How to get the ARN of an SSM Document in CloudFormation?

Question

I have a CloudFormation template that creates an AWS::Events::Rule and an AWS::SSM::Document. I need to provide a list of Targets for the SSM::Rule, but each target expects an ARN:

mySSMDocument:
  Type: AWS::SSM::Document
  Properties:
    DocumentType: 'Command'
    Content:
      schemaVersion: '2.2'
      description: "Code that will be run on EC2"
      mainSteps:
        - action: "aws:runShellScript"
          name: runShellScript
          inputs:
            runCommand:
              - 'Some command to execute'
myEventRule:
  Type: AWS::Events::Rule
  Properties:
    Description: "A description for the Rule."
    EventPattern: 
      source:
        - "aws.autoscaling"
      detail-type:
        - "EC2 Instance-terminate Lifecycle Action"
      detail:
        AutoScalingGroupName:
          - !Ref 'someAutoScalingGroupInThisTemplate'
    RoleArn: 'some role ARN'
    State: "ENABLED"
    Targets:
      - Id: "some-unique-id"
        Arn: <-- This is the value that I need to fill in.
        RunCommandParameters:
          RunCommandTargets:
            - Key: "tag: Name"
              Values:
                - 'The name of the EC2 machine'

I think that I need to replace the <-- This is the value that I need to fill in. with the ARN of mySSMDocument, but I don't see any way to retrieve this value from within the template itself. The documentation does not specify any GetAtt functionality on SSM::Document that allows to get the ARN. Anyone know how to solve this issue?

Answer 1

This is ARN pattern of Document

arn:${Partition}:ssm:${Region}:${Account}:document/${DocumentName}

example:

arn:aws:ssm:us-east-2:12345678912:document/demoooo

You can use Ref function to get name of document, then Sub to create final ARN

refer: https://docs.aws.amazon.com/IAM/latest/UserGuide/list_awssystemsmanager.html#awssystemsmanager-resources-for-iam-policies