- A
Keep the ASG in one subnet/AZ, but enable ALB stickiness to reduce session interruption.
Why wrong: Stickiness affects session routing but does not create targets in a second AZ during an AZ outage.
- B
Update the ASG to launch instances across subnets in at least two Availability Zones and ensure ALB health checks target an application-ready path.
Spreading instances across multiple AZs ensures the ALB can route to healthy targets even when one AZ fails.
- C
Add a NAT gateway in the public subnets so instances can reach the internet during maintenance events.
Why wrong: NAT affects outbound connectivity, not target placement or health during an AZ outage.
- D
Create a second ALB in the same Availability Zone and route traffic using DNS failover.
Why wrong: DNS failover adds complexity and still depends on healthy targets; without multi-AZ instances, failover won’t help much.
SAA-C03 Design Resilient Architectures Practice Question
This SAA-C03 practice question tests your understanding of design resilient architectures. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A stateless web API runs on EC2 instances behind an Application Load Balancer (ALB). The Auto Scaling group (ASG) currently uses subnets from only one Availability Zone, even though the ALB spans two Availability Zones. During maintenance of that single AZ, the ALB remains up but clients see timeouts because there are no healthy targets. Which change most directly improves resilience against an AZ failure?
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
Update the ASG to launch instances across subnets in at least two Availability Zones and ensure ALB health checks target an application-ready path.
The most direct fix for AZ failure resilience is to distribute the ASG across multiple Availability Zones. With the ALB already spanning two AZs, if the ASG only launches instances in one AZ, a failure of that AZ leaves the ALB with zero healthy targets, causing timeouts. By configuring the ASG to launch instances in at least two AZs and setting ALB health checks to an application-ready path, the ALB can route traffic to healthy instances in the surviving AZ, maintaining availability.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 subnet/AZ, but enable ALB stickiness to reduce session interruption.
Why it's wrong here
Stickiness affects session routing but does not create targets in a second AZ during an AZ outage.
- ✓
Update the ASG to launch instances across subnets in at least two Availability Zones and ensure ALB health checks target an application-ready path.
Why this is correct
Spreading instances across multiple AZs ensures the ALB can route to healthy targets even when one AZ fails.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Add a NAT gateway in the public subnets so instances can reach the internet during maintenance events.
Why it's wrong here
NAT affects outbound connectivity, not target placement or health during an AZ outage.
- ✗
Create a second ALB in the same Availability Zone and route traffic using DNS failover.
Why it's wrong here
DNS failover adds complexity and still depends on healthy targets; without multi-AZ instances, failover won’t help much.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may think adding a second ALB or enabling stickiness solves the problem, when the real issue is that the ASG is not distributing instances across multiple Availability Zones, leaving the ALB with no healthy targets during an AZ outage.
Detailed technical explanation
How to think about this question
Under the hood, an ALB routes traffic only to healthy targets registered in its target group, which are EC2 instances in the ASG. If the ASG is confined to one AZ, the ALB's cross-zone load balancing feature cannot help because there are no targets in the other AZ. Health checks at the application path (e.g., /health) ensure the ALB only sends traffic to instances that are fully ready, preventing timeouts from misconfigured or overloaded targets. In practice, distributing instances across AZs also protects against data center-level failures and is a core tenet of high-availability design in AWS.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
What to study next
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FAQ
Questions learners often ask
What does this SAA-C03 question test?
Design Resilient Architectures — This question tests Design Resilient Architectures — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Update the ASG to launch instances across subnets in at least two Availability Zones and ensure ALB health checks target an application-ready path. — The most direct fix for AZ failure resilience is to distribute the ASG across multiple Availability Zones. With the ALB already spanning two AZs, if the ASG only launches instances in one AZ, a failure of that AZ leaves the ALB with zero healthy targets, causing timeouts. By configuring the ASG to launch instances in at least two AZs and setting ALB health checks to an application-ready path, the ALB can route traffic to healthy instances in the surviving AZ, maintaining availability.
What should I do if I get this SAA-C03 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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