Ref - AWS CloudFormation (original) (raw)

The intrinsic function Ref returns the value of a specified parameter, resource, or another intrinsic function. This function is commonly used to create references between resources within a CloudFormation template.

Declaration

JSON

{ "Ref" : "logicalName" }

{ "Ref" : "IntrinsicFunction" }

YAML

Syntax for the full function name:

Ref: logicalName

Ref:
   IntrinsicFunction

Syntax for the short form:

!Ref logicalName

!Ref
   IntrinsicFunction

Parameters

logicalName

The logical name of the resource or parameter you want to reference.

IntrinsicFunction

The intrinsic function that resolves to a valid string. It should contain references to parameters or identifiers, and should not contain resource logical identifiers.

Return value

The return value of Ref depends on the type of entity being referenced:

• When you specify a parameter's logical name, it returns the value of the parameter. For more information, see CloudFormation template Parameters syntax.
• When you specify a resource's logical name, it returns a value that you use to identify that resource. Usually, that's the name of the resource. However, for some resources, an identifier is returned that has another significant meaning in the context of the resource. For example, the AWS::EC2::EIP resource returns the IP address, and the AWS::EC2::Instance returns the instance ID. For more information about Ref return values for a resource, see the documentation for that resource in the Resource and property reference.
• When you specify an intrinsic function, it returns the output of that function.

Examples

Create references between resources

The following resource declaration for an Elastic IP address needs the instance ID of an EC2 instance. It uses the Ref function to specify the instance ID of the MyEC2Instance resource declared elsewhere in the template.

JSON

{
  "AWSTemplateFormatVersion":"2010-09-09",
  "Resources":{
  
      ...
  
    "MyEIP":{
      "Type":"AWS::EC2::EIP",
      "Properties":{
        "InstanceId":{
          "Ref":"MyEC2Instance"
        }
      }
    }
  }
}

YAML

AWSTemplateFormatVersion: 2010-09-09
Resources:

  ...

  MyEIP:
    Type: AWS::EC2::EIP
    Properties:
      InstanceId: !Ref MyEC2Instance

Return a resource identifier as stack output

The following examples show how to use the Ref function to return the name of an Amazon S3 bucket with the logical name MyBucket as stack output.

JSON

{
  "AWSTemplateFormatVersion":"2010-09-09",
  "Resources":{
    "MyBucket":{
      "Type":"AWS::S3::Bucket",
      "Properties":{
        "BucketName":{
          "Fn::Sub": "${AWS::StackName}-mybucket"
        }
      }
    }
  },
  "Outputs":{
    "BucketNameOutput":{
      "Description":"The name of the S3 bucket",
      "Value":{
        "Ref":"MyBucket"
      }
    }
  }
}

YAML

AWSTemplateFormatVersion: 2010-09-09
Resources:
  MyBucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: !Sub ${AWS::StackName}-mybucket

Outputs:
  BucketNameOutput:
    Description: The name of the S3 bucket
    Value: !Ref MyBucket

UseFn::Join intrinsic function inside Ref function

Note

When you use the AWS::LanguageExtensions transform, you can useRef in combination with other intrinsic functions. For supported functions, see Supported functions.

The following examples show how to set identifiers of resources using theFn::Sub intrinsic function, conditions, and the input for theStage parameter. The Ref and theFn::GetAtt functions then reference the appropriate values, based on the stage. Fn::Sub is first used with Fn::GetAtt to obtain the ARN of the appropriate Amazon SQS queue to set the dimensions of the Amazon CloudWatch alarm. Next, Fn::Join is used with Ref to create the name of the SNS topic for the AlarmActions property.

JSON

{
    "AWSTemplateFormatVersion": "2010-09-09",
    "Transform": "AWS::LanguageExtensions",
    "Parameters": {
        "Stage": {
            "Type": "String",
            "Default": "Dev",
            "AllowedValues": [
                "Dev",
                "Prod"
            ]
        }
    },
    "Conditions": {
        "isProd": {
            "Fn::Equals": [
                {"Ref": "Stage"},
                "Prod"
            ]
        },
        "isDev": {
            "Fn::Equals": [
                {"Ref": "Stage"},
                "Dev"
            ]
        }
    },
    "Resources": {
        "DevQueue": {
            "Type": "AWS::SQS::Queue",
            "Condition": "isDev",
            "Properties": {
                "QueueName": {"Fn::Sub": "My${Stage}Queue"}
            }
        },
        "ProdQueue": {
            "Type": "AWS::SQS::Queue",
            "Condition": "isProd",
            "Properties": {
                "QueueName": {"Fn::Sub": "My${Stage}Queue"}
            }
        },
        "DevTopic": {
            "Condition": "isDev",
            "Type": "AWS::SNS::Topic"
        },
        "ProdTopic": {
            "Condition": "isProd",
            "Type": "AWS::SNS::Topic"
        },
        "MyAlarm": {
            "Type": "AWS::CloudWatch::Alarm",
            "Properties": {
                "AlarmDescription": "Alarm if queue depth grows beyond 10 messages",
                "Namespace": "AWS/SQS",
                "MetricName": "ApproximateNumberOfMessagesVisible",
                "Dimensions":[
                    {
                        "Name": {"Fn::Sub": "${Stage}Queue"},
                        "Value": {"Fn::GetAtt": [{"Fn::Sub": "${Stage}Queue"}, "QueueName"]}
                    }
                ],
                "Statistic": "Sum",
                "Period": 300,
                "EvaluationPeriods": 1,
                "Threshold": 10,
                "ComparisonOperator": "GreaterThanThreshold",
                "AlarmActions": [
                    {
                        "Ref": {"Fn::Join": ["", [{"Ref": "Stage"}, "Topic"]]}
                    }
                ]
            }
        }
    }
}

YAML

AWSTemplateFormatVersion: 2010-09-09
Transform: AWS::LanguageExtensions
Parameters:
  Stage:
    Type: String
    Default: Dev
    AllowedValues:
      - Dev
      - Prod
Conditions:
  isProd: !Equals 
    - !Ref Stage
    - Prod
  isDev: !Equals 
    - !Ref Stage
    - Dev
Resources:
  DevQueue:
    Type: AWS::SQS::Queue
    Condition: isDev
    Properties:
      QueueName: !Sub My${Stage}Queue
  ProdQueu:
    Type: AWS::SQS::Queue
    Condition: isProd
    Properties:
      QueueName: !Sub My${Stage}Queue
  DevTopic:
    Condition: isDev
    Type: AWS::SNS::Topic
  ProdTopic:
    Condition: isProd
    Type: AWS::SNS::Topic
  MyAlarm:
    Type: AWS::CloudWatch::Alarm
    Properties:
      AlarmDescription: Alarm if queue depth grows beyond 10 messages
      Namespace: AWS/SQS
      MetricName: ApproximateNumberOfMessagesVisible
      Dimensions:
        - Name: !Sub '${Stage}Queue'
          Value: !GetAtt 
            - !Sub '${Stage}Queue'
            - QueueName
      Statistic: Sum
      Period: 300
      EvaluationPeriods: 1
      Threshold: 10
      ComparisonOperator: GreaterThanThreshold
      AlarmActions:
        - !Ref 
          'Fn::Join':
            - ''
            - - !Ref Stage
              - Topic

Supported functions

When you use the AWS::LanguageExtensions transform, you can use the following functions within the Ref function.

• Fn::Base64
• Fn::FindInMap
• Fn::If
• Fn::ImportValue
• Fn::Join
• Fn::Sub
• Fn::ToJsonString
• Ref