SOA-C02 Cost and Performance Optimization Practice Question

A company runs a web application on EC2 instances with Elastic Load Balancing. They notice that costs are higher than expected. What is the MOST cost-effective way to optimize costs while maintaining high availability?

⚠ Common exam trap

A common mix-up: candidates confuse Spot Instances with a general cost-saving answer — candidates forget that Spot's interruption model makes it unsuitable when the question explicitly requires high availability.

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 combination of Reserved Instances for baseline traffic and On-Demand for spikes.

Reserved Instances provide a significant discount (up to ~72% versus On-Demand) in exchange for a one- or three-year commitment, making them ideal for the predictable baseline load. On-Demand instances then absorb traffic spikes without a long-term commitment, so the workload stays highly available while the blended cost is minimized. This hybrid model is the standard AWS cost-optimization pattern for steady-state web tiers behind an ELB.

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 Spot Instances for all traffic.
Why it's wrong here
Spot Instances can provide up to a 90% discount, but they are interruptible: EC2 can reclaim them with a two-minute warning when it needs the capacity. Running all traffic on Spot means any recycling of capacity would withdraw instances from the web tier, causing immediate availability gaps or complete downtime. For a production web application that must remain responsive, relying solely on Spot is unacceptable because the risk of interruption is inherent, not an edge case.
✗
Use On-Demand instances for all traffic.
Why it's wrong here
On-Demand Instances give you per-second billing without any upfront commitment, which is convenient, but it is also the most expensive per-hour pricing model. Paying On-Demand rates 24/7 for the predictable baseline portion of the workload means you forfeit the savings offered by Reserved Instances or Savings Plans. Because the baseline is constant and known, running it on On-Demand is a waste of cost optimization opportunity, even though On-Demand is appropriate for the unpredictable spike portion.
✓
Use a combination of Reserved Instances for baseline traffic and On-Demand for spikes.
Why this is correct
This is the correct approach because Reserved Instances provide a significant hourly discount in exchange for a one- or three-year commitment, making them ideal for the steady, predictable baseline web traffic. On-Demand Instances, though more expensive, are billed per second with no commitment, so they are perfect for absorbing temporary spikes without over-provisioning or risking capacity loss. By covering the minimum long-term load with Reserved Instances and adding On-Demand Instances only when load increases, you minimize spend while maintaining availability and elasticity — the core goal of cost-aware architecture.
✗
Reduce the number of instances to one.
Why it's wrong here
Reducing the web application to a single EC2 instance eliminates redundant capacity, creating a single point of failure. If that instance crashes, experiences instance health degradation, or its Availability Zone fails, the application becomes unreachable until manual recovery. It also leaves no headroom for even small traffic bursts, so normal load fluctuation could saturate the instance and cause latency spikes or HTTP 503 errors. This configuration sacrifices both high availability and scalability, which are foundational requirements for a production web application.

Go deeper

SOA-C02 Cost and Performance Optimization Practice Question

Why each option matters

Option-by-option breakdown

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