How do I pass CommaDelimitedList parameters to nested stacks in AWS CloudFormation?

Short description

You can't pass values of type CommaDelimitedList to a nested stack. Instead, use the Fn::Join intrinsic function in your parent stack to convert type CommaDelimitedList to type String.

Resolution

The following example shows you how to pass a list of SecurityGroupIds from a parent stack to a nested stack.

1.    Open the JSON or YAML file of your parent stack, and then set the Type of SecurityGroupIds to CommaDelimitedList.

In the Resources section of the JSON file, the Fn::Join function returns the combined string. In the Resources section of the YAML file, the !Join function returns the combined string. In both JSON and YAML files, the combined string converts the SecurityGroupIds parameter type from CommaDelimitedList to String.

Example parent JSON file:

{
  "AWSTemplateFormatVersion": "2010-09-09",
  "Parameters": {
    "SubnetId": {
      "Type": "AWS::EC2::Subnet::Id"
    },
    "SecurityGroupIds": {
      "Type": "List<AWS::EC2::SecurityGroup::Id>"
    },
    "KeyName": {
      "Type": "AWS::EC2::KeyPair::KeyName"
    },
    "ImageId": {
      "Type": "String"
    }
  },
  "Resources": {
    "Instance": {
      "Type": "AWS::CloudFormation::Stack",
      "Properties": {
        "TemplateURL": "https://s3.amazonaws.com/cloudformation-templates-us-east-2/nested.yml",
        "Parameters": {
          "SubnetId": {
            "Ref": "SubnetId"
          },
          "SecurityGroupIds": {
            "Fn::Join": [
              ",",
              {
                "Ref": "SecurityGroupIds"
              }
            ]
          },
          "KeyName": {
            "Ref": "KeyName"
          },
          "ImageId": {
            "Ref": "ImageId"
          }
        }
      }
    }
  }
}

Example parent YAML file:

AWSTemplateFormatVersion: 2010-09-09
Parameters:
  SubnetId:
    Type: 'AWS::EC2::Subnet::Id'
  SecurityGroupIds:
    Type: 'List<AWS::EC2::SecurityGroup::Id>'
  KeyName:
    Type: 'AWS::EC2::KeyPair::KeyName'
  ImageId:
    Type: String
Resources:
  Instance:
    Type: 'AWS::CloudFormation::Stack'
    Properties:
      TemplateURL: 'https://s3.amazonaws.com/cloudformation-templates-us-east-2/nested.yml'
      Parameters:
        SubnetId: !Ref SubnetId
        SecurityGroupIds: !Join 
          - ','
          - !Ref SecurityGroupIds
        KeyName: !Ref KeyName
        ImageId: !Ref ImageId

Note: If you pass two subnets, such as ["subnet-aaaa, subnet-bbbb"], the output of Fn::Join is {"subnet-aaaa, subnet-bbbb"}.

2.    In the JSON or YAML file of your nested stack, set the Type of SecurityGroupIds to CommaDelimitedList.

Example nested JSON file:

{
  "AWSTemplateFormatVersion": "2010-09-09",
  "Parameters": {
    "SubnetId": {
      "Type": "String"
    },
    "SecurityGroupIds": {
      "Type": "CommaDelimitedList"
    },
    "KeyName": {
      "Type": "String"
    },
    "ImageId": {
      "Type": "String"
    }
  },
  "Resources": {
    "Ec2instance": {
      "Type": "AWS::EC2::Instance",
      "Properties": {
        "ImageId": {
          "Ref": "ImageId"
        },
        "KeyName": {
          "Ref": "KeyName"
        },
        "SecurityGroupIds": {
          "Ref": "SecurityGroupIds"
        },
        "SubnetId": {
          "Ref": "SubnetId"
        }
      }
    }
  }
}

Example nested YAML file:

AWSTemplateFormatVersion: 2010-09-09
Parameters:
  SubnetId:
    Type: String
  SecurityGroupIds:
    Type: CommaDelimitedList
  KeyName:
    Type: String
  ImageId:
    Type: String
Resources:
  Ec2instance:
    Type: 'AWS::EC2::Instance'
    Properties:
      ImageId: !Ref ImageId
      KeyName: !Ref KeyName
      SecurityGroupIds: !Ref SecurityGroupIds
      SubnetId: !Ref SubnetId

Note: In the nested stack, the combined string from the parent stack is passed to SecurityGroupIds as CommaDelimitedList. For example, the value {"sg-aaaaa, sg-bbbbb"} is converted back to ["sg-aaaaa", "sg-bbbbb"]. Therefore, SecurityGroupIds must be directly referenced by SecurityGroupIds: !Ref SecurityGroupIds and not as a list of strings.

Related information

Working with nested stacks

AWS::CloudFormation::Stack