- A
Use all Spot Instances with a simple scaling policy.
Why wrong: Spot Instances can be interrupted, risking performance degradation during spikes. Simple scaling may not respond well to rapid spikes.
- B
Use all On-Demand Instances with a scheduled scaling policy.
Why wrong: Scheduled scaling is not effective for unpredictable spikes. All On-Demand is more expensive than a mix.
- C
Use a mix of Reserved Instances for baseline and On-Demand for spikes, with a target tracking scaling policy.
Correct. Reserved Instances cover steady-state load at a discount, On-Demand handles spikes, and target tracking auto-scales to maintain utilization.
- D
Use only Reserved Instances with a manual scaling policy.
Why wrong: Reserved Instances alone may be over-provisioned for spikes leading to waste, or under-provisioned for spikes causing performance issues. Manual scaling is inflexible.
Optimize EC2 Costs for Unpredictable Traffic Spikes
This SOA-C02 practice question tests your understanding of cost and performance optimization. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. A key principle to apply: reserved Instances provide significant discounts for predictable, long-term capacity.. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company runs a web application on Amazon EC2 instances. The application's traffic pattern is unpredictable, often spiking to 3x normal load for short periods. The SysOps administrator needs to ensure that the application can handle spikes without performance degradation while minimizing costs. Which combination of purchasing options and scaling strategies should the administrator use?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Use a mix of Reserved Instances for baseline and On-Demand for spikes, with a target tracking scaling policy.
Option C is correct because it combines Reserved Instances for the predictable baseline load, which reduces costs through a significant discount, with On-Demand Instances to handle unpredictable spikes. The target tracking scaling policy automatically adjusts capacity based on a target metric (e.g., average CPU utilization), ensuring performance is maintained during spikes without manual intervention or over-provisioning.
Key principle: Reserved Instances provide significant discounts for predictable, long-term capacity.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Use all Spot Instances with a simple scaling policy.
Why it's wrong here
Spot Instances can be interrupted, risking performance degradation during spikes. Simple scaling may not respond well to rapid spikes.
- ✗
Use all On-Demand Instances with a scheduled scaling policy.
Why it's wrong here
Scheduled scaling is not effective for unpredictable spikes. All On-Demand is more expensive than a mix.
- ✓
Use a mix of Reserved Instances for baseline and On-Demand for spikes, with a target tracking scaling policy.
Why this is correct
Correct. Reserved Instances cover steady-state load at a discount, On-Demand handles spikes, and target tracking auto-scales to maintain utilization.
Related concept
Reserved Instances provide significant discounts for predictable, long-term capacity.
- ✗
Use only Reserved Instances with a manual scaling policy.
Why it's wrong here
Reserved Instances alone may be over-provisioned for spikes leading to waste, or under-provisioned for spikes causing performance issues. Manual scaling is inflexible.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates might choose all Spot Instances (Option A) thinking they are cheapest, but they overlook the risk of interruption during spikes, which violates the requirement to 'handle spikes without performance degradation.'
Detailed technical explanation
How to think about this question
Reserved Instances provide a capacity reservation and a discount of up to 72% over On-Demand, making them ideal for steady-state workloads. Target tracking scaling policies use a CloudWatch alarm to maintain a target metric (e.g., 50% CPU), automatically adding or removing instances via Auto Scaling, which is essential for unpredictable traffic patterns. In contrast, Spot Instances have no guarantee of availability and can be terminated with a 2-minute warning, making them unsuitable for handling spikes that require consistent performance.
KKey Concepts to Remember
- Reserved Instances provide significant discounts for predictable, long-term capacity.
- On-Demand Instances offer flexible, pay-as-you-go capacity for variable workloads.
- Target tracking scaling policies adjust instance count to maintain a specified metric level.
- Combining RIs for baseline and On-Demand for spikes optimizes cost and performance.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Reserved Instances provide significant discounts for predictable, long-term capacity.
Real-world example
How this comes up in practice
A startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
What to study next
Got this wrong? Here's your next step.
Review reserved Instances provide significant discounts for predictable, long-term capacity., then practise related SOA-C02 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this SOA-C02 question test?
Cost and Performance Optimization — This question tests Cost and Performance Optimization — Reserved Instances provide significant discounts for predictable, long-term capacity..
