SAP-C02 Design for New Solutions Practice Question
A company is designing a serverless application using AWS Lambda functions that process messages from an Amazon SQS queue. The Lambda function sometimes experiences throttling, causing messages to be sent to the dead-letter queue (DLQ). The company wants to minimize throttling and ensure that messages are processed in order. What should the solutions architect do?
⚠ Common exam trap
Candidates often confuse provisioned concurrency (which reduces cold starts) with reserved concurrency (which guarantees execution capacity), and assuming standard queues can maintain order when they only offer best-effort ordering.
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 FIFO SQS queue and configure the Lambda function with reserved concurrency.
Using an SQS FIFO queue guarantees strict message ordering, and reserved concurrency on the Lambda function prevents throttling by allocating a fixed number of concurrent executions exclusively for this function. This combination ensures messages are processed in order without being throttled, avoiding unnecessary DLQ deliveries.
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 a standard SQS queue and configure a Lambda function with a higher concurrency limit.
Why it's wrong here
Standard queues do not guarantee ordering.
- ✗
Use an SQS FIFO queue with provisioned concurrency on the Lambda function.
Why it's wrong here
Provisioned concurrency is for Lambda, but for SQS-triggered functions, reserved concurrency is appropriate.
- ✗
Increase the batch size in the Lambda event source mapping and use a standard queue.
Why it's wrong here
Larger batch size does not prevent throttling and does not guarantee order.
- ✓
Use a FIFO SQS queue and configure the Lambda function with reserved concurrency.
Why this is correct
FIFO queues preserve order. Reserved concurrency prevents throttling by ensuring enough capacity.
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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