AWS DVA-C02 Drill: Lambda Event Invocation - Understanding S3 Event Integration

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 DVA-C02 candidates, the confusion often lies in how event sources like S3 communicate directly with Lambda functions and the role of permissions in the invocation process. In production, this is about knowing exactly how resource-based policies and event notification limitations impact whether your Lambda function runs as expected. Let’s drill down.

The Certification Drill (Simulated Question)

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Scenario

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TechNova, an online marketing startup, has built a system to automate processing new media uploads. They created an AWS Lambda function designed to send alerts through Amazon SNS whenever a new media file larger than 50 MB is added to their media-uploads S3 bucket. The developer verified the Lambda function locally using the AWS CLI test event. However, after configuring S3 event notifications on the bucket, uploading a large 3,000 MB media file does not trigger the Lambda function.

The Requirement:

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Determine the most likely cause why the Lambda function wasn’t triggered upon uploading the large file.

The Options

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• A) Amazon S3 does not send event notifications for files larger than 1,000 MB.
• B) The Lambda function’s resource-based policy lacks permission allowing S3 to invoke it.
• C) AWS Lambda functions cannot be triggered directly by S3 event notifications.
• D) The S3 bucket must be set to public to allow event notifications to invoke Lambda.

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Correct Answer

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B

Quick Insight: The Developer Imperative

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Understanding AWS Lambda invocation is critical. While S3 can trigger Lambda on object creation events, this requires appropriate resource-based permissions allowing S3 to invoke the Lambda function. Without these, the function won’t launch regardless of file size or bucket visibility.

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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Option B

The Winning Logic

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The core reason the Lambda function does not invoke despite uploading such a large file is the lack of proper permissions. When you configure S3 as an event source to invoke Lambda, a resource-based policy must explicitly grant S3 permission to invoke the Lambda function. If this permission is missing, the invoke request from S3 is denied silently and the function never triggers.

Here’s why:

• S3 event notifications trigger Lambda asynchronously but the call is a direct invoke made by the S3 service.
• Lambda resource-based policies control which services or principals may invoke your function.
• Specifically, you need a permission statement like this attached to your Lambda function policy:

{
  "Sid": "AllowS3Invoke",
  "Effect": "Allow",
  "Principal": {
    "Service": "s3.amazonaws.com"
  },
  "Action": "lambda:InvokeFunction",
  "Resource": "arn:aws:lambda:region:account-id:function:FunctionName",
  "Condition": {
    "ArnLike": {
      "AWS:SourceArn": "arn:aws:s3:::media-uploads"
    }
  }
}

Without this policy, S3’s invocation attempt is rejected regardless of file size.

The Trap (Distractor Analysis):

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• Why not A?
  S3 event notifications are not limited by the size of the uploaded object. You can trigger Lambda for objects larger than 1,000 MB without issue, as long as permissions and configurations are correct.

• Why not C?
  AWS Lambda functions can indeed be directly invoked by S3 event notifications. This is a common, documented integration pattern.

• Why not D?
  S3 bucket visibility (public vs private) does not impact event notification delivery. Event notifications work on private buckets, provided credentials and permissions are set up correctly.

The Technical Blueprint

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Relevant AWS CLI Command to Add Permission for S3 Invocation

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aws lambda add-permission --function-name MediaAlertFunction \
  --statement-id AllowS3Invoke \
  --action "lambda:InvokeFunction" \
  --principal s3.amazonaws.com \
  --source-arn arn:aws:s3:::media-uploads

This command lets your Lambda function trust S3 events from the specific bucket.

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 confirm your Lambda function has an explicit resource-based policy allowing the triggering service—like S3—to invoke it.”

Real World

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“In real projects, forgetting to update Lambda permissions when hooking up a new event source leads to silent failures that are hard to debug. Proper automation and validation of these permissions in deployment pipelines save headaches.”

(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.