EC2 purchasing options: SAA-C03 study notes and practice questions
What to know about EC2 purchasing options for the SAA-C03 exam, from the Design Cost-Optimized Architectures domain (20% of the exam guide), then 3 free questions to test it. Part of the SAA-C03 study guide.
EC2 pricing questions usually describe a mixed workload. The trick is to split it into parts and pay for each part the cheapest way that still meets its needs.
The options
Option
Commitment
Discount (up to)
Fits
On-Demand
None
none
Unpredictable or short-term use, new workloads
Compute Savings Plans
$/hour for 1 or 3 years
~66%
Steady use that may change instance family, size, Region or OS, or move to Fargate or Lambda
EC2 Instance Savings Plans
$/hour for one instance family in one Region, 1 or 3 years
~72%
Steady use that will stay in one family and Region
Standard Reserved Instances
Instance type, Region, 1 or 3 years
~72%
Same as Instance Savings Plans; can be sold on the RI Marketplace
Convertible Reserved Instances
1 or 3 years, exchangeable
~66%
Steady use where you want to exchange instance attributes
Spot Instances
None; can be interrupted with a 2-minute warning
~90%
Fault-tolerant, flexible work: batch, CI, stateless web tiers, big data
On-Demand Capacity Reservations
None (billed whether used or not)
none (combine with Savings Plans)
Guaranteeing capacity in a specific zone, for example for a known event
Dedicated Hosts
On-Demand or reserved
varies
Per-socket or per-core software licenses, compliance needing a physical server
Dedicated Instances
On-Demand or reserved
varies
Hardware not shared with other customers, without host-level control
Key facts
Savings Plans apply automatically to matching usage; Compute Savings Plans also cover Fargate and Lambda.
Spot best practices: be flexible across many instance types and zones, use the price-capacity-optimized allocation strategy, and checkpoint work so an interruption only loses a little.
Auto Scaling groups can mix On-Demand (for a base) and Spot (for the rest), across several instance types.
Capacity Reservations don’t give a discount on their own, but Savings Plans and regional RIs apply to them.
How to choose
Steady 24/7 baseline that may move to containers: Compute Savings Plan.
Nightly batch jobs that can restart: Spot.
Short-term project, a few weeks: On-Demand.
License tied to physical cores: Dedicated Hosts.
Must have capacity in one zone on a specific date: On-Demand Capacity Reservation.
Common traps
Spot for work that can’t be interrupted, such as a single-instance database.
Reserved Instances when the workload will change instance family. Compute Savings Plans are more flexible.
Dedicated Hosts for “isolation” when Dedicated Instances (or nothing extra) would do. They cost more.
A genomics research team runs a batch pipeline every night from 22:00.
It needs about 400 vCPUs for roughly 6 hours and must finish by 07:00, which leaves about 3 hours of slack.
Jobs save a checkpoint every 10 minutes and can resume on any instance. The pipeline runs well on many compute optimized and general purpose instance types, and the capacity is not needed during the day.
Which purchasing approach gives the LOWEST cost while meeting the deadline?
Choose one answer.
AA. Use a Spot-only Auto Scaling group spanning many instance types and all Availability Zones, with the price-capacity-optimized strategy.
BB. Use a Spot-only Auto Scaling group with one instance type in one Availability Zone, with the lowest-price allocation strategy.
CC. Buy a 1-year Compute Savings Plan sized for 400 vCPUs of On-Demand usage, and run the pipeline on On-Demand Instances each night.
DD. Run the pipeline on On-Demand Instances that a scheduled scaling action launches at 22:00 nightly and ends when the jobs finish.
Show the answer and explanation
Answer: A
A. Right: Correct. Spot Instances give the deepest discount, and checkpointing means an interruption costs at most a few minutes of work. Drawing on many instance types and zones with price-capacity-optimized picks pools with spare capacity, which lowers the chance of interruptions eating into the slack. Learn more: Best practices for Amazon EC2 Spot (opens in a new tab)
C. Wrong: A Savings Plan is an hourly commitment, and each hour's commitment cannot be carried over to another hour. The pipeline runs about 6 hours a day, so most of the commitment would be paid for and unused. Spot pricing is cheaper for interruptible night-time work. Learn more: Understanding how Savings Plans apply to your usage (opens in a new tab)
D. Wrong: This avoids paying for idle daytime capacity and never risks interruption, but every hour is charged at full On-Demand price. The jobs checkpoint and have slack, so they can safely use Spot capacity at a much lower price. Learn more: Amazon EC2 billing and purchasing options (opens in a new tab)
Explanation
Checkpointed, restartable jobs with slack in the schedule are a good fit for Spot Instances. How Spot is requested matters: being flexible across many instance types and Availability Zones, with the price-capacity-optimized strategy, reduces interruptions and capacity shortfalls that could threaten the deadline. Commitments such as Savings Plans suit steady round-the-clock use, not 6 hours a night.
