Jeff’s Note #
Unlike generic exam drills, ADH digs into this scenario from the viewpoint of a Lead Developer focusing on AWS Lambda and SQS integration.
For AWS DVA-C02 candidates, the common confusion lies in understanding how concurrency is managed with event source mappings from SQS and how that interacts with Lambda’s own concurrency controls. In production, it’s about knowing precisely how to throttle calls to external APIs without sacrificing throughput or causing message delays. Let’s drill in.
The Certification Drill (Simulated Question) #
Scenario #
Horizon Analytics, a data insights startup, uses an AWS Lambda function to process customer data ingested via an Amazon SQS standard queue. Each Lambda invocation reads messages from the queue and calls a third-party analytics API over HTTP to enrich data. However, the API provider enforces a strict limit of two concurrent requests from Horizon Analytics to avoid service degradation.
The Requirement: #
Ensure the Lambda function processing the SQS messages does not exceed two concurrent API calls at any time, effectively limiting concurrency without causing message loss or undue delay.
The Options #
- A) Configure provisioned concurrency of two on the Lambda function.
- B) Configure a batch size of two on the Amazon SQS event source mapping for the Lambda function.
- C) Configure Lambda event filtering to process two messages from Amazon SQS at every invocation.
- D) Configure a maximum concurrency of two on the Amazon SQS event source mapping for the Lambda function.
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Correct Answer #
D) Configure a maximum concurrency of two on the Amazon SQS event source mapping for the Lambda function.
Quick Insight: The Developer’s Imperative #
Lambda functions triggered by SQS pollers can scale out to many concurrent instances depending on the queue volume and batch size. Controlling the Maximum Concurrency setting on the event source mapping is the AWS-native way to limit how many Lambda function instances run simultaneously. This ensures you never exceed the hard API concurrency limit.
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 #
The key to controlling concurrent API calls invoked by Lambda processing SQS lies in Lambda’s event source mapping configuration:
- The maximum concurrency property on the SQS event source mapping explicitly throttles how many Lambda instances can be invoked concurrently for that queue.
- Setting this to 2 ensures only two function executions at once, matching the third-party API concurrency limit.
- This throttle happens at the poller level, so messages beyond concurrency capacity wait in SQS until a Lambda slot frees up.
- This approach integrates cleanly with Lambda’s scaling model and avoids unnecessary simultaneous HTTP calls.
The Trap (Distractor Analysis): #
-
Why not A? Provisioned concurrency keeps a fixed number of Lambda execution environments ready to reduce cold starts. It does not throttle concurrent executions triggered by SQS. It might keep environments warm but cannot limit concurrency to two.
-
Why not B? Batch size controls how many messages the function reads per invocation, not total concurrent executions. Even if batch size is 2, multiple concurrent Lambda executions could happen, exceeding the API limit.
-
Why not C? Lambda event filtering applies to filtering messages from event sources like SNS or EventBridge, but not to batch size or concurrency control for SQS triggers.
The Technical Blueprint #
# CLI snippet to set maximum concurrency on SQS Event Source Mapping for the Lambda function
aws lambda update-event-source-mapping \
--uuid <mapping-uuid> \
--maximum-concurrency 2
The Comparative Analysis #
| Option | API Complexity | Performance | Use Case |
|---|---|---|---|
| A | Low — Property unrelated to concurrency | Keeps environments warm, no concurrency limit | Reduces cold starts but not suited for throttling |
| B | Low — Batch size controls messages per invocation | Might be inefficient if concurrency unknown | Useful to optimize message processing throughput but not concurrency cap |
| C | Not applicable — Event filtering not for concurrency | No direct control on requests | Invalid use case for SQS + Lambda concurrency control |
| D | Medium — Requires managing event source mapping | Precise concurrency limit enforcement | Best fit to throttle concurrent Lambda executions, suited for external API limits |
Real-World Application (Practitioner Insight) #
Exam Rule #
For the exam, always choose Maximum Concurrency on the Event Source Mapping when you need to control concurrency limits of Lambda functions triggered by SQS.
Real World #
In production, you might combine this with adaptive batch sizes and retries to balance throughput with API rate limits and cost controls.
(CTA) Stop Guessing, Start Mastering #
Disclaimer
This is a study note based on simulated scenarios for the AWS DVA-C02 exam.