SOA-C02 Cost and Performance Optimization Practice Question

A company runs a web application on a fleet of Amazon EC2 instances behind an Application Load Balancer. The application has predictable traffic patterns with high traffic during business hours and low traffic at night. The SysOps administrator wants to reduce compute costs while ensuring the application remains responsive during peak hours. The administrator has already implemented Auto Scaling based on CPU utilization. Which additional action should the administrator take to optimize costs?

⚠ Common exam trap

A common mix-up: candidates think increasing the minimum instance count (Option C) improves responsiveness, but it actually increases costs during low-traffic periods without addressing the cost optimization goal.

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

✓

Purchase Reserved Instances for the baseline capacity and use Spot Instances for the additional capacity during peak hours

It combines Reserved Instances for predictable baseline capacity (lower cost per hour) with Spot Instances for elastic peak demand, leveraging Auto Scaling to handle variable traffic. This hybrid approach reduces compute costs compared to using On-Demand instances for all capacity, while maintaining responsiveness during peak hours.

Answer analysis

Option-by-option breakdown

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

✗
Use On-Demand instances only
Why it's wrong here
Using On-Demand Instances exclusively for a predictable, steady-state web workload is cost-suboptimal because this pricing model carries no discount for sustained usage or any commitment. The baseline capacity runs continuously and can be covered with Reserved Instances (or a Savings Plan) at up to 72% lower compute cost, while the peak-hour surge can be served by Spot Instances, which cost up to 90% less than On-Demand. Without these options, you pay the full On-Demand rate for every hour, including the majority of hours when there is no peak load, directly inflating the company's AWS bill.
✓
Purchase Reserved Instances for the baseline capacity and use Spot Instances for the additional capacity during peak hours
Why this is correct
This approach minimizes costs by applying the highest discount (Reserved Instances) to the steady-state capacity and leveraging the cost savings of Spot Instances for the flexible, peak-demand capacity. Auto Scaling can be configured to launch Spot Instances as needed, providing both cost efficiency and performance.
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Increase the minimum number of instances in the Auto Scaling group
Why it's wrong here
Raising the Auto Scaling group's minimum instance count does not directly address peak-hour capacity—it only raises the always-running floor, meaning extra instances remain active during low-demand periods and accumulate hourly On-Demand costs without being used. While it may make the group more responsive to sudden spikes, a minimum that is too high wastes resources 24/7, whereas the actual peak load should be handled by scaling policies that launch additional instances (ideally Spot Instances) only when demand increases. Thus, this approach increases baseline cost without optimizing the cost of the surplus capacity.
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Use Dedicated Hosts to reduce licensing costs
Why it's wrong here
Dedicated Hosts are primarily used for compliance or to reduce software licensing costs when using certain server-bound licenses. They are typically more expensive than shared tenancy and do not directly address compute cost optimization for a standard web application.

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SOA-C02 Cost and Performance Optimization Practice Question

Why each option matters

Option-by-option breakdown

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