A streaming company runs its customer-facing API on 10 m6i.xlarge instances 24 hours a day and expects this baseline to last at least 3 years.
Within the next year, it plans to move the API to Graviton-based m7g instances, and later possibly to containers on AWS Fargate. A few times a month, unpredictable traffic surges need up to 6 extra instances for a few hours. The API cannot tolerate interruptions.
Which TWO purchasing choices give the LOWEST total cost? (Choose two.)
Choose two answers.
AA. Buy a 3-year Compute Savings Plan with an hourly commitment that covers the usage of the 10 baseline instances.
BB. Run the surge instances as On-Demand Instances that the Auto Scaling group launches only while they are needed.
CC. Buy 3-year Standard Reserved Instances for 10 m6i.xlarge instances to cover the baseline at the deepest discount.
DD. Run the extra surge instances as Spot Instances across several instance types to get the largest discount on them.
EE. Buy a 3-year EC2 Instance Savings Plan for the m6i family with a commitment that covers all 16 instances.
Show the answer and explanation
Answers: A, B
A. Right: Correct. Compute Savings Plans apply automatically whatever the instance family, and also to Fargate, so the discount follows the API from m6i to m7g and on to containers. Sizing the commitment to the steady baseline means it is fully used every hour. Learn more: Savings Plans types (opens in a new tab)
B. Right: Correct. The surges are short and unpredictable, so paying On-Demand only while they run avoids committing to capacity that sits idle most of the month. On-Demand Instances are not interrupted, which the API requires. Learn more: Amazon EC2 billing and purchasing options (opens in a new tab)
C. Wrong: Standard RIs offer up to 72% off, but they apply only to the instance configuration bought and cannot be exchanged for another family or used for Fargate. After the move to m7g, the m6i reservations would go unused for the rest of the term. Learn more: Compute Savings Plans and Reserved Instances (opens in a new tab)
D. Wrong: Spot Instances are the cheapest way to add capacity, but AWS can reclaim them with a two-minute warning. The API cannot tolerate interruptions, and a surge is exactly when losing capacity would hurt most. On-Demand is the safe choice for the surges. Learn more: Best practices for Amazon EC2 Spot (opens in a new tab)
E. Wrong: EC2 Instance Savings Plans are tied to one instance family in one Region, so the discount stops applying after the move to m7g. Covering all 16 instances also commits to surge capacity that runs only a few hours a month, wasting most of that commitment. Learn more: Savings Plans types (opens in a new tab)
Explanation
Match each part of the workload to a pricing model. The steady baseline suits a 3-year commitment, and because the company will change instance family and may move to Fargate, the Compute Savings Plan is the commitment that keeps applying. Short, unpredictable surges that cannot be interrupted suit On-Demand, which costs nothing when idle.
A company spends a steady amount every hour on compute and expects this to continue for at least 3 years.
Over that time it plans to move many workloads from Amazon EC2 to AWS Fargate and AWS Lambda, switch EC2 instance families as new generations arrive, and possibly move some workloads to another AWS Region. It wants a discount that follows these changes automatically, without manual exchanges.
Which purchase option should it choose?
Choose one answer.
AA. Buy EC2 Instance Savings Plans for the current instance families in the current Region, with a 3-year term.
BB. Buy Convertible Reserved Instances with a 3-year term, and exchange them whenever instance families change.
CC. Buy Standard Reserved Instances with a 3-year term for the instance types that the company runs today.
DD. Buy Compute Savings Plans with a 3-year term and an hourly commitment that matches the steady baseline.
Show the answer and explanation
Answer: D
A. Wrong: EC2 Instance Savings Plans give the deepest Savings Plans discount, but each plan is tied to one instance family in one Region. They do not apply to Fargate or Lambda, so the discount would shrink as workloads move or change family. Learn more: Savings Plans types (opens in a new tab)
B. Wrong: Convertible Reserved Instances can be exchanged for another instance family, operating system or tenancy, but only by manual exchanges and only within the same Region. They never apply to Fargate or Lambda, so they miss two of the company's plans. Learn more: Exchange Convertible Reserved Instances (opens in a new tab)
C. Wrong: Standard Reserved Instances give a large discount for a fixed instance configuration in one Region. They cannot follow a change of instance family or Region, and they do not apply to Fargate or Lambda, so much of the commitment would go unused. Learn more: Compute Savings Plans and Reserved Instances (opens in a new tab)
D. Right: Correct. Compute Savings Plans apply automatically to EC2 usage in any instance family, size, Region, operating system or tenancy, and also to Fargate and Lambda. The company commits to an hourly spend, and the discount follows the workloads as they change. Learn more: Compute Savings Plans and Reserved Instances (opens in a new tab)
Explanation
Only Compute Savings Plans apply automatically across instance families and Regions and to Fargate and Lambda, in exchange for an hourly spend commitment over 1 or 3 years. EC2 Instance Savings Plans and Reserved Instances can give bigger discounts, but they are tied to EC2, to a Region, and (except by manual exchange) to an instance family.
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