AWS CloudFormation YAML template - appending to list parameter types.

README.md

AWS CloudFormation YAML template - appending to list parameter types

Documenting this here, as I often forget (what I have found) is the best way to do this at the moment.

For example, you have a list of two existing security groups given to a stack and wish to create (and use) a third - attaching all to an ALB:

AWSTemplateFormatVersion: "2010-09-09"
Description: "Example template"

Parameters:
  VPC:
    Type: "AWS::EC2::VPC::Id"

  ALBSubnetList:
    Type: "List<AWS::EC2::Subnet::Id>"

  securityGroupIdList:
    Type: "List<AWS::EC2::SecurityGroup::Id>"

Resources:
  ALBInstance:
    Type: "AWS::ElasticLoadBalancingV2::LoadBalancer"
    Properties:
      Name: "My ALB"
      Scheme: "internal"
      SecurityGroups: !Split
        - ","
        - !Sub
          - "${idList},${ALBSecurityGroup}"
          - idList: !Join [",",!Ref "securityGroupIdList"]
      Subnets: !Ref "ALBSubnetList"
      
  ALBSecurityGroup:
    Type: "AWS::EC2::SecurityGroup"
    Properties:
      GroupDescription: "My new ALB security group"
      SecurityGroupIngress:
        - CidrIp: "0.0.0.0/0"
          FromPort: 443
          IpProtocol: "tcp"
          ToPort: 443
      VpcId: !Ref "VPC"

What's happening here:

• Taking given securityGroupIdList list of strings and using !Join to create a single string delimited with commas.
• Next, using !Sub we join this string (with a comma) to our new group resource ID of ALBSecurityGroup.
• Finally, re-split via !Split the complete string on commas, returning result as a list of strings passed to SecurityGroups.