AWS Certified Solutions Architect – Associate (SAA-C03) — Question 421

A company is running a custom application on Amazon EC2 On-Demand Instances. The application has frontend nodes that need to run 24 hours a day, 7 days a week and backend nodes that need to run only for a short time based on workload. The number of backend nodes varies during the day.

The company needs to scale out and scale in more instances based on workload.

Which solution will meet these requirements MOST cost-effectively?

Answer options

• A. Use Reserved Instances for the frontend nodes. Use AWS Fargate for the backend nodes.
• B. Use Reserved Instances for the frontend nodes. Use Spot Instances for the backend nodes.
• C. Use Spot Instances for the frontend nodes. Use Reserved Instances for the backend nodes.
• D. Use Spot Instances for the frontend nodes. Use AWS Fargate for the backend nodes.

Correct answer: B

Explanation

Reserved Instances are ideal for the frontend nodes because they run continuously (24/7) and can benefit from significant long-term discounts. Spot Instances are the most cost-effective option for the backend nodes because they can scale up and down dynamically to handle short-term, variable workloads at a fraction of the cost of On-Demand or Fargate. Using Spot Instances for the frontend is risky due to potential interruptions, and using Reserved Instances for the backend would result in wasted capacity when workloads are low.