SAA-C03 Design Resilient Architectures Practice Question

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 with CloudWatch (1-minute metrics) only increases the frequency of metric collection and alarms; it has no effect on fault tolerance or instance replacement. A single EC2 instance still resides in one Availability Zone, so an AZ outage, hardware failure, or maintenance event terminates the instance and takes the dashboard offline until manual intervention or a separate recovery mechanism acts. Detailed monitoring can help you react faster to performance issues, but it cannot restart, replace, or relocate the instance, leaving the application with a single point of failure.

• ✓

  Subnets in at least two Availability Zones with health checks enabled

  Why this is correct

  Placing subnets in at least two Availability Zones (AZs) and attaching health checks to the Auto Scaling group allows the group to detect and replace unhealthy instances while maintaining desired capacity across AZs. If one AZ fails, the remaining healthy instances in the other AZ continue serving traffic, and Auto Scaling launches new instances in the surviving AZs to compensate. Health checks (ELB or EC2 status checks) drive the replacement process, and distributing subnets across AZs makes the architecture resilient to both instance-level and AZ-level failures, which is the key requirement for a fault-tolerant trading dashboard.

• ✗

  All instances in one larger subnet

  Why it's wrong here

  Expanding a subnet to include more IP addresses does not change the underlying physical location; a subnet exists entirely within a single Availability Zone. If all EC2 instances are launched into that one larger subnet, they all share the same AZ, so any failure of that AZ—power loss, network partition, or cooling failure—takes every instance down simultaneously. Larger subnets only provide more capacity for launching instances, not geographical or fault-domain diversity, and therefore do not address availability or redundancy requirements.

• ✗

  A Network Load Balancer in one subnet

  Why it's wrong here

  A Network Load Balancer (NLB) is a regional service that can route traffic to targets across multiple AZs; however, if the NLB itself is placed in a single subnet (single AZ), it becomes a single point of failure at the network layer. Even if the backend instances are in multiple AZs, an AZ failure that includes the NLB's subnet will make the load balancer unreachable, and the entire application loses connectivity. NLB nodes must be deployed in at least two AZs—or the NLB must automatically have a node in each enabled AZ—to provide high availability; simply having a load balancer in one subnet adds a dependency that defeats resilience.

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