AWS Certified Solutions Architect – Associate (SAA-C02) — Question 793

A company hosts a two-tier application on Amazon EC2 instances and Amazon RDS. The application's demand varies based on the time of day. The load is minimal after work hours and on weekends. The EC2 instances run in an EC2 Auto Scaling group that is configured with a minimum of two instances and a maximum of five instances. The application must be available at all times, but the company is concerned about overall cost.
Which solution meets the availability requirement MOST cost-effectively?

Answer options

• A. Use all EC2 Spot Instances. Stop the RDS database when it is not in use.
• B. Purchase EC2 Instance Savings Plans to cover five EC2 instances. Purchase an RDS Reserved DB Instance.
• C. Purchase two EC2 Reserved Instances. Use up to three additional EC2 Spot Instances as needed. Stop the RDS database when it is not in use.
• D. Purchase EC2 Instance Savings Plans to cover two EC2 instances. Use up to three additional EC2 On-Demand Instances as needed. Purchase an RDS Reserved DB Instance.

Correct answer: D

Explanation

Option D is the most cost-effective and reliable solution because EC2 Instance Savings Plans cover the baseline of two instances that run continuously, while On-Demand Instances handle the variable load without risking termination. Purchasing an RDS Reserved DB Instance provides a significant discount for the database, which must remain running continuously to meet the 24/7 availability requirement. Options A and C are incorrect because stopping the RDS database violates the constant availability requirement, and Spot Instances can be reclaimed by AWS, risking application downtime.