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 CI/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 MySQL 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:

Using thecfn-signalhelper script allows the EC2 instance to communicate whether the user data execution is successful or not. TheWaitOnResourceSignalsupdate policy ensures CloudFormation waits for a signal before completing the stack update. If the signal fails or is not received within the timeout, CloudFormation will fail the deployment.

CloudWatch alarms and SNS notifications are useful for monitoring and alerting but do not directly influence the success or failure of the CloudFormation stack update. CloudFormation does not consider CloudWatch alarms to signal success or failure in the deployment process.

Lifecycle hooks help control the state of EC2 instances within Auto Scaling groups, but they do not directly signal user data execution success or failure. CloudFormation does not directly consider lifecycle hooks for controlling deployment success in this context.

Although CloudWatch logs and Lambda can be used to monitor and react to EC2 instance startup issues, this solution is overly complex and does not provide a direct method to signal CloudFormation failure. TheSignalResourceAPI is not the recommended approach in this scenario.

2025-10-15

1 Comment

  1. Amy
    Author

    I map out 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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