You need to perform a blue/green deployment for a three‑tier app on EC2 with RDS. Blue and green environments each have their own launch template, Auto Scaling group, and target group; the ALB can route to either target group and Route 53 points at the ALB. The deployment requirement is to switch all traffic at once from blue to green once the green instances have the new software. What should you do?

The correct approach for an all-at-once blue/green deployment involves preparing the new "green" environment completely before switching traffic. A rolling restart of the green Auto Scaling group ensures all green instances receive the new software without downtime for the currently active "blue" environment. Once the green environment is verified, updating the Application Load Balancer (ALB) to route all traffic to the green target group immediately switches all users to the new version. Incorrect options: Switching ALB traffic to green before deploying software to green instances would direct users to an outdated or non-functional environment. Updating the blue launch template and restarting blue instances is a traditional in-place deployment, not a blue/green strategy, and would cause downtime or performance degradation for users. Changing Route 53 DNS to point to a new ALB endpoint is unnecessary and more complex when the existing ALB can simply re-route to a different target group.