How can the DevOps engineer ensure that the CloudFormation deployment will fail if the user data fails to successfully finish running?

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A DevOps engineer has implemented a Cl/CD pipeline to deploy an AWS CloudFormation template that provisions a web application.The web application consists of an Application Load Balancer (ALB), a target group, a launch template that uses an Amazon Linux 2 AMI, an Auto Scaling group of Amazon EC2 instances, a security group, and an Amazon RDS for MySOL database.The launch template includes user data that specifies a script to install and start the application.The initial deployment of the application was successful.The DevOps engineer made changes to update the version of the application with the user data.The CI/CD pipeline has deployed a new version of the template.However, the health checks on the ALB are now failing.The health checks have marked all targets as unhealthy.During investigation, the DevOps engineer notices that the CloudFormation stack has a status of UPDATE_COMPLETE.However, when the DevOps engineer connects to one of the EC2 instances and checks /var/log/messages, the DevOps engineer notices that the Apache web server failed to start successfully because of a configuration error.

How can the DevOps engineer ensure that the CloudFormation deployment will fail if the user data fails to successfully finish running?

Use the cfn-signal helper script to signal success or failure to CloudFormation. Use the WaitOnResourceSignals update policy within the CloudFormation template. Set an appropriate timeout for the update policy.

Create an Amazon CloudWatch alarm for the UnhealthyHostCount metric. Include an appropriate alarm threshold for the target group. Create an Amazon Simple Notification Service (Amazon SNS) topic as the target to signal success or failure to CloudFormation.

Create a lifecycle hook on the Auto Scaling group by using the AWS::AutoScaling::LifecycleHook resource. Create an Amazon Simple Notification Service (Amazon SNS) topic as the target to signal success or failure to CloudFormation. Set an appropriate timeout on the lifecycle hook.

Use the Amazon CloudWatch agent to stream the cloud-init logs. Create a subscription filter that includes an AWS Lambda function with an appropriate invocation timeout. Configure the Lambda function to use the SignalResource API operation to signal success or failure to CloudFormation.

Explanations:

The cfn-signal helper script is used to send a success or failure signal to CloudFormation. The WaitOnResourceSignals update policy ensures that CloudFormation waits for the signal before considering the stack update complete. This setup ensures the deployment will fail if the user data fails to run successfully.

CloudWatch alarms for UnhealthyHostCount monitor the ALB’s health but do not provide direct integration with CloudFormation to signal success or failure. SNS topics cannot directly signal CloudFormation without additional setup like cfn-signal.

While lifecycle hooks can pause the instance creation process in an Auto Scaling group, they do not automatically signal CloudFormation about the success or failure of the user data. An SNS topic does not suffice to signal CloudFormation without using cfn-signal or similar mechanisms.

The CloudWatch agent and Lambda function setup would involve complex, unnecessary steps. The SignalResource API is not the most direct way to handle failure signaling for user data. The simpler and more appropriate solution is using cfn-signal.

2025-09-30

1 Comment

  1. Rachel
    Author

    I systematize that the answer is:
    Use the cfn-signal helper script to signal success or failure to CloudFormation. Use the WaitOnResourceSignals update policy within the CloudFormation template. Set an appropriate timeout for the update policy.

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