Using CloudFormation Nested Change Sets

Every year, just in time for re:invent, AWS announces a plethora of upgrades to the services in the AWS ecosphere. This year is no different, even though re:invent will be completely virtual.

For anyone using CloudFormation and nested stacks, one of the major frustrations has been the inability to see what changes are being applied to the nested stacks when doing a stack update. In case you missed it, on November 18, 2020, AWS released a change to CloudFormation allowing users to see the changesets being applied to the nested stacks.

What’s a nested stack you ask?

A nested stack is when one CloudFormation stack has another stack as a resource. Consider this fragment:

Parameters:
  VpcId:
  # select the VPC the stack is being deployed in.
    Type: AWS::EC2::VPC::Id
    Description: Select the VPC
  BucketName:
    Type: String

Resources:
  ClusterA:
    Type: AWS::ECS::Cluster
    Properties:
      ClusterName: clusterA
      ClusterSettings:
        - Name: containerInsights
          Value: enabled
  CloudwatchExportsBucket:
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateURL: https://bucket.s3.amazonaws.com/template.yaml
      Parameters:
        BucketName: !Ref BucketName

This fragment creates the ECS Cluster resource named ClusterA, and then uses another CloudFormation stack to create the LoadBalancer resource by using the AWS::CloudFormation::Stack resource type. The nested stack template must exist in an S3 bucket accessible to CloudFormation. Assuming the templates are deployed to an S3 bucket, we can use either the CloudFormation console or the AWS CLI to create the stack and its resources.

aws cloudformationcreate-stack --stack-name my-sample-changeset --template-url https://labr-cfn.s3.amazonaws.com/parent.yaml --parameters "ParameterKey=BucketName,ParameterValue=my-sample-changset"
{
    "StackId": "arn:aws:cloudformation:us-east-1:548985610555:stack/my-sample-changeset/dea1ea90-319a-11eb-a380-0e2470ba0d5b"
}

We can use the console to verify the stack:

Verifying the stack creation

With the resources created, what happens when we want to apply a change to those resources?

Change Sets