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Difficulty: HardAWS Serverless Application Model (SAM)

A cloud engineering team is migrating a legacy payment service to a serverless architecture on AWS. To ensure safe deployments, they intend to implement a canary rollout where 10%10\% of traffic is shifted to the new version for 1010 minutes before the remaining traffic is cut over. They write the following AWS SAM template:

yaml
Transform: AWS::Serverless-2016-10-31

Resources:
ProcessPaymentFunction:
Type: AWS::Serverless::Function
Properties:
Handler: index.handler
Runtime: nodejs18.x
CodeUri: ./payment
DeploymentPreference:
Type: Canary10Percent10Minutes

After deploying the template, the team observes that the application traffic shifts to the new function version immediately, completely bypassing the 1010-minute canary phase.

What is the root cause of this behavior?

  1. The AutoPublishAlias property is omitted from the function properties, preventing AWS SAM from generating the Lambda alias and CodeDeploy resources required for traffic shifting.Answer
  2. B
    The IAM role assigned to the function lacks a trust relationship allowing the AWS CodeDeploy service principal to assume it.
  3. C
    The execution timeout of the function is set to a value shorter than the 1010-minute canary window, causing the deployment to fail back to All-at-Once routing.
  4. D
    The Canary10Percent10Minutes deployment configuration is exclusive to Amazon ECS task sets and cannot be applied to serverless functions.

Answer

The AutoPublishAlias property is omitted from the function properties, which prevents AWS SAM from generating the Lambda alias and AWS CodeDeploy resources required for gradual traffic shifting.
The correct answer is correct because AWS SAM requires the AutoPublishAlias property to be defined in order to set up gradual deployments. AutoPublishAlias instructs SAM to publish new versions of the function and create a Lambda alias pointing to them. CodeDeploy shifts traffic between these versions on the alias. If AutoPublishAlias is omitted, SAM will update the function directly, resulting in an immediate traffic shift.

Step-by-Step Solution

1
Analyze how AWS SAM implements gradual deployment preferences using AWS CodeDeploy under the hood.
Identified that AWS CodeDeploy requires a specific target Lambda alias to shift traffic between two underlying Lambda function versions.
Traffic routing cannot occur directly on the function's static ARN or the $LATEST version.
2
Examine the provided template properties for the AWS::Serverless::Function resource.
Observed that the template defines DeploymentPreference but lacks the AutoPublishAlias property under Properties.
Checking if all required properties are declared to allow SAM to synthesize the CodeDeploy resources.
3
Determine the outcome of omitting AutoPublishAlias during the CloudFormation transformation phase.
Without AutoPublishAlias, AWS SAM does not generate the Lambda alias resource or the CodeDeploy deployment group, leading to direct updates on $LATEST and causing traffic to shift immediately.
Explaining the root cause of the immediate traffic cutover.

Key Concept

AWS SAM Gradual Lambda Deployments with CodeDeploy and AutoPublishAlias
Estimated Time:2m 0s
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Topics

AWS Serverless Application Model (SAM)

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