A stateless web application runs on 10 On-Demand EC2 instances in an Auto Scaling group. The service requires at least 6 instances to satisfy traffic demands. What is the most cost-effective way to ensure the application stays available while meeting the minimum instance requirement?
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Correct answer: Use a Spot Fleet but reserve an On-Demand capacity of 6 instances..
Why this is the answer
The most cost-effective solution is to use a Spot Fleet with a reserved On-Demand capacity of 6 instances. This ensures the critical 6 instances are always available at the higher On-Demand price, while the remaining instances (up to the total of 10) can be provisioned as lower-cost Spot Instances. Spot Instances can be interrupted, but the On-Demand base guarantees service availability. Modifying the Auto Scaling group to use only On-Demand instances for 6 to 10 instances is less cost-effective as all instances would run at the higher On-Demand price. Setting a minimum of 1 and maximum of 6 On-Demand instances doesn't meet the requirement for up to 10 instances. Using a Spot Fleet with a target capacity of 6 instances is risky because all 6 could be interrupted, failing to meet the availability requirement.
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