SAA-C03 Design Resilient Architectures Practice Question

Exhibit

Auto Scaling group configuration:
- Desired capacity: 4
- VPC subnets: subnet-0a11 (us-east-1a) only
- Health check type: ELB

Application Load Balancer configuration:
- Enabled subnets: subnet-0a11 (us-east-1a), subnet-0b22 (us-east-1b)

Incident note:
- A planned test stopped all instances in us-east-1a and the application became unavailable.

Based on the exhibit, a web application must stay available if one Availability Zone fails. What is the best change to improve resilience?

⚠ Common exam trap

The trap here is that candidates often focus on scaling up (more instances or larger instances) or changing the load balancer type, missing the fundamental requirement of distributing resources across multiple Availability Zones to achieve AZ-level resilience.

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

Add a subnet in another Availability Zone to the Auto Scaling group and keep the ALB spanning both AZs.

Adding a subnet in another Availability Zone (AZ) to the Auto Scaling group and keeping the ALB spanning both AZs ensures that if one AZ fails, the ALB can route traffic to healthy instances in the other AZ. This is the standard pattern for building multi-AZ resilient architectures with Auto Scaling and ALB, as it eliminates the single point of failure at the AZ level.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
Increase the desired capacity to 8 instances in the same subnet.
Why it's wrong here
More instances in one Availability Zone do not protect against the Zone itself failing. Capacity increases, but resilience does not.
✓
Add a subnet in another Availability Zone to the Auto Scaling group and keep the ALB spanning both AZs.
Why this is correct
This places application instances across multiple Availability Zones, which protects the stateless tier from a single-AZ failure. The ALB already spans two AZs, so the missing piece is the Auto Scaling group using subnets in more than one AZ. That allows AWS to replace unhealthy instances and continue serving traffic from the surviving Zone.
✗
Replace the Application Load Balancer with a Network Load Balancer.
Why it's wrong here
A different load balancer type does not fix the single-AZ compute placement problem. The application still has no surviving instances in another Zone.
✗
Move the instances to a larger instance type with more CPU and memory.
Why it's wrong here
Bigger instances can improve performance, but they do not add Availability Zone redundancy. A larger instance in one AZ is still a single point of failure.

Visual reference

Go deeper

SAA-C03 Design Resilient Architectures Practice Question

Exhibit

Why each option matters

Option-by-option breakdown

Related to this question