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 why Lambda functions deployed via CloudFormation don’t update with unchanged S3 object keys. In production, this is about knowing exactly how CloudFormation’s resource update detection works and when to force a new version deployment. Let’s drill down.
The Certification Drill (Simulated Question) #
Scenario #
A startup called BrightLabs is implementing a serverless backend for their mobile app using AWS CloudFormation. The stack provisions an Amazon S3 bucket configured for static website hosting, an API Gateway to handle requests, and several AWS Lambda functions for business logic. The developers upload the zipped Lambda source code to a designated S3 bucket and reference the S3 object key in the CloudFormation template under the Lambda function’s code property.
However, after changing the Lambda source code and updating the CloudFormation stack, they notice that the Lambda function’s code is not updated—CloudFormation does not trigger a replacement or update of the function code because the S3 object key remains the same. They want to fix this issue to ensure Lambda functions always reflect the latest code after stack updates.
The Requirement: #
How should the BrightLabs developers modify their deployment process to ensure that each CloudFormation stack update triggers an update to the Lambda function code when the source code changes?
The Options #
- A) Create a new Lambda function alias before updating the CloudFormation stack.
- B) Change the S3 object key or specify the S3 object version in the CloudFormation template before updating the CloudFormation stack.
- C) Upload the zipped source code to a different S3 bucket before updating the CloudFormation stack.
- D) Associate a code signing configuration with the Lambda function before updating the CloudFormation stack.
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Correct Answer #
B
Quick Insight: The Developer Imperative #
When managing Lambda source code deployments using CloudFormation and S3, CloudFormation treats the S3 object key as an immutable identifier. If the key does not change, CloudFormation believes the source is unchanged, and thus the Lambda function is not updated. To force an update, either the S3 object key or the S3 object version must change so the template reflects a different artifact.
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 B
The Winning Logic #
CloudFormation uses the S3 object key and optionally the object version to detect changes to the Lambda deployment package. If the S3 object key remains the same and object version is not specified, CloudFormation assumes the function code hasn’t changed and thus does not update the Lambda function. To fix this:
- After changing the Lambda code, upload the new zip with either a new S3 key (e.g., app-v2.zip) or enable S3 versioning on the bucket and reference the new object version in the CloudFormation template.
- This ensures CloudFormation detects a change in the deployment artifact and updates the Lambda function accordingly during stack update.
The Trap (Distractor Analysis): #
- Why not A? Creating a new Lambda alias does not trigger code updates; aliases control version pointers but do not change the function source code itself.
- Why not C? Uploading to another bucket is unnecessary and cumbersome. The key point is changing the S3 object identifier or version, not switching buckets.
- Why not D? Code signing is a security feature to validate Lambda packages; it does not influence whether CloudFormation detects a code update.
The Technical Blueprint #
# Assuming S3 bucket versioning is enabled, upload new code with the same key:
aws s3 cp lambda-function.zip s3://brightlabs-lambda-code/function-code.zip
# Retrieve the new object version ID:
aws s3api head-object --bucket brightlabs-lambda-code --key function-code.zip
# Update CloudFormation template snippet:
Resources:
MyLambdaFunction:
Type: AWS::Lambda::Function
Properties:
Code:
S3Bucket: brightlabs-lambda-code
S3Key: function-code.zip
S3ObjectVersion: "new-version-id-from-head-object"
# Update stack:
aws cloudformation update-stack --stack-name brightlabs-stack --template-body file://template.yaml
The Comparative Analysis (Developer Focus) #
| Option | API Complexity | Performance Impact | Use Case |
|---|---|---|---|
| A | Low | No code change effect | Helpful for routing traffic, not code updates |
| B | Moderate – need to track object version | Ensures latest code deploys, efficient | Preferred method for Lambdas updated via S3/CFN |
| C | Moderate – bucket management overhead | More complicated, redundant | Not generally recommended |
| D | Higher, involves code signing configs | Adds security, no update trigger | Use for security validation, unrelated to update triggering |
Real-World Application (Practitioner Insight) #
Exam Rule #
For the exam, always pick changing the S3 object key or specifying the S3 object version when you see Lambda code deployed via CloudFormation referencing S3.
Real World #
In production, enabling S3 versioning for Lambda deployment buckets is highly recommended. It automates version tracking and avoids manual key renaming, reducing errors and deployment friction.
(CTA) Stop Guessing, Start Mastering #
Disclaimer
This is a study note based on simulated scenarios for the AWS DVA-C02 exam.