Purchase options (Savings Plans, Reserved Instances, Spot): SAP-C03 study notes and practice questions
What to know about Purchase options (Savings Plans, Reserved Instances, Spot) for the SAP-C03 exam, from the Cost-Optimized Architecture Design domain (15% of the exam guide), then 1 free question to test it. Part of the SAP-C03 study guide.
SAP-C03 describes a fleet’s usage shape (steady baseline, daily peaks, batch jobs) and asks for the purchase mix that cuts cost without risking availability. The answer is usually a layered mix.
The options
Option
Commitment
Best for
On-Demand
None
Spiky, short-lived or unknown usage; the top of the curve
Compute Savings Plans
$/hour for 1 or 3 years
Steady spend that may change instance family, size, Region, OS, or move between EC2, Fargate and Lambda
EC2 Instance Savings Plans
$/hour for 1 or 3 years, one instance family in one Region
Steady usage on a family you won’t leave; deeper discount than Compute Savings Plans
Standard Reserved Instances
1 or 3 years, instance attributes
Largely replaced by Savings Plans for EC2; still the model for RDS, ElastiCache, OpenSearch and Redshift reserved nodes
Guaranteed capacity in a specific AZ, for example for DR or a launch event
Payment options for Savings Plans and RIs are no upfront, partial upfront and all upfront; more upfront and the 3-year term give the deepest discount.
Facts that decide answers
Savings Plans give a discount, not capacity. If the scenario needs capacity guaranteed in an AZ, use an On-Demand Capacity Reservation (or a zonal RI). Capacity Reservations are billed at On-Demand rates, and Savings Plans or Regional RIs then discount them.
Compute Savings Plans also cover Fargate and Lambda, so they suit a company moving from EC2 to containers or serverless during the term.
Database Savings Plans are a 1-year hourly commitment that covers Aurora, RDS, DynamoDB, ElastiCache, DocumentDB, Neptune, Keyspaces, Timestream and DMS, and keep the discount when you change engine, Region or move to serverless. See database costs.
SageMaker AI Savings Plans cover SageMaker AI instance usage, separate from Compute Savings Plans.
In an organization, commitments bought in one account share across the consolidated bill by default. Buy them centrally (often in the management or a dedicated account) and turn sharing off only where a business unit must keep its own discount.
Size the commitment to the steady baseline using Cost Explorer recommendations and utilization and coverage reports.
Spot best practice: diversify across many instance types and AZs, use the price-capacity-optimized allocation strategy, react to the interruption notice and rebalance recommendation, and checkpoint long jobs. Auto Scaling groups with a mixed instances policy combine an On-Demand base with Spot above it.
Fargate Spot and EC2 Spot in ECS or EKS capacity providers bring the same savings to containers.
How to choose
Scenario wording
Pick
“Steady 24/7 usage, plans to move to containers or Lambda”
Compute Savings Plans
“Steady usage on one instance family in one Region, maximum discount”
EC2 Instance Savings Plans (3-year, all upfront if cash allows)
“Must be sure capacity exists in a specific AZ during failover”
On-Demand Capacity Reservation
“Batch jobs that can restart, lowest cost”
Spot with diversified instance types
“Stable baseline plus unpredictable peaks”
Savings Plans for the baseline, On-Demand or Spot above it
Common traps
Buying Savings Plans for the peak instead of the baseline.
Expecting a Savings Plan to reserve capacity.
Spot for a stateful database or a job that cannot be interrupted.
Buying commitments before rightsizing; rightsize first, then commit (see rightsizing).
A retailer runs a steady baseline of EC2 instances and some AWS Fargate tasks in two Regions. Its engineers expect to move workloads between instance families and to adopt Graviton over the next year, so the commitment must not be tied to one instance type. Both Regions run about the same amount of steady load.
Finance wants the lowest committed hourly rate for that baseline, and it accepts a one-year commitment with no upfront payment.
Which purchase option meets these requirements?
Choose one answer.
AA. Buy Standard Reserved Instances for the current instance types in each Region, so that the hourly rate is locked for every type the team runs today.
BB. Keep the baseline On-Demand and rely on the standard hourly rate, because flexibility matters more than a commitment for this workload.
CC. Run the entire baseline on Spot Instances, so every hour is billed at the lowest Spot price available in the two Regions.
DD. Buy a one-year Compute Savings Plan at the hourly spend the baseline needs, which applies to EC2 across instance families and Regions, and to eligible Fargate usage.
Show the answer and explanation
Answer: D
A. Wrong: Standard Reserved Instances are tied to an instance type and Region, so the discount is lost when the team moves to a new family or to Graviton. They also do not cover Fargate tasks.
B. Wrong: On-Demand gives full flexibility but no discount, and the finance team asked for the lowest committed rate on a steady baseline. This option does not reduce cost.
C. Wrong: Spot capacity can be reclaimed with a short warning, which does not suit a steady baseline that must stay up. The lower price comes with interruptions the team has not accepted.
D. Right: Correct. A Compute Savings Plan discounts EC2 regardless of instance family, size, Region or operating system, and it also covers Fargate. The commitment follows the workloads as the team changes instance types.
Explanation
A one-year Compute Savings Plan gives the lowest committed rate on a steady baseline while following the team across instance families and Regions. Standard Reserved Instances are tied to one type, and Spot or On-Demand each fail the stability or cost requirement.
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