The answer is to configure the Auto Scaling group with subnets in at least two Availability Zones and enable health checks. This is correct because distributing EC2 instances across multiple AZs ensures the application can tolerate the failure of an entire Availability Zone, as the Auto Scaling group will automatically launch replacement instances in the remaining healthy AZs. The key technical concept here is that an Auto Scaling group tied to a single AZ becomes a single point of failure; by spanning multiple AZs and using ELB health checks, the group can detect and replace failed instances without manual intervention. On the SAA-C03 exam, this scenario tests your understanding of high-availability architecture patterns, often paired with a trap where candidates mistakenly choose a single-AZ configuration with larger instance types. Remember the mnemonic: “Two AZs plus health checks equals fault tolerance”—if you see only one subnet, the design is not resilient to an AZ outage.
SAA-C03 Design Resilient Architectures Practice Question
This SAA-C03 practice question tests your understanding of design resilient architectures. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: auto Scaling Groups distribute instances across specified subnets.. 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 trading dashboard runs on EC2 instances behind an Application Load Balancer. The design must tolerate the failure of one Availability Zone. What should the Auto Scaling group configuration include? The architecture review board prefers a managed AWS-native control.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Subnets in at least two Availability Zones with health checks enabled
Option B is correct because distributing EC2 instances across at least two Availability Zones (AZs) ensures that the application remains available if one AZ fails. The Auto Scaling group must include subnets in multiple AZs and use health checks (e.g., ELB health checks) to automatically replace unhealthy instances. This configuration meets the requirement for fault tolerance and aligns with AWS-managed best practices for high availability.
Key principle: Auto Scaling Groups distribute instances across specified subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
A single EC2 instance with detailed monitoring
Why it's wrong here
Monitoring does not provide redundancy.
✓
Subnets in at least two Availability Zones with health checks enabled
Why this is correct
An Auto Scaling group spanning multiple AZs can replace unhealthy instances and maintain capacity during an AZ failure.
Related concept
Auto Scaling Groups distribute instances across specified subnets.
✗
All instances in one larger subnet
Why it's wrong here
One AZ remains a single point of failure.
✗
A Network Load Balancer in one subnet
Why it's wrong here
A load balancer in one AZ does not make the application resilient to AZ failure.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse 'scaling' with 'resilience' and think that a single large subnet or a different load balancer type (NLB) provides AZ fault tolerance, but only multi-AZ subnet configuration with health checks ensures automatic recovery from an AZ failure.
Detailed technical explanation
How to think about this question
Auto Scaling groups distribute instances across the subnets specified in the launch configuration or template. When health checks are enabled (e.g., ELB health checks), the group monitors instance health via HTTP/HTTPS pings or TCP connections; if an instance fails health checks, it is terminated and replaced. Under the hood, the group uses the Availability Zone balancer to maintain an even distribution of instances across selected AZs, ensuring that a single AZ failure does not reduce capacity below the desired count.
KKey Concepts to Remember
Auto Scaling Groups distribute instances across specified subnets.
Subnets in an ASG should span at least two Availability Zones for fault tolerance.
Health checks are vital for ASGs to detect and replace unhealthy instances.
ASGs automatically launch new instances in healthy AZs during an AZ failure.
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
Auto Scaling Groups distribute instances across specified subnets.
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
Got this wrong? Here's your next step.
Review auto Scaling Groups distribute instances across specified subnets., then practise related SAA-C03 questions on the same topic to reinforce the concept.
Design Resilient Architectures — This question tests Design Resilient Architectures — Auto Scaling Groups distribute instances across specified subnets..
What is the correct answer to this question?
The correct answer is: Subnets in at least two Availability Zones with health checks enabled — Option B is correct because distributing EC2 instances across at least two Availability Zones (AZs) ensures that the application remains available if one AZ fails. The Auto Scaling group must include subnets in multiple AZs and use health checks (e.g., ELB health checks) to automatically replace unhealthy instances. This configuration meets the requirement for fault tolerance and aligns with AWS-managed best practices for high availability.
What should I do if I get this SAA-C03 question wrong?
Review auto Scaling Groups distribute instances across specified subnets., then practise related SAA-C03 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
Auto Scaling Groups distribute instances across specified subnets.
About these practice questions
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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 ticket booking system runs on EC2 instances behind an Application Load Balancer. The design must tolerate the failure of one Availability Zone. What should the Auto Scaling group configuration include?
medium
✓ A.Subnets in at least two Availability Zones with health checks enabled
B.All instances in one larger subnet
C.A Network Load Balancer in one subnet
D.A single EC2 instance with detailed monitoring
Why A: Option A is correct because an Auto Scaling group configured with subnets in at least two Availability Zones and health checks enabled ensures that if one AZ fails, EC2 instances in the remaining AZs continue to serve traffic. The Application Load Balancer distributes requests across healthy instances in multiple AZs, and the Auto Scaling group replaces failed instances in the affected AZ, maintaining capacity. This design meets the requirement to tolerate the failure of one Availability Zone.
Variation 2. A ticket booking system runs on EC2 instances behind an Application Load Balancer. The design must tolerate the failure of one Availability Zone. What should the Auto Scaling group configuration include? The team wants the control to be enforceable during normal operations.
medium
✓ A.Subnets in at least two Availability Zones with health checks enabled
B.All instances in one larger subnet
C.A Network Load Balancer in one subnet
D.A single EC2 instance with detailed monitoring
Why A: Option A is correct because distributing subnets across at least two Availability Zones ensures that if one AZ fails, the Auto Scaling group can launch replacement instances in the remaining AZ(s) to maintain capacity. Enabling health checks on the Application Load Balancer allows the Auto Scaling group to detect and replace unhealthy instances, enforcing resilience during normal operations without manual intervention.
Variation 3. A ticket booking system runs on EC2 instances behind an Application Load Balancer. The design must tolerate the failure of one Availability Zone. What should the Auto Scaling group configuration include? The design must avoid adding custom operational scripts.
medium
✓ A.Subnets in at least two Availability Zones with health checks enabled
B.All instances in one larger subnet
C.A Network Load Balancer in one subnet
D.A single EC2 instance with detailed monitoring
Why A: Option A is correct because placing subnets in at least two Availability Zones ensures that if one AZ fails, the Auto Scaling group can launch instances in the remaining healthy AZ, maintaining application availability. Health checks integrated with the Application Load Balancer allow the Auto Scaling group to automatically replace unhealthy instances without custom scripts, aligning with the requirement to avoid operational overhead.
Variation 4. A ticket booking system runs on EC2 instances behind an Application Load Balancer. The design must tolerate the failure of one Availability Zone. What should the Auto Scaling group configuration include? The architecture review board prefers a managed AWS-native control.
medium
✓ A.Subnets in at least two Availability Zones with health checks enabled
B.All instances in one larger subnet
C.A Network Load Balancer in one subnet
D.A single EC2 instance with detailed monitoring
Why A: Option A is correct because an Auto Scaling group configured with subnets in at least two Availability Zones ensures that if one AZ fails, the remaining AZ(s) can continue to serve traffic. Health checks on the EC2 instances allow the Auto Scaling group to detect and replace unhealthy instances, maintaining the desired capacity across the surviving AZs. This aligns with the requirement for a managed AWS-native control to tolerate an AZ failure.
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