AWS CloudFormation cfn-init and cfn-signal Overview

• cfn-init: Used to retrieve and interpret resource metadata, install packages, create files, and start services.
• cfn-signal: A script that is used to signal CloudFormation whether the EC2 instance has been configured successfully.

Using cfn-signal with WaitCondition

• WaitCondition: A CloudFormation resource that pauses the stack creation until it receives a signal indicating success or failure.
• Creation Policy: Attached to resources like EC2 instances or Auto Scaling groups to specify a timeout and count for the expected signals.

Workflow

1. CloudFormation launches an EC2 instance.
2. The instance runs cfn-init to configure itself.
3. After running cfn-init, the instance sends a signal using cfn-signal.
4. The signal is sent to a WaitCondition resource in the CloudFormation template.
5. The WaitCondition waits for a success or failure signal within a specified timeout.

CloudFormation Template Snippet

AWSTemplateFormatVersion: '2010-09-09'
Description: EC2 instance with cfn-init and cfn-signal
Parameters:
  KeyName:
    Description: Name of an existing EC2 KeyPair to enable SSH access
    Type: 'AWS::EC2::KeyPair::KeyName'
Resources:
  MyEC2Instance:
    Type: 'AWS::EC2::Instance'
    Metadata:
      'AWS::CloudFormation::Init':
        config:
          files:
            "/home/ec2-user/hello.txt":
              content: "Hello, World!"
    Properties:
      InstanceType: t2.micro
      ImageId: ami-0abcdef1234567890  # Update this with a valid AMI ID
      KeyName: !Ref KeyName
      UserData:
        Fn::Base64: !Sub |
          #!/bin/bash -xe
          yum update -y aws-cfn-bootstrap
          /opt/aws/bin/cfn-init -v --stack ${AWS::StackName} --resource MyEC2Instance --region ${AWS::Region}
          /opt/aws/bin/cfn-signal -e $? --stack ${AWS::StackName} --resource MyEC2Instance --region ${AWS::Region}
  WaitHandle:
    Type: 'AWS::CloudFormation::WaitConditionHandle'
  WaitCondition:
    Type: 'AWS::CloudFormation::WaitCondition'
    DependsOn: MyEC2Instance
    Properties:
      Handle: !Ref WaitHandle
      Timeout: '300'

Execution Steps

1. Create a stack with the CloudFormation template.
2. Monitor the creation of resources like the security group and EC2 instance.
3. Observe the WaitCondition resource's status.
4. Once the EC2 instance finishes its bootstrap and signals back, the WaitCondition status will change to complete.

Outcome