SOA-C02 Cost and Performance Optimization Practice Question

A company runs an e-commerce application on Amazon EC2 instances behind an Auto Scaling group. The application has a predictable baseline load from 8 AM to 8 PM daily and low load overnight. The SysOps administrator wants to optimize costs while ensuring sufficient capacity for the baseline load. Which purchasing option and scaling strategy should the administrator use?

⚠ Common exam trap

Test-takers frequently assume Spot Instances are always the cheapest option, but they fail to recognize that the predictable baseline load requires guaranteed availability, which Spot Instances cannot provide due to potential interruptions.

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 Reserved Instances for the predicted baseline and On-Demand for any unexpected spikes.

Reserved Instances provide a significant discount (up to 72%) over On-Demand for predictable, steady-state workloads like the 8 AM to 8 PM baseline. On-Demand instances then cover any unexpected spikes without requiring upfront commitment, ensuring cost optimization while maintaining capacity for the predictable load.

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 On-Demand instances for the baseline and Spot Instances for any additional capacity.
Why it's wrong here
This mix inverts the appropriate usage: the steady-state baseline should be covered by Reserved Instances to capture the largest discount, while the unpredictable spike capacity should use On-Demand to ensure availability. Paying full On-Demand price for the always-on portion forfeits up to 72% savings, and using Spot for surges is risky because Spot Instances can be reclaimed with a two-minute warning—exactly when you need capacity most.
✓
Use Reserved Instances for the predicted baseline and On-Demand for any unexpected spikes.
Why this is correct
Reserved Instances should back the predictable baseline because they offer a substantial discount (up to 72% compared to On-Demand) for a commitment you know you will use, while On-Demand covers unexpected spikes without requiring a long-term contract. This combination minimizes cost on the steady-state load while retaining the flexibility to launch extra capacity at any moment, and it avoids the interruption risk of Spot for the mission-critical spikes.
✗
Use Dedicated Hosts for all instances to maximize cost savings.
Why it's wrong here
Dedicated Hosts are physical EC2 servers reserved for your use, and while they help with licensing or compliance requirements, they are significantly more expensive than Reserved Instances because you pay for the entire host even when instances are idle. For a standard e-commerce workload there is no software licensing or regulatory need for dedicated hardware, so this choice would increase costs rather than maximize savings.
✗
Use Spot Instances for all instances to minimize costs.
Why it's wrong here
Spot Instances are the cheapest purchasing option, but they are interruptible and can be reclaimed by EC2 with as little as two minutes' notice when AWS needs the capacity. A predictable baseline workload that requires high availability cannot tolerate these interruptions, and using Spot for all instances would put the e-commerce site at risk of sudden shutdowns and lost transactions.

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SOA-C02 Cost and Performance Optimization Practice Question

Why each option matters

Option-by-option breakdown

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