AWS DVA-C02 Drill: Lambda Traffic Shifting - Alias Weighted Routing vs. Triggers

Jeff’s Note

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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 mixing up Lambda triggers, proxy integrations, and alias-based traffic routing. In production, this is about knowing exactly which Lambda construct supports native weighted traffic distribution. Let’s drill down.”

The Certification Drill (Simulated Question)

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Scenario

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A software engineering team at StreamWave Media is deploying a critical bug fix to their video transcoding Lambda function that processes thousands of requests daily. The lead developer has successfully validated the patched code in the staging environment. To minimize risk, the team wants to implement a canary deployment strategy where only 10% of production traffic initially hits the new code version, while 90% continues using the stable version. If no errors are detected after monitoring metrics for 24 hours, they’ll shift 100% of traffic to the new version.

The Requirement:

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Implement a gradual rollout mechanism that splits traffic between two Lambda function versions (90% to stable, 10% to new) without requiring changes to client applications or invoking services.

The Options

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• A) Update the Lambda code and create a new version of the Lambda function. Create a Lambda function trigger. Configure the traffic weights in the trigger between the two Lambda function versions. Send 90% of the traffic to the production version, and send 10% of the traffic to the new version.
• B) Create a new Lambda function that uses the updated code. Create a Lambda alias for the production Lambda function. Configure the Lambda alias to send 90% of the traffic to the production Lambda function, and send 10% of the traffic to the Test Lambda function.
• C) Update the Lambda code and create a new version of the Lambda function. Create a Lambda proxy integration. Configure the Lambda proxy to split traffic between the two Lambda function versions. Send 90% of the traffic to the Production version, and send 10% of the traffic to the new version.
• D) Update the Lambda code and create a new version of the Lambda function. Create a Lambda function alias. Configure the traffic weights in the Lambda alias between the two Lambda function versions. Send 90% of the traffic to the Production version, and send 10% of the traffic to the new version.

Correct Answer

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Option D.

Quick Insight: The Developer Deployment Imperative

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For DVA-C02: This tests your understanding of Lambda’s native traffic shifting capabilities. The exam expects you to know that Lambda Aliases support weighted routing between versions through the RoutingConfig parameter in the AWS Lambda API. Triggers are event sources (S3, DynamoDB Streams), not traffic routing mechanisms. Proxy integrations relate to API Gateway, not Lambda version management.

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: Update Lambda code → Create new version → Create alias → Configure traffic weights in alias

The Winning Logic

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This solution correctly leverages Lambda Aliases with weighted routing, which is the AWS-native mechanism for canary and blue/green deployments:

• Lambda Versions: When you update function code and publish, AWS creates an immutable version (e.g., version 5 becomes your new code, version 4 remains the stable production code).
• Lambda Alias: Acts as a pointer to one or more versions. The alias ARN remains constant, so clients don’t need updates.
• RoutingConfig API Parameter: The update-alias command accepts --routing-config to specify AdditionalVersionWeights, enabling traffic splitting:

aws lambda update-alias \
  --function-name videoTranscoder \
  --name PROD \
  --function-version 5 \
  --routing-config AdditionalVersionWeights={"4"=0.9}

This configuration sends 10% of invocations to version 5 (the alias’s primary target) and 90% to version 4.

• CloudWatch Metrics: Lambda automatically publishes separate metrics for each version behind the alias, allowing you to compare error rates and duration between canary and stable.

The Trap (Distractor Analysis):

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• Why not Option A (Lambda Triggers)?
  Lambda triggers are event sources (S3 bucket notifications, EventBridge rules, SQS queues), not traffic management tools. Triggers define what invokes the function, not how to split invocations between versions. No traffic-weights parameter exists in trigger configurations.

• Why not Option B (Separate Lambda Functions)?
  Creating a new function violates the immutability principle of versions. You’d have two separate functions with different ARNs, requiring client-side routing logic. Additionally, aliases can only route between versions of the same function, not between two different functions.

• Why not Option C (Lambda Proxy Integration)?
  Lambda proxy integration is an API Gateway feature that forwards the entire HTTP request to Lambda as a JSON event. It’s an invocation pattern, not a deployment strategy. API Gateway can perform traffic splitting between Lambda functions using stage variables, but that’s not what this option describes—a “Lambda proxy” doesn’t configure traffic weights between versions.

The Technical Blueprint

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Developer Implementation Pattern (AWS CLI + SDK):

# Step 1: Update function code
aws lambda update-function-code \
  --function-name videoTranscoder \
  --zip-file fileb://function-v2.zip

# Step 2: Publish new version
NEW_VERSION=$(aws lambda publish-version \
  --function-name videoTranscoder \
  --description "Bug fix for H.264 encoder" \
  --query 'Version' --output text)

# Step 3: Update alias with weighted routing
aws lambda update-alias \
  --function-name videoTranscoder \
  --name PROD \
  --function-version $NEW_VERSION \
  --routing-config AdditionalVersionWeights={"4"=0.9}

# Step 4: Monitor CloudWatch metrics
aws cloudwatch get-metric-statistics \
  --namespace AWS/Lambda \
  --metric-name Errors \
  --dimensions Name=FunctionName,Value=videoTranscoder:PROD \
  --start-time 2025-02-15T00:00:00Z \
  --end-time 2025-02-15T23:59:59Z \
  --period 3600 \
  --statistics Sum

# Step 5: Shift 100% traffic after validation
aws lambda update-alias \
  --function-name videoTranscoder \
  --name PROD \
  --function-version $NEW_VERSION \
  --routing-config AdditionalVersionWeights={}

Equivalent SDK Code (Python Boto3):

import boto3

lambda_client = boto3.client('lambda')

# Publish new version
response = lambda_client.publish_version(
    FunctionName='videoTranscoder',
    Description='Bug fix for H.264 encoder'
)
new_version = response['Version']

# Update alias with 90/10 split
lambda_client.update_alias(
    FunctionName='videoTranscoder',
    Name='PROD',
    FunctionVersion=new_version,
    RoutingConfig={
        'AdditionalVersionWeights': {
            '4': 0.9  # 90% to version 4, 10% to new_version
        }
    }
)

The Comparative Analysis

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Key Differentiator for DVA-C02:

• Alias routing is the only Lambda-native feature that splits traffic between versions.
• Alternatives like AWS CodeDeploy can automate this process (using LambdaLinear10PercentEvery10Minutes deployment preferences), but Option D describes the underlying mechanism CodeDeploy itself uses.

Real-World Application (Practitioner Insight)

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Exam Rule

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“For the DVA-C02 exam, when you see gradual rollout, canary deployment, or traffic shifting for Lambda, always pick Alias with weighted routing. Ignore triggers, proxy integrations, or separate functions.”

Real World

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“In production systems at scale, we typically automate this using AWS SAM (AutoPublishAlias with DeploymentPreference) or CodeDeploy:

Resources:
  VideoTranscoderFunction:
    Type: AWS::Serverless::Function
    Properties:
      AutoPublishAlias: live
      DeploymentPreference:
        Type: Canary10Percent5Minutes  # Automated traffic shifting
        Alarms:
          - !Ref TranscoderErrorAlarm

We also integrate CloudWatch Alarms to automatically roll back if error rates exceed thresholds. For high-stakes deployments (financial transactions, healthcare), we use Lambda’s Reserved Concurrency on the canary version to guarantee resource isolation and prevent the new version from consuming all function capacity if it has a memory leak.”

Stop Guessing, Start Mastering

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Disclaimer

This is a study note based on simulated scenarios for the AWS DVA-C02 exam. Always refer to the official AWS documentation for the most current service capabilities and best practices.