AWS DVA-C02 Drill: Lambda & ALB Integration - Permission Pitfalls

Jeff’s Note

#

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 understanding the invocation permissions required when registering Lambda as an ALB target. In production, this is about knowing exactly how Lambda integrates with ALB beyond just registration, especially IAM permissions for invocation. Let’s drill down.

The Certification Drill (Simulated Question)

#

Scenario

#

BrightApps Inc., a SaaS startup specializing in real-time analytics, wants to route HTTP POST requests through an Application Load Balancer (ALB) directly to a Lambda function for serverless processing. The lead developer uses the AWS CLI to register the Lambda function as a target group for the ALB. However, when users send POST requests via the ALB endpoint, the Lambda function is never invoked, and no errors are logged in the Lambda console.

The Requirement:

#

Identify why the Lambda function isn’t triggered when requests come through the ALB, despite successful target registration using the CLI.

The Options

#

• A) A Lambda function cannot be registered as a target for an ALB.
• B) A Lambda function can be registered with an ALB using the AWS Management Console only.
• C) The permissions to invoke the Lambda function are missing.
• D) Cross-zone load balancing is not enabled on the ALB.

Google adsense

#

leave a comment:

Correct Answer

#

C) The permissions to invoke the Lambda function are missing.

Quick Insight: The Developer Imperative

#

Lambda integration with ALB requires explicit permission allowing the ALB service to invoke the Lambda function. Without that, the ALB cannot trigger the function even if the target group registration succeeds.

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 C

The Winning Logic

#

Lambda functions can be registered as targets for ALB, both via CLI and Console, so A and B are incorrect. Cross-zone load balancing affects request distribution among ALB nodes, but does not prevent invocation, so D is a red herring. The key detail is that the Lambda function needs permissions granted to the ALB’s Elastic Load Balancing service principal (i.e., elasticloadbalancing.amazonaws.com) to invoke it. This is done by adding a resource-based policy to the Lambda function’s permissions.

Without this permission, the ALB target registration appears to succeed, but runtime invocation silently fails because the service lacks invoke rights. This often trips up developers who only associate Lambda permissions with API Gateway or direct invocations through SDKs.

The Trap (Distractor Analysis):

#

• Why not A? Lambda is supported as an ALB target since late 2018 — this is a common misunderstanding.
• Why not B? CLI and CloudFormation support Lambda targets equally; no console-only limitation exists.
• Why not D? Cross-zone load balancing is unrelated to Lambda invocation; it affects traffic routing among ALB nodes, not whether Lambda triggers.

The Technical Blueprint

#

Developer CLI Command to Add Lambda Invoke Permission for ALB

#

aws lambda add-permission \
  --function-name MyProcessingLambda \
  --statement-id alb-invoke-permission \
  --action lambda:InvokeFunction \
  --principal elasticloadbalancing.amazonaws.com \
  --source-arn arn:aws:elasticloadbalancing:region:account-id:loadbalancer/app/my-alb/50dc6c495c0c9188 \
  --region us-east-1

Replace the placeholders accordingly. This resource-based policy enables the ALB to invoke the Lambda securely.

The Comparative Analysis

#

Real-World Application (Practitioner Insight)

#

Exam Rule

#

For the exam, always verify resource-based Lambda permissions when integrating with services like ALB and API Gateway.

Real World

#

In production, some teams forget to automate Lambda permission addition as part of infrastructure deployment, which blocks integrations silently. Using Infrastructure as Code (e.g., CloudFormation or Terraform) reduces this risk.

(CTA) Stop Guessing, Start Mastering

#

Disclaimer

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