SAA-C03 practice question: EC2 Auto Scaling (1 of 3)
Design Resilient Architectures A free question from our original SAA-C03 practice exam. Try it, then open the answer to see why each option is right or wrong.
An online learning platform runs its web application on two Amazon EC2 instances in a single Availability Zone behind an Application Load Balancer (ALB).
Sometimes the application on one instance stops responding, although the instance keeps passing its EC2 status checks. The ALB stops sending it traffic, but the instance stays in place until an engineer replaces it, and the site runs at half capacity.
The company wants the application to survive the loss of an Availability Zone and to replace failed instances automatically.
Which TWO actions should a solutions architect take? (Choose two.)
Choose two answers.
AA. Create an Auto Scaling group that uses subnets in at least two Availability Zones, and enable the same zones on the ALB.
BB. Configure the Auto Scaling group to use Elastic Load Balancing health checks in addition to the default EC2 status checks.
CC. Create a CloudWatch alarm on each instance's StatusCheckFailed_System metric that runs the EC2 recover action when it fails.
DD. Lower the target group's health check interval and unhealthy threshold so that failed instances are taken out of service sooner.
EE. Launch both instances into a spread placement group so that each one runs on a separate rack with its own network and power.
B. Right: Correct. By default, Auto Scaling looks only at EC2 status checks, which a hung application still passes. With ELB health checks turned on, an instance that the load balancer reports as unhealthy is marked unhealthy and replaced automatically. Learn more: About the health checks for your Auto Scaling group (opens in a new tab)
C. Wrong: The recover action moves an instance to new hardware when the system status check fails because of an AWS infrastructure problem. Here the instance passes its status checks while the application hangs, so the alarm never fires. It also does nothing for an Availability Zone outage. Learn more: Configure CloudWatch action based recovery on an EC2 instance (opens in a new tab)
E. Wrong: A spread placement group reduces correlated hardware failures, but it does not replace instances whose application has hung. Keeping both instances in the same zone still leaves the site exposed to a zone outage. Multi-AZ Auto Scaling covers both needs. Learn more: Placement strategies for your placement groups (opens in a new tab)
Explanation
Two separate gaps need closing. An Auto Scaling group across at least two Availability Zones handles the loss of a zone. Turning on ELB health checks for that group handles the hung application: EC2 status checks alone would keep reporting the instance as healthy, so Auto Scaling would never replace it, while the ALB on its own only routes around failed targets.
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