SAA-C03 Design Cost-Optimized Architectures Practice Question

A retail company runs an e-commerce platform on a fleet of Amazon EC2 instances behind an Application Load Balancer. Traffic follows a predictable pattern: high during business hours and very low overnight. The operations team wants to reduce EC2 costs without affecting availability during peak hours. The instances currently run continuously and are managed by an Auto Scaling group with a minimum capacity of 4 and a maximum of 20. Which solution will meet these requirements MOST cost-effectively?

⚠ Common exam trap

The trap here is assuming that a discount purchasing option or a reactive scaling policy will automatically reduce cost, when the real issue is that the fleet keeps running at full baseline capacity during predictably low-traffic hours.

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

✓

Configure a scheduled scaling action on the Auto Scaling group to reduce the desired capacity during off-peak hours and increase it before peak hours.

For workloads with a predictable daily or weekly traffic pattern, scheduled scaling is the most direct way to align capacity with demand. It reduces the number of running instances during known low-traffic periods without waiting for a metric-based policy to react. Reserved Instances or target tracking do not eliminate the cost of idle overnight capacity in this scenario.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
Enable detailed monitoring on all instances and create a target tracking scaling policy based on CPU utilization.
Why it's wrong here
Target tracking based on CPU can react to changes, but for a predictable schedule it may lag behind the actual traffic pattern and still leave instances running during the low period. Detailed monitoring adds CloudWatch cost and does not guarantee that capacity drops overnight. A scheduled action is more precise and cost-effective for a known daily pattern.
✗
Purchase 4 Standard Reserved Instances for a 3-year term and let the Auto Scaling group launch On-Demand instances beyond that baseline.
Why it's wrong here
Reserved Instances provide a billing discount for a committed baseline, but they do not reduce the number of running instances overnight. The fleet would still run at least four instances 24/7, so the low-traffic hours would continue to incur charges for capacity that is not needed. This does not address the predictable daily dip in demand.
✗
Replace the Auto Scaling group with a larger number of smaller instances to improve granularity of scaling.
Why it's wrong here
Changing instance size does not change the fundamental cost driver: the total instance-hours consumed. Without a scaling policy that reduces capacity during off-peak hours, the fleet still runs continuously. Smaller instances may even increase management overhead and licensing costs, but they do not by themselves reduce cost for a predictable daily traffic pattern.
✓
Configure a scheduled scaling action on the Auto Scaling group to reduce the desired capacity during off-peak hours and increase it before peak hours.
Why this is correct
Scheduled scaling lets you set the desired capacity based on a known schedule, so the Auto Scaling group can scale down when traffic is predictably low and scale up before the peak. This directly matches capacity to demand and avoids paying for idle instances during off-peak hours, making it the most cost-effective option for a predictable pattern.

Go deeper

SAA-C03 Design Cost-Optimized Architectures Practice Question

Why each option matters

Option-by-option breakdown

Related to this question