Question 635 of 302
Multi-AZ Auto Scaling Group: Surviving an Availability Zone Outage
A web application runs on an Amazon EC2 Auto Scaling group (ASG) behind an Application Load Balancer (ALB). The ALB is configured to use at least two Availability Zones (AZs), but the ASG currently uses subnets in only one AZ. If that AZ becomes unavailable, the application stops serving requests. Which change most directly improves resilience to an AZ outage?
Quick Answer
The answer is to place the Auto Scaling group across multiple Availability Zones by configuring it with subnets in at least two AZs. This directly eliminates the single point of failure because a Multi-AZ Auto Scaling group for AZ resilience ensures that if one AZ goes down, the remaining AZs continue running instances to handle traffic. The Application Load Balancer is already set up for multi-AZ, but the ASG’s single-AZ subnet placement is the bottleneck—this is a classic SAA-C03 trap where candidates focus on the ALB and forget the ASG must also span AZs. The exam tests your understanding that resilience requires both the load balancer and the compute layer to be distributed. A simple memory tip: “ALB spans AZs, so must your ASG—don’t let one subnet sink your app.”
⚠ Common exam trap
Many candidates assume the ALB’s multi-AZ configuration automatically protects the application, overlooking that the ASG must also span multiple AZs to provide compute redundancy.
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
✓
Place the ASG across multiple Availability Zones by configuring it with subnets in at least two AZs.
Distributing an Auto Scaling group across multiple Availability Zones (AZs) ensures that if one AZ fails, the remaining AZs continue to serve traffic. The Application Load Balancer (ALB) is already configured for at least two AZs, but the ASG’s single-AZ subnet placement creates a single point of failure. By adding subnets in at least two AZs to the ASG, the application becomes resilient to an AZ outage without any other architectural changes.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Keep the ASG in one Availability Zone, but reduce ALB health check intervals.
Why it's wrong here
Reducing health check intervals only changes how quickly the ALB marks targets unhealthy. It does not create capacity in other AZs, so the ALB still has no healthy instances to route to when the only ASG AZ fails.
- ✓
Place the ASG across multiple Availability Zones by configuring it with subnets in at least two AZs.
Why this is correct
An ASG launches instances into the AZs of the subnets you specify. By placing the ASG in at least two AZs, the ALB can route traffic to healthy targets in the remaining AZ(s) if one AZ fails, enabling recovery as new instances maintain desired capacity.
- ✗
Switch the load balancer from an ALB to an NLB to remove HTTP health check dependency.
Why it's wrong here
Changing from ALB to NLB does not address the root cause: there is no instance capacity in other AZs. Without multi-AZ compute capacity, any load balancer cannot serve traffic during the AZ outage.
- ✗
Add an Amazon SQS queue to buffer requests during failures.
Why it's wrong here
A queue can help with asynchronous work, but a typical web request/response path still requires immediate compute capacity to accept and handle requests. Queueing does not eliminate the loss of in-AZ instances when the only ASG AZ fails.
Visual reference
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Same concept, more angles
1 more way this is tested on SAA-C03
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company hosts a web application on Amazon EC2 instances in an Auto Scaling group behind an Application Load Balancer (ALB). The ALB and the Auto Scaling group are currently deployed in only one Availability Zone (AZ). The business wants the application to keep running if that AZ has an outage. What is the best change?
easy- A.Increase the desired capacity in the existing Availability Zone to handle all traffic during an outage.
- ✓ B.Deploy the ALB and the Auto Scaling group across at least two Availability Zones so healthy targets remain.
- C.Enable longer ALB health check intervals so failing instances are detected more slowly.
- D.Switch from the ALB to an Internet Gateway so instances can fail over to the public internet.
Why B: Deploying the ALB and Auto Scaling group across at least two Availability Zones (AZs) ensures that if one AZ fails, the ALB can route traffic to healthy EC2 instances in the remaining AZ(s). This is the fundamental AWS best practice for high availability: an ALB is a regional service that requires targets in multiple AZs to survive an AZ outage, and the Auto Scaling group must also span those AZs to maintain capacity. Without multi-AZ deployment, a single AZ failure makes the entire application unavailable regardless of instance health checks.
Last reviewed: Jun 11, 2026
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