SAA-C03 Design Resilient Architectures Practice Question
A web application runs on an Auto Scaling group (ASG) behind an Application Load Balancer (ALB). The ASG is currently attached to subnets in only two Availability Zones (AZs). During a planned maintenance window, one AZ becomes unavailable for about 25 minutes. Monitoring shows that targets in the remaining AZ go healthy, and the ALB/target group health checks report normal. However, users still experience intermittent connection failures and slower responses during the AZ outage. What change will most directly improve resilience against an AZ loss while keeping the same ALB-based design?
⚠ Common exam trap
The trap here is that candidates may focus on connection-level settings (idle timeout) or health check behavior, missing the fundamental architectural need for multi-AZ redundancy to maintain capacity during an AZ outage.
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
✓
Extend the ASG to use subnets in three AZs so there is placement redundancy during an AZ outage, while continuing to keep traffic behind the ALB.
B is correct because deploying the ASG across three Availability Zones (AZs) ensures that when one AZ becomes unavailable, the remaining two AZs can handle the full traffic load without overloading the instances. This placement redundancy directly addresses the intermittent connection failures and slower responses, as the ALB can distribute traffic only to healthy targets in the remaining AZs, maintaining capacity and performance. The current two-AZ setup lacks sufficient buffer capacity, causing the single remaining AZ to become overwhelmed during the outage.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Set the ASG min capacity to 0 so instances can be recreated faster when an AZ recovers.
Why it's wrong here
Lowering min capacity to 0 makes it more likely the ASG can run with too little (or no) capacity in the remaining AZ(s) when one AZ is down. That can increase request failures and performance issues until capacity scales back, and it does not guarantee the ALB can maintain sufficient healthy targets during the outage.
- ✓
Extend the ASG to use subnets in three AZs so there is placement redundancy during an AZ outage, while continuing to keep traffic behind the ALB.
Why this is correct
An AZ outage reduces the number of AZs where the ASG can place instances. With only two AZs, losing one significantly limits capacity and can cause temporary shortages and uneven load distribution, even if existing targets are marked healthy. Expanding the ASG to subnets in three (or more) AZs provides additional placement options so the ASG can maintain the desired number of instances across the remaining AZ(s). The ALB will continue routing only to healthy targets, and the system is more likely to sustain stable response times during the outage.
- ✗
Increase the ALB idle timeout to 120 seconds to reduce connection drops.
Why it's wrong here
Changing the ALB idle timeout affects long-idle TCP/HTTP connections, but it does not create additional healthy targets when one AZ is unavailable. If intermittent failures are driven by reduced capacity/instance count during the outage, idle timeout alone will not reliably fix the problem.
- ✗
Disable health checks on the target group so instances are not deregistered during the maintenance window.
Why it's wrong here
Disabling health checks prevents the ALB from removing unhealthy instances, which can increase error rates by routing to targets that are failing or overloaded. It also does not address the root resilience gap: insufficient AZ coverage in the ASG.
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