The ‘alfred’ helper and the AWS CloudFormation parameter files - AWS Control Tower
Example

The ‘alfred’ helper and the AWS CloudFormation parameter files

CfCT provides you with a mechanism known as the alfred helper to get the value for an SSM Parameter Store key that's defined in the AWS CloudFormation template. Using the alfred helper, you can use values that are stored in the SSM Parameter Store and without updating the AWS CloudFormation template. For more information, see What is an AWS CloudFormation template? in the AWS CloudFormation User Guide.

Important

The alfred helper has two limitations. Parameters are available only in the home region of the AWS Control Tower management account. As a best practice, consider working with values that don't change from stack instance to stack instance. When the 'alfred' helper retreives parameters, it chooses a random stack instance from the stack set that exports the variable.

Example

Suppose that you have two AWS CloudFormation stack sets. Stack set 1 has one stack instance and deploys to one account in one Region. It creates an Amazon VPC and subnets in an availability zone, and the VPC ID and subnet ID must be passed into stack set 2 as parameter values. Before the VPC ID and subnet ID can be passed to stack set 2, the VPC ID and subnet ID must be stored in stack set 1 using AWS:::SSM::Parameter. For more information, see AWS:::SSM::Parameter in the AWS CloudFormation User Guide.

AWS CloudFormation stack set 1:

In the following snippet, the alfred helper can gets value for the VPC ID and subnet ID from the parameter store and pass them as input to the StackSet state machine. 

VpcIdParameter:
    Type: AWS::SSM::Parameter
    Properties:
      Name: '/stack_1/vpc/id'
      Description: Contains the VPC id
      Type: String
      Value: !Ref MyVpc

SubnetIdParameter:
    Type: AWS::SSM::Parameter
    Properties:
      Name: '/stack_1/subnet/id'
      Description: Contains the subnet id
      Type: String
      Value: !Ref MySubnet

AWS CloudFormation stack set 2:

The snippet shows the parameters that are specified in the AWS CloudFormation stack 2 manifest.yaml file. 

parameters:
      - parameter_key: VpcId
        parameter_value: $[alfred_ssm_/stack_1/vpc/id]
      - parameter_key: SubnetId
        parameter_value: $[alfred_ssm_/stack_1/subnet/id]

AWS CloudFormation stack set 2.1:

The snippet shows that you can list alfred_ssm properties to support parameters of type CommaDelimitedList. For more information, see Parameters in the AWS CloudFormation User Guide. 

parameters:
      - parameter_key: VpcId # Type: String
        parameter_value: $[alfred_ssm_/stack_1/vpc/id']
      - parameter_key: SubnetId # Type: String
        parameter_value: $[ alfred_ssm_/stack_1/subnet/id']
      - parameter_key: AvailablityZones # Type: CommaDelimitedList
        parameter_value:
  - "$[alfred_ssm_/availability_zone_1]"
 - "$[alfred_ssm_/availability_zone_2]"