What is the correct answer to this question?
The correct answer is: Use a mix of Reserved Instances for baseline and On-Demand for spikes, with a target tracking scaling policy. — Option C is correct because it combines Reserved Instances for the predictable baseline load, which reduces costs through a significant discount, with On-Demand Instances to handle unpredictable spikes. The target tracking scaling policy automatically adjusts capacity based on a target metric (e.g., average CPU utilization), ensuring performance is maintained during spikes without manual intervention or over-provisioning.
What should I do if I get this SOA-C02 question wrong?
Review reserved Instances provide significant discounts for predictable, long-term capacity., then practise related SOA-C02 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
Reserved Instances provide significant discounts for predictable, long-term capacity.
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Same concept, more angles
3 more ways this is tested on SOA-C02
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company runs a web application on a fleet of Amazon EC2 instances behind an Application Load Balancer. The application has predictable traffic patterns with high traffic during business hours and low traffic at night. The SysOps administrator wants to reduce compute costs while ensuring the application remains responsive during peak hours. The administrator has already implemented Auto Scaling based on CPU utilization. Which additional action should the administrator take to optimize costs?
medium- A.Use On-Demand instances only
- ✓ B.Purchase Reserved Instances for the baseline capacity and use Spot Instances for the additional capacity during peak hours
- C.Increase the minimum number of instances in the Auto Scaling group
- D.Use Dedicated Hosts to reduce licensing costs
Why B: Option B is correct because it combines Reserved Instances for predictable baseline capacity (lower cost per hour) with Spot Instances for elastic peak demand, leveraging Auto Scaling to handle variable traffic. This hybrid approach reduces compute costs compared to using On-Demand instances for all capacity, while maintaining responsiveness during peak hours.
Variation 2. A company runs a production web application on Amazon EC2 instances behind an Application Load Balancer. The application experiences variable traffic patterns, with peak usage during business hours. The company wants to optimize costs while maintaining performance. What should the SysOps administrator do?
medium- A.Increase the EC2 instance sizes to handle peak load at all times.
- B.Purchase Reserved Instances for all instances to get the lowest hourly rate.
- ✓ C.Use a combination of On-Demand and Spot Instances, with On-Demand for baseline traffic and Spot for burst capacity.
- D.Use Dedicated Hosts to ensure consistent performance.
Why C: The correct answer is C. A combination of On-Demand and Spot Instances allows the company to handle variable traffic cost-effectively: On-Demand Instances cover baseline traffic with guaranteed availability, while Spot Instances provide burst capacity at lower cost. Option A is wrong because increasing instance sizes for peak load leads to over-provisioning and higher costs during off-peak times. Option B is wrong because Reserved Instances require a 1- or 3-year commitment and are not cost-effective for variable traffic since they run all the time. Option D is wrong because Dedicated Hosts are expensive and unnecessary for this workload; they provide physical isolation, not cost optimization.
Variation 3. A company runs a production web application on Amazon EC2 instances behind an Application Load Balancer. The application experiences unpredictable traffic spikes. The operations team wants to optimize costs while maintaining performance during spikes. Which solution is MOST cost-effective?
medium- A.Use Spot Instances to handle the spikes.
- ✓ B.Use Compute Savings Plans for the baseline and run the spikes on On-Demand instances.
- C.Purchase Reserved Instances for the expected baseline capacity.
- D.Use Dedicated Hosts for the entire workload.
Why B: Option B is the most cost-effective solution because Compute Savings Plans offer a flexible discount (up to 66%) in exchange for a commitment to a consistent amount of compute usage ($/hour) for 1 or 3 years. This covers EC2, Lambda, and Fargate across any instance family, region, or OS. By applying Savings Plans to the baseline workload, the company can save significantly while using On-Demand instances to handle unpredictable spikes without interruption risk. Option A (Spot Instances) is incorrect because Spot instances can be interrupted with short notice, making them unsuitable for a production web application that requires high availability. Option C (Reserved Instances) is less flexible because they are tied to a specific instance family and region, which may not accommodate changes in instance types needed during spikes. Option D (Dedicated Hosts) is expensive and provides no cost optimization for variable workloads, as it bills by the host regardless of usage.
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Last reviewed: Jun 11, 2026
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