AWS Certified Solutions Architect – Associate (SAA-C02) — Question 789

A company runs a stateless web application in production on a group of Amazon EC2 On-Demand Instances behind an Application Load Balancer. The application experiences heavy usage during an 8-hour period each business day. Application usage is moderate and steady overnight. Application usage is low during weekends.
The company wants to minimize its EC2 costs without affecting the availability of the application.
Which solution will meet these requirements?

Answer options

• A. Use Spot Instances for the entire workload
• B. Use Reserved Instances for the baseline level of usage. Use Spot Instances for any additional capacity that the application needs.
• C. Use On-Demand Instances for the baseline level of usage. Use Spot Instances for any additional capacity that the application needs.
• D. Use Dedicated Instances for the baseline level of usage. Use On-Demand Instances for any additional capacity that the application needs.

Correct answer: B

Explanation

Reserved Instances provide a significant discount for the predictable, continuous baseline workload that runs overnight and on weekends. Since the web application is stateless, Spot Instances are the most cost-effective choice to handle the additional peak traffic during business hours, as any potential interruption can be gracefully handled by the Application Load Balancer without affecting overall availability. Using On-Demand or Dedicated Instances for the baseline is unnecessarily expensive, and relying entirely on Spot Instances for the entire workload risks complete service unavailability if capacity is reclaimed.