Reference existing resources and Systems Manager parameters with CloudFormation-supplied parameter types

When creating your template, you can create parameters that require users to input identifiers of existing AWS resources or Systems Manager parameters by using specialized parameter types provided by CloudFormation.

Overview

In CloudFormation, you can use parameters to customize your stacks by providing input values during stack creation or update. This feature makes your templates reusable and flexible across different scenarios.

Parameters are defined in the Parameters section of a CloudFormation template. Each parameter has a name and a type, and can have additional settings such as a default value and allowed values. For more information, see Parameters.

The parameter type determines the kind of input value the parameter can accept. For example, Number only accepts numeric values, while String accepts text input.

CloudFormation provides several additional parameter types that you can use in your templates to reference existing AWS resources and Systems Manager parameters.

These parameter types fall into two categories:

Once your parameters are defined in the Parameters section, you can reference parameter values throughout your CloudFormation template using the Ref function.

Example

The following example shows a template that uses the following parameter types.

To create a stack from this template, you must specify an existing VPC ID, subnet ID, and key pair name from your account. You can also specify an existing Parameter Store key that references the desired AMI ID or keep the default value of /aws/service/ami-amazon-linux-latest/amzn2-ami-hvm-x86_64-gp2. This public parameter is an alias for the regional AMI ID for the latest Amazon Linux 2 AMI. For more information about public parameters, see Discovering public parameters in Parameter Store in the AWS Systems Manager User Guide.

{
    "Parameters": {
        "VpcId": {
            "Description": "ID of an existing Virtual Private Cloud (VPC).",
            "Type": "AWS::EC2::VPC::Id"
        },
        "PublicSubnetId": {
            "Description": "ID of an existing public subnet within the specified VPC.",
            "Type": "AWS::EC2::Subnet::Id"
        },
        "KeyName": {
            "Description": "Name of an existing EC2 key pair to enable SSH access to the instance.",
            "Type": "AWS::EC2::KeyPair::KeyName"
        },
        "AMIId": {
            "Description": "Name of a Parameter Store parameter that stores the ID of the Amazon Machine Image (AMI).",
            "Type": "AWS::SSM::Parameter::Value<AWS::EC2::Image::Id>",
            "Default": "/aws/service/ami-amazon-linux-latest/amzn2-ami-hvm-x86_64-gp2"
        }
    },
    "Resources": {
        "InstanceSecurityGroup": {
            "Type": "AWS::EC2::SecurityGroup",
            "Properties": {
                "GroupDescription": "Enable SSH access via port 22",
                "VpcId": { "Ref": "VpcId" },
                "SecurityGroupIngress": [
                    {
                        "IpProtocol": "tcp",
                        "FromPort": 22,
                        "ToPort": 22,
                        "CidrIp": "0.0.0.0/0"
                    }
                ]
            }
        },
        "Ec2Instance": {
            "Type": "AWS::EC2::Instance",
            "Properties": {
                "KeyName": { "Ref": "KeyName" },
                "ImageId": { "Ref": "AMIId" },
                "NetworkInterfaces": [
                    {
                        "AssociatePublicIpAddress": "true",
                        "DeviceIndex": "0",
                        "SubnetId": { "Ref": "PublicSubnetId" },
                        "GroupSet": [{ "Ref": "InstanceSecurityGroup" }]
                    }
                ]
            }
        }
    },
    "Outputs": {
        "InstanceId": {
            "Value": { "Ref": "Ec2Instance" }
        }
    }
}

Parameters:
  VpcId:
    Description: ID of an existing Virtual Private Cloud (VPC).
    Type: 'AWS::EC2::VPC::Id'
  PublicSubnetId:
    Description: ID of an existing public subnet within the specified VPC.
    Type: 'AWS::EC2::Subnet::Id'
  KeyName:
    Description: Name of an existing EC2 KeyPair to enable SSH access to the instance.
    Type: 'AWS::EC2::KeyPair::KeyName' 
  AMIId:
    Description: Name of a Parameter Store parameter that stores the ID of the Amazon Machine Image (AMI).
    Type: 'AWS::SSM::Parameter::Value<AWS::EC2::Image::Id>'
    Default: '/aws/service/ami-amazon-linux-latest/amzn2-ami-hvm-x86_64-gp2'
Resources:
  InstanceSecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: Enable SSH access via port 22
      VpcId: !Ref VpcId
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: 22
          ToPort: 22
          CidrIp: 0.0.0.0/0
  Ec2Instance:
    Type: 'AWS::EC2::Instance'
    Properties:
      KeyName: !Ref KeyName
      ImageId: !Ref AMIId
      NetworkInterfaces:
        - AssociatePublicIpAddress: "true"
          DeviceIndex: "0"
          SubnetId: !Ref PublicSubnetId
          GroupSet:
            - !Ref InstanceSecurityGroup
Outputs:
  InstanceId:
    Value: !Ref Ec2Instance

AWS CLI command to create the stack

The following create-stack command creates a stack based on the example template.

aws cloudformation create-stack --stack-name MyStack \
  --template-body file://sampletemplate.json \
  --parameters \
ParameterKey="VpcId",ParameterValue="vpc-a123baa3" \
ParameterKey="PublicSubnetId",ParameterValue="subnet-123a351e" \
ParameterKey="KeyName",ParameterValue="MyKeyName" \
ParameterKey="AMIId",ParameterValue="MyParameterKey"

To use a parameter type that accepts a list of strings , such as List<AWS::EC2::Subnet::Id>, you must escape the commas inside the ParameterValue with a double backslash, as shown in the following example.

--parameters ParameterKey="SubnetIDs",ParameterValue="subnet-5ea0c127\\,subnet-6194ea3b\\,subnet-c87f2be0"

Considerations

It's strongly recommended that you use dynamic references to restrict access to sensitive configuration definitions, such as third-party credentials. For more information, see Get values stored in other services using dynamic references.

If you want to allow template users to specify values from different AWS accounts, don't use AWS-specific parameter types. Instead, define parameters of type String or CommaDelimitedList.

There are a few things to keep in mind with Systems Manager parameter types:

Supported AWS-specific parameter types

CloudFormation supports the following AWS-specific types:

Supported Systems Manager parameter types

CloudFormation supports the following Systems Manager parameter types:

Unsupported Systems Manager parameter types

CloudFormation doesn't support the following Systems Manager parameter type:

In addition, CloudFormation doesn't support defining template parameters as SecureString Systems Manager parameter types. However, you can specify secure strings as parameter values for certain resources. For more information, see Get values stored in other services using dynamic references.