Jeff’s Note #
Unlike generic exam dumps, ADH analyzes this scenario through the lens of a Real-World Lead Developer.
For DVA-C02 candidates, the confusion often lies in how to properly manage Lambda concurrency in response to external API rate limiting. In production, this is about mastering event source mapping controls and knowing the difference between concurrency throttling and retry settings. Let’s drill down.
The Certification Drill (Simulated Question) #
Scenario #
TechNova Inc. runs an event-driven application where an Amazon Simple Queue Service (SQS) queue feeds messages to an AWS Lambda function. This function transforms the messages and then makes HTTP API calls to an external partner’s service. Recently, the volume of messages has spiked, and the partner’s API is returning frequent HTTP 429 Too Many Requests errors. This throttling causes a backlog of unprocessed messages and degrades overall throughput.
The Requirement: #
You need to fix the application so that it respects the third-party API’s documented rate limits and reduces 429 errors while maintaining the highest possible message processing throughput without losing messages.
The Options #
- A) Increase the SQS event source mapping batch size to process more messages per Lambda invocation.
- B) Configure provisioned concurrency on the Lambda function to match the third-party API’s rate limit.
- C) Increase the retry attempts and maximum event age in the Lambda function’s asynchronous invocation settings.
- D) Set the maximum concurrency on the SQS event source mapping aligned to the third-party API’s rate limits.
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Correct Answer #
D
Quick Insight: The Developer Imperative #
The key is controlling the in-flight Lambda executions triggered by SQS, so the concurrency aligns with the external API’s rate limits. Setting maximum concurrency on the event source mapping directly throttles Lambda invocations, preventing excess API calls that cause HTTP 429s.
Content Locked: The Expert Analysis #
You’ve identified the answer. But do you know the implementation details that separate a Junior from a Senior?
The Expert’s Analysis #
Correct Answer #
Option D
The Winning Logic #
Lambda functions triggered by SQS scale automatically based on the number of messages and throughput demand. However, external APIs often impose strict rate limits. Setting maximum concurrency on the event source mapping restricts the number of Lambda instances polling the queue simultaneously. This effectively caps API calls within allowed limits and reduces HTTP 429 errors by correctly respecting the partner’s rate limits.
- Setting max concurrency applies backpressure to SQS—messages stay in the queue until Lambda capacity is available.
- This approach avoids overloading the downstream API, preserving message durability without data loss.
- It is a more direct and reliable control than increasing batch size or adjusting retries.
The Trap (Distractor Analysis): #
- Why not A? Increasing batch size causes each Lambda invocation to attempt processing more messages. This can amplify throttling since more API calls happen concurrently within each batch, worsening 429 errors.
- Why not B? Provisioned concurrency pre-warms Lambda instances but does not limit concurrency to respect API rate limits; it can actually lead to more simultaneous API calls, aggravating throttling.
- Why not C? Increasing retries and max event age helps message durability but does not prevent excessive concurrent requests causing throttling—it only delays handling.
The Technical Blueprint #
# Example CLI command to set max concurrency on SQS event source mapping
aws lambda update-event-source-mapping \
--uuid <event-source-mapping-uuid> \
--maximum-concurrency <max-concurrency-aligned-to-api-limit>
The Comparative Analysis #
| Option | API Complexity | Performance Impact | Use Case |
|---|---|---|---|
| A | Low | May increase API throttling | Suitable only if API scales with batch size (not here) |
| B | Medium | Does not limit concurrent calls; may worsen 429s | Used for startup latency improvements, not throttling control |
| C | Medium | Improves durability but no throttling control | Useful for async error handling, not rate limit management |
| D | Low | Controls concurrency, reduces 429 errors | Best for aligning Lambda calls with API rate limits |
Real-World Application (Practitioner Insight) #
Exam Rule #
For the exam, always pick maximum concurrency on SQS event source mappings when dealing with third-party API rate limits returning HTTP 429 errors.
Real World #
In real projects, you might combine max concurrency with exponential backoff retry logic in the Lambda function to elastically handle spikes without overwhelming the external API.
(CTA) Stop Guessing, Start Mastering #
Disclaimer
This is a study note based on simulated scenarios for the DVA-C02 exam.