AWS DVA-C02 Drill: SQS and Lambda Integration - Handling Duplicate Messages

Jeff’s Note

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Unlike generic exam dumps, ADH analyzes this scenario through the lens of a Real-World Lead Developer.

For AWS DVA-C02 candidates, the confusion often lies in understanding SQS message delivery semantics combined with Lambda invocation timeouts. In production, this is about knowing exactly how SQS visibility timeout and Lambda timeouts interplay, triggering potential duplicate message processing. Let’s drill down.

The Certification Drill (Simulated Question)

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Scenario

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At CloudVana, an enterprise SaaS provider, the backend processes user notifications by reading messages from an Amazon Simple Queue Service (SQS) standard queue. A Lambda function is triggered to poll messages from the queue and send transactional emails via Amazon Simple Email Service (SES). Recently, during spikes in user notification requests, customers have reported receiving duplicate emails.

The Requirement:

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Identify the most likely technical reasons behind the duplicate email deliveries to prevent future occurrences.

The Options

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• A) Standard SQS queues guarantee at-least-once message delivery, which can cause duplicates under load.
• B) Standard SQS provides exactly-once processing, so duplicates are due to application logic errors.
• C) Amazon SES DomainKeys Identified Mail (DKIM) signing is misconfigured, causing message duplication.
• D) The SQS queue’s visibility timeout is less than or equal to the Lambda function’s execution timeout.
• E) Amazon SES bounce rate is excessively high, resulting in repeated attempts to send the same email.

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Correct Answer

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A and D

Quick Insight: The Developer Imperative

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• Amazon SQS Standard queues guarantee at-least-once delivery which means duplicates can occur naturally.
• Lambda functions processing messages can inadvertently cause duplicates if the function timeout exceeds or matches the SQS visibility timeout, causing messages to become visible and retried before the Lambda completes successfully.

Content Locked: The Expert Analysis

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You’ve identified the answer. But do you know the implementation details that separate a Junior from a Senior?

The Expert’s Analysis

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Correct Answer

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Options A and D

The Winning Logic

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• Option A:
  Amazon SQS Standard queues do not guarantee exactly-once delivery; instead, they guarantee at-least-once delivery. Network glitches, Lambda retries, or client-side polling can cause the same message to be delivered multiple times. This is by design to ensure reliability and availability, especially under high traffic.

• Option D:
  If the visibility timeout of an SQS message is set too low (less than or equal to the Lambda’s maximum execution timeout), a Lambda function might still be processing a message when SQS makes it visible again. This causes the function to pick up and process a message twice, resulting in duplicate emails.

The Trap (Distractor Analysis):

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• Option B:
  This is incorrect as Standard SQS queues do not support exactly-once processing. Only FIFO queues have exactly-once semantics.

• Option C:
  DKIM misconfiguration would cause email authentication failures or spam issues, not duplicate sending.

• Option E:
  A high bounce rate impacts SES reputation and throttling but does not cause duplicated email sends inherently.

The Technical Blueprint

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Code Snippet Illustrating Lambda Timeout and Visibility Timeout Setup

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# Check current visibility timeout (default 30 seconds)
aws sqs get-queue-attributes --queue-url https://sqs.us-east-1.amazonaws.com/123456789012/MyQueue --attribute-names VisibilityTimeout

# Set visibility timeout for 60 seconds
aws sqs set-queue-attributes --queue-url https://sqs.us-east-1.amazonaws.com/123456789012/MyQueue --attributes VisibilityTimeout=60

# Ensure Lambda function timeout is less than visibility timeout (e.g., 55 seconds)

The Comparative Analysis

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Real-World Application (Practitioner Insight)

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Exam Rule

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For the exam, always remember: Standard SQS = at-least-once delivery semantic. Expect duplicates when using Standard queues.

Real World

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In production, to avoid duplicates in critical email workflows, engineers often switch to FIFO queues or implement idempotency in the Lambda function and downstream components.

(CTA) Stop Guessing, Start Mastering

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Disclaimer

This is a study note based on simulated scenarios for the AWS DVA-C02 exam.