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 team wants the control to be enforceable during normal operations.
⚠ Common exam trap
A common mix-up: candidates confuse high availability (spanning multiple AZs) with fault tolerance at the instance level, mistakenly thinking a single instance with monitoring or a single subnet can survive 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 if one AZ fails, the Auto Scaling group can maintain capacity in the remaining AZ(s). Enabling health checks allows the group to detect instance failures and automatically replace them, providing fault tolerance. This configuration meets the requirement to tolerate a single AZ failure while remaining enforceable during normal operations.
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 (CloudWatch metrics at 1-minute frequency) only provides visibility into CPU, memory, and status checks; it does nothing to replace an instance that fails. A single EC2 instance is a single point of failure — if its underlying host crashes or its Availability Zone goes down, the dashboard becomes unreachable despite having monitoring enabled. Even with an EC2 Auto-Recovery alarm, the instance is restarted on a new host in the same AZ and will be down for minutes, and it still cannot survive an AZ-wide failure.
- ✓
Subnets in at least two Availability Zones with health checks enabled
Why this is correct
Deploying subnets (and therefore EC2 instances) in at least two Availability Zones removes any single AZ as a point of failure. An Auto Scaling group distributed across those subnets works with a load balancer's health checks to detect unhealthy instances, terminate them, and launch replacement capacity in a healthy AZ. If one AZ fails entirely, the ASG can continue to meet desired capacity in the surviving AZ and the load balancer routes traffic away from unhealthy targets, keeping the dashboard available.
- ✗
All instances in one larger subnet
Why it's wrong here
A subnet is always contained within one Availability Zone; making a subnet 'larger' by expanding its CIDR does not change its zone boundary. Placing all instances in a single large subnet means they all share the same physical infrastructure and same failure domain. If that AZ suffers a loss of power or network connectivity, every instance goes down together — increasing instance count or subnet size provides zero additional resilience.
- ✗
A Network Load Balancer in one subnet
Why it's wrong here
A Network Load Balancer in a single subnet is bound to exactly one Availability Zone and has a node only in that zone. If that AZ fails, the NLB endpoint becomes unreachable and it cannot forward traffic, even if backend instances in other AZs are healthy (and in this design there are no other configured target AZs anyway). Health checks on the NLB help route around unhealthy instances, but they cannot help when the load balancer's own single node is down — the NLB must itself be enabled in multiple AZs (subnets) to be resilient.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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