AWS DVA-C02 Drill: Lambda Event Handling - Understanding S3 Event Retries on Timeout

Jeff’s Note

#

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 how asynchronous event sources such as S3 trigger Lambda retries and what happens on timeout exceptions. In production, this is about knowing exactly whether Lambda automatically retries, discards, or routes failed events to a dead letter queue when configured with default settings. Let’s drill down.

The Certification Drill (Simulated Question)

#

Scenario

#

At NextGenForms, an innovative SaaS company specializing in digital document workflows, users upload scanned forms to a central Amazon S3 bucket via a responsive web interface. Upon each new file arrival, an AWS Lambda function asynchronously processes the documents—extracting metadata and indexing them for search. Recently, the development team noticed that some Lambda invocations intermittently time out under heavier workloads.

By default, the Lambda function is deployed with no special error handling or dead letter queue. The team wants to understand what happens to the incoming S3 event notification when their Lambda execution times out before completing processing.

The Requirement:

#

Determine the behavior of the S3 bucket event notification that triggers the Lambda function when the Lambda times out, given default configuration and no custom error handling settings.

The Options

#

• A) A notification about the failed event is automatically sent as an email through Amazon SNS.
• B) The S3 event is sent to the default Dead Letter Queue configured for the Lambda function.
• C) The S3 event is retried by Lambda multiple times until processing succeeds.
• D) The event is discarded by Lambda after two retry attempts and not processed further.

Google adsense

#

leave a comment:

Correct Answer

#

D) The event is discarded by Lambda after two retry attempts and not processed further.

Quick Insight: The Developer Imperative

#

When Lambda is invoked asynchronously by S3 and the function times out, Lambda automatically retries the invocation twice. If the function still fails, the event is dropped by default unless you configure a Dead Letter Queue (DLQ) or use asynchronous invocation destinations.

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

#

When S3 triggers a Lambda function asynchronously, Lambda handles retry attempts automatically. Specifically, Lambda retries twice upon failure (including timeouts), making for up to three total invocation attempts. If after these retries the function continues to fail, Lambda silently discards the event unless custom error handling is configured.

• Lambda does NOT send failure notifications directly via SNS unless you explicitly code this or set up destinations.
• There is no “default Dead Letter Queue” automatically set for Lambda functions; DLQs must be explicitly configured.
• Lambda retries exactly twice after the initial failure for asynchronous invocation events, not indefinitely.
• Once all retries are exhausted without success, the event disappears from the processing pipeline, meaning it’s dropped.

The Trap (Distractor Analysis):

#

• Why not A? SNS notification for failure is not automatic; this requires explicit setup of Lambda Destinations or SNS-based error handling.
• Why not B? Lambda functions do not have a default DLQ enabled; it must be manually attached.
• Why not C? Lambda retries twice and then stops; it does not retry infinitely until success.

The Technical Blueprint

#

Developer-Focused Code Snippet (CLI to configure DLQ)

#

Here’s how you’d explicitly add a Dead Letter Queue to your Lambda function to capture failed asynchronous events:

aws lambda update-function-configuration \
  --function-name ProcessDocumentFunction \
  --dead-letter-config TargetArn=arn:aws:sqs:us-east-1:123456789012:MyDeadLetterQueue

By adding this config, failed events after retries will be sent to the SQS queue instead of being discarded.

The Comparative Analysis

#

Real-World Application (Practitioner Insight)

#

Exam Rule

#

“For the exam, always remember that Lambda asynchronous invocations have two retries by default, and events are discarded unless you configure DLQs or destinations.”

Real World

#

“In production, we rarely allow event loss—DLQs or Lambda Destinations are essential for reliability, error tracking, and alerting.”

(CTA) Stop Guessing, Start Mastering

#

Disclaimer

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