- A
Set the ASG min capacity to 0 so instances can be recreated faster when an AZ recovers.
Why wrong: 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.
- B
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.
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.
- C
Increase the ALB idle timeout to 120 seconds to reduce connection drops.
Why wrong: 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.
- D
Disable health checks on the target group so instances are not deregistered during the maintenance window.
Why wrong: 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.
Quick Answer
The answer is to extend the Auto Scaling group to use subnets in three Availability Zones. This directly addresses the intermittent failures and slower responses because with only two AZs, losing one forces the remaining single AZ to handle 100% of the traffic, quickly overwhelming its instances and degrading performance. By adding a third AZ, you create placement redundancy: during an AZ outage, the ALB distributes traffic across two healthy AZs, each carrying only 50% of the load, preserving capacity and response times. On the SAA-C03 exam, this scenario tests your understanding of how ASG and ALB designs must account for burst capacity and not just health checks—a common trap is assuming two AZs are sufficient for resilience. The key insight is that health checks may pass, but performance fails if the remaining AZ lacks headroom. Memory tip: think “Three is the key to capacity redundancy—two AZs is a single point of performance failure.”
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 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?
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.
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.
- ✗
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.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
Under the hood, the ALB uses a round-robin algorithm to distribute traffic across healthy targets in all registered AZs. When an AZ is lost, the ALB automatically stops routing to targets in that AZ, but if only two AZs are used, the remaining AZ must handle 100% of the traffic, potentially exceeding instance capacity and causing latency or timeouts. AWS recommends using at least three AZs for production workloads to absorb the loss of one AZ without performance degradation, as the remaining two AZs can each handle 50% of the original load.
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: 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.
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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Same concept, more angles
2 more ways 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 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?
medium- A.Keep the ASG in one subnet/AZ, but enable ALB stickiness to reduce session interruption.
- ✓ 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.
- C.Add a NAT gateway in the public subnets so instances can reach the internet during maintenance events.
- D.Create a second ALB in the same Availability Zone and route traffic using DNS failover.
Why B: Option B is correct because it directly addresses the single point of failure: the ASG only launches instances in one AZ, so when that AZ fails, the ALB has no healthy targets to route traffic to, causing timeouts. By configuring the ASG to span at least two AZs, the ALB can distribute traffic to healthy instances in the remaining AZ during maintenance, ensuring high availability. The ALB health check must target an application-ready path (e.g., /health) to accurately detect instance health and avoid routing requests to impaired instances.
Variation 2. 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?
medium- A.Keep the ASG in one subnet/AZ, but enable ALB stickiness to reduce session interruption.
- ✓ 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.
- C.Add a NAT gateway in the public subnets so instances can reach the internet during maintenance events.
- D.Create a second ALB in the same Availability Zone and route traffic using DNS failover.
Why B: 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.
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Last reviewed: Jun 11, 2026
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