Ref, short for Reference is used extensively in the Resources and Output sections of the CloudFormation template.

You can use the Ref intrinsic function in 2 contexts –

•  Parameter: If the input to the Ref function is a parameter it returns the value of the parameter.
  
  
• Resource name: If the input to the Ref function is the logical name of an AWS resource, it returns the physical name of the resource.

  
  This can sound a bit confusing. So let me clarify.
  
  Every resource created in your stack needs to be uniquely identifiable. Certain AWS resources can be assigned a user-friendly name, called a Logical ID in your template.
  
  This is great, but how do you identify resources without a Logical ID?
  
  To avoid this confusion, CloudFormation, by default, assigns every resource a Physical ID. You then use the logical name in the Ref function to determine the physical name.
  
  

The declaration format for Ref Intrinsic function is as shown below.

JSON Format: { "Ref" : "LogicalName" }

YAML Format: !Ref LogicalName

Example on using CloudFormation Ref function as a Parameter

In the below example, we create an Amazon Relational Database Service (RDS) instance using a CloudFormation template.

CloudFormation Ref function is used as a parameter to pass Username and Password values from the Parameters section of the template to the Resources section of the template.

 
JSON Format
{
 "Parameters": {
    "DBUser": {
      "NoEcho": "true",
      "Description" : "The database admin account username",
      "Type": "String",
      "MinLength": "1",
      "MaxLength": "16",
      "AllowedPattern" : "[a-zA-Z][a-zA-Z0-9]*",
      "ConstraintDescription" : "must begin with a letter and contain only alphanumeric characters."
    },

    "DBPassword": {
      "NoEcho": "true",
      "Description" : "The database admin account password",
      "Type": "String",
      "MinLength": "8",
      "MaxLength": "41",
      "AllowedPattern" : "[a-zA-Z0-9]*",
      "ConstraintDescription" : "must contain only alphanumeric characters."
    }
  },

  "Resources" : {
    "myDB" : {
      "Type" : "AWS::RDS::DBInstance",
      "Properties" : {
        "AllocatedStorage" : "100",
        "DBInstanceClass" : "db.t2.small",
        "Engine" : "MySQL",
        "Iops" : "1000",
        "MasterUsername" : { "Ref" : "DBUser" },
        "MasterUserPassword" : { "Ref" : "DBPassword" }
      }
    }
  }
}
 

YAML Format
Parameters:
  DBUser:
    NoEcho: 'true'
    Description: The database admin account username
    Type: String
    MinLength: '1'
    MaxLength: '16'
    AllowedPattern: '[a-zA-Z][a-zA-Z0-9]*'
    ConstraintDescription: must begin with a letter and contain only alphanumeric characters.
  DBPassword:
    NoEcho: 'true'
    Description: The database admin account password
    Type: String
    MinLength: '8'
    MaxLength: '41'
    AllowedPattern: '[a-zA-Z0-9]*'
    ConstraintDescription: must contain only alphanumeric characters.
Resources:
  myDB:
    Type: 'AWS::RDS::DBInstance'
    Properties:
      AllocatedStorage: '100'
      DBInstanceClass: db.t2.small
      Engine: MySQL
      Iops: '1000'
      MasterUsername: !Ref DBUser
      MasterUserPassword: !Ref DBPassword
 

Example on using Ref function used as a Resource

This second example installs and deploys WordPress on an EC2 instance.

The CloudFormation Ref function is used as a resource to obtain the Physical ID from the Logical ID for the VPC.

JSON Format
{
    "Parameters": {
        "VpcId": {
            "Type": "AWS::EC2::VPC::Id",
            "Description": "VpcId of your existing Virtual Private Cloud (VPC)",
            "ConstraintDescription": "must be the VPC Id of an existing Virtual Private Cloud."
        }
    },
    "Resources": {
        "ALBTargetGroup": {
            "Type": "AWS::ElasticLoadBalancingV2::TargetGroup",
            "Properties": {
                "HealthCheckPath": "/wordpress/wp-admin/install.php",
                "HealthCheckIntervalSeconds": 10,
                "HealthCheckTimeoutSeconds": 5,
                "HealthyThresholdCount": 2,
                "Port": 80,
                "Protocol