Questions 7 to 9
Explanations for questions 7 to 9
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Question 7
An e-commerce website runs on Amazon EC2 instances behind an Application Load Balancer (ALB). The application is stateless and elastic and scales from a minimum of 10 instances up to a maximum of 200 instances. For at least 80% of the time, at least 40 instances are required.
Which solution should be used to minimize costs?
- Purchase Reserved Instances to cover 200 instances.
- Purchase Reserved Instances to cover 80 instances. Use Spot Instances to cover the remaining instances.
- Purchase On-Demand Instances to cover 40 instances. Use Spot Instances to cover the remaining instances.
- Purchase Reserved Instances to cover 40 instances. Use On-Demand and Spot Instances to cover the remaining instances.
Correct Answer: 4
Explanation: In this case, at least 40 instances are required 80% of the time, for which Reserved Instances are a good option. Reserved Instances also provide discounts of up to 72% as compared to On-Demand Instances. For the remainder, On-Demand and Spot Instances should be used. Spot can be used as the application is stateless and it will minimize costs, and On-Demand can be used when Spot Instances aren’t available or when the price is excessive.
INCORRECT: “Purchase Reserved Instances to cover 200 instances.” is incorrect as the extra instances above 40 instances are only used less than 20% of the time. It would be better to reserve 40 instances only.
INCORRECT: “Purchase Reserved Instances to cover 80 instances. Use Spot Instances to cover the remaining instances.” is incorrect as only 40 instances should be reserved as these are used 80% of the time. The remainder should be Spot Instances.
INCORRECT: “Purchase On-Demand Instances to cover 40 instances. Use Spot Instances to cover the remaining instances.” is incorrect as on-demand instances will not minimize costs. For the instances that will be required at a minimum, Reserved Instances should be used.
CORRECT: “Purchase Reserved Instances to cover 40 instances. Use On-Demand and Spot Instances to cover the remaining instances.” is the correct answer.
References: https://aws.amazon.com/ec2/pricing/reserved-instances/
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