZ-A. Which is now unreachable.
You didn't have a highly available architecture. You had the illusion of one.
