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CLF-048
Which purchasing option is most appropriate for reducing the cost of EC2 instances that run for a long period of time?
Choices
Explanation and answer
Read the explanation and correct answer (spoiler — try answering first)
Correct answer: C. Reserved Instances
Explanation
Reserved Instances are a purchasing option that lets you use services such as EC2 at a lower price than on-demand pricing in exchange for committing to a usage period.
Per-choice explanations
A. Security Group
A Security Group is a virtual firewall. It is not a cost-saving purchasing option.
B. AWS CloudTrail
CloudTrail is a service that records API call history.
C. Reserved Instances
Correct. They are used to reduce the cost of workloads that run continuously.
D. Amazon S3 Standard
S3 Standard is an S3 storage class, not an EC2 purchasing option.
Practical Use
Reserved Instances (RIs) are a purchasing option that lets you use resources such as Amazon EC2 at a significantly discounted price compared with on-demand pricing in exchange for committing to a 1-year or 3-year usage period. Three payment options are available: All Upfront, Partial Upfront, and No Upfront. The larger the upfront payment, the higher the discount rate. An RI is a reservation that applies to a combination of instance type, Region, OS, and tenancy, and is effective for workloads that run continuously for long periods. On the exam, Reserved Instances are the expected answer for the prompt asking which purchasing option reduces the cost of EC2 instances that run for a long time. Common distractors include AWS CloudTrail (audit logs), Amazon S3 Standard (a storage class), and Security Group (a virtual firewall). None of these are purchasing options, and they have other roles as services or features. The keywords "purchasing option" and "cost reduction" help separate them. In practice, cost optimization for Amazon EC2 is done by combining the four options of Reserved Instances, Savings Plans, Spot Instances, and on-demand. A typical pattern is to assign RIs or Savings Plans to always-on production workloads, Spot to development environments and batch jobs, and on-demand to workloads with hard-to-predict demand. Savings Plans are similar to RIs as a long-term commitment discount, but they differ in that they commit to a specific dollar-per-hour usage amount regardless of instance type, providing more flexibility than RIs. CLF-C02 frequently asks about the differences among the three options of Reserved Instances, Savings Plans, and Spot Instances and the workload characteristics that suit each one.