SAA-C03 Design Cost-Optimized Architectures Practice Question
A company runs a REST API on AWS Lambda behind Amazon API Gateway. The API is used by internal clients during a two-hour batch window each night and is completely idle the rest of the day. The team is concerned about the cost of API Gateway and wants to minimize it without changing the API contract for clients. Which change should a solutions architect recommend?
⚠ Common exam trap
The trap here is reaching for performance-oriented features such as provisioned concurrency or API caching when the workload is idle most of the day and the actual goal is to reduce per-request and idle-time cost.
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 an HTTP API instead of the REST API so requests are billed at the lower HTTP API rate.
The workload is bursty and idle most of the day, so a per-request pricing model is the right foundation and provisioned or always-on resources should be avoided. Moving from a REST API to an HTTP API lowers the per-request charge while keeping the same client-facing contract, so the nightly batch pays less and the idle hours cost nothing. Caching and provisioned concurrency would both add continuous charges.
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 an HTTP API instead of the REST API so requests are billed at the lower HTTP API rate.
Why this is correct
HTTP APIs cost substantially less per million requests than REST APIs while preserving the request-response contract for clients. Because the API is only invoked during the nightly batch window, the pay-per-request model means near-zero cost during the 22 idle hours, and the lower per-request price directly reduces spend without altering client behavior.
- ✗
Replace API Gateway and Lambda with an Application Load Balancer forwarding to an Auto Scaling group of EC2 instances.
Why it's wrong here
An Application Load Balancer is billed hourly and there are data processing charges, and the EC2 instances would need to run or scale up for the nightly window. This trades a pay-per-request serverless model for always-on infrastructure, which is more expensive for a workload that only runs two hours per day.
- ✗
Keep the REST API but enable API caching with a one-hour time-to-live.
Why it's wrong here
API Gateway caching charges an hourly rate based on the cache size and is billed whether or not requests arrive. Because the API is idle for most of the day, the cache would sit unused and generate continuous charges, making the total cost higher than paying per request during the short nightly batch window.
- ✗
Switch the API from a REST API to an HTTP API and configure the Lambda function with provisioned concurrency.
Why it's wrong here
HTTP APIs are cheaper per request than REST APIs and are a valid modernization, but provisioned concurrency keeps execution environments warm continuously and bills for that readiness even during the 22 idle hours each day. For a batch-window workload, provisioned concurrency adds cost rather than removing it and does not address the idle-time pricing model.
Quick reference
Cloud Service Model Comparison
| Model | You Manage | Provider Manages | Examples |
|---|---|---|---|
| IaaS | OS, runtime, apps, data | Hardware, hypervisor, networking | EC2, Azure VMs, GCP Compute Engine |
| PaaS | Apps and data | OS, runtime, middleware, hardware | Elastic Beanstalk, Azure App Service |
| SaaS | Data and settings only | Everything else | Microsoft 365, Salesforce, Workday |
| FaaS / Serverless | Function code only | Infra, scaling, runtime | Lambda, Azure Functions, Cloud Run |
| CaaS | Containers and apps | Kubernetes, OS, hardware | EKS, AKS, GKE |
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Amazon Web Services exam blueprint
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