SAA-C03 Design Resilient Architectures Practice Question
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.
⚠ Common exam trap
Test-takers frequently 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.
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
✓
Subnets in at least two Availability Zones with health checks enabled
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.
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
Detailed monitoring only reduces the CloudWatch metric interval to 1 minute; it does nothing to keep the application available if the instance fails or the underlying hardware degrades. A single EC2 instance is a single point of failure, so any OS crash, software fault, or Host maintenance event will take the dashboard offline. To achieve resilience, you need multiple instances managed by an Auto Scaling group that can automatically replace unhealthy instances.
- ✓
Subnets in at least two Availability Zones with health checks enabled
Why this is correct
Placing the Auto Scaling group's subnets in at least two Availability Zones ensures that if one AZ becomes unavailable, the remaining AZs still have healthy instances to serve traffic. Health checks (either EC2 status checks or Elastic Load Balancing target health checks) allow the ASG to detect failed instances and replace them while maintaining the desired capacity. This architecture survives both individual instance crashes and full AZ outages, providing high availability for the trading dashboard.
- ✗
All instances in one larger subnet
Why it's wrong here
A subnet is always contained within a single Availability Zone, so making the subnet larger simply concentrates all instances in one physical location. If that AZ fails, every instance in the larger subnet becomes unreachable, taking the dashboard down completely. Increasing instance count in one subnet does not add fault tolerance; you must distribute instances across distinct AZs to avoid a single point of failure.
- ✗
A Network Load Balancer in one subnet
Why it's wrong here
An NLB deployed in only one subnet lives entirely in one Availability Zone, and its endpoints are tied to that AZ. Even though the NLB can route traffic to instances in a single subnet, a failure or network partition affecting that AZ will make the load balancer itself unreachable. For resilience, the NLB must have load balancer nodes in multiple AZs and be configured with target instances in those AZs to survive an AZ-level outage.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This SAA-C03 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SAA-C03 exam.