AWS DVA-C02 Drill: Event-Driven vs Scheduled Lambda Invocation - Choosing Cost-Effective Automation

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 when to trigger serverless functions: event-driven vs scheduled invocation. In production, this is about knowing exactly how to balance cost and functional correctness when automating Lambda start times without unnecessary overhead or poor scalability. Let’s drill down.

The Certification Drill (Simulated Question)

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Scenario

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TechSolutions Inc. operates a regional chain of retail stores. Each store uploads its daily sales data to a centralized Amazon S3 bucket at a fixed time every day. TechSolutions has created an AWS Lambda function designed to process all uploaded sales reports collectively and then writes summary analytics to a DynamoDB table. They want the analysis process to begin once daily at a predetermined time, after all stores have uploaded their reports.

The Requirement:

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Design the MOST cost-effective solution to invoke the Lambda function once daily at the specified time.

The Options

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• A) Configure an S3 event notification to invoke the Lambda function every time a store uploads a sales report.
• B) Create an AWS Step Functions state machine that invokes the Lambda function once daily at the scheduled time.
• C) Configure the Lambda function to run continuously and internally check the time to start processing only at the specified time daily.
• D) Create an Amazon EventBridge scheduled rule that invokes the Lambda function once daily at the predetermined time.

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Correct Answer

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D

Quick Insight: The Developer Imperative

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• Lambda functions should be triggered by the most appropriate event source to optimize cost and efficiency.
• Running Lambda “continuously” is not possible and even if simulated, it is highly inefficient and expensive.
• Polling S3 upload events for each file will invoke Lambda multiple times, complicating aggregation logic.
• Using EventBridge scheduled triggers provides a straightforward, cost-effective and scalable mechanism for predictable, time-based Lambda invocation.

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 D

The Winning Logic

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Option D uses an Amazon EventBridge scheduled rule to trigger the Lambda function at a precise, predefined time once per day. This approach is cost-effective because:

• The Lambda function runs only once per day, avoiding multiple executions that would result from event-driven triggers from each individual S3 object upload.
• EventBridge scheduled events provide native cron-like scheduling with no need to maintain custom timer logic.
• It keeps processing logic simple: collect all reports from S3 uploaded before the scheduled time, then run one analysis pass.
• Lambda functions scale on invocation demand; running them continuously (Option C) is not possible or recommended.
• Step Functions (Option B) adds orchestration complexity and cost that aren’t justified for a simple daily trigger.

The Trap (Distractor Analysis)

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• Why not A?
  Invoking Lambda on every S3 PUT means one execution per report—complicating aggregation, increasing costs, and risking race conditions. You need aggregation logic external to Lambda invocations.

• Why not B?
  Step Functions can schedule via EventBridge but add additional costs and complexity. It’s overkill for a single invocation without multi-step orchestration.

• Why not C?
  Lambda does not support continuous execution or internal timers to wait until a schedule. This would dramatically increase cost or require unsuitable workarounds.

The Technical Blueprint

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Code snippet: EventBridge Rule to Invoke Lambda Daily

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aws events put-rule \
  --name "DailyLambdaTrigger" \
  --schedule-expression "cron(0 20 * * ? *)"

aws lambda add-permission \
  --function-name AnalyzeSalesReports \
  --statement-id "AllowEventBridgeInvoke" \
  --action "lambda:InvokeFunction" \
  --principal events.amazonaws.com \
  --source-arn arn:aws:events:<region>:<account-id>:rule/DailyLambdaTrigger

aws events put-targets \
  --rule "DailyLambdaTrigger" \
  --targets "Id"="1","Arn"="arn:aws:lambda:<region>:<account-id>:function:AnalyzeSalesReports"

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 pick EventBridge scheduled rules when you need to start Lambda functions on a fixed schedule.

Real World

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In production, if report uploads vary, you might combine EventBridge for scheduling and S3 event triggers with DynamoDB for incremental updates, but cost and complexity grow accordingly.

(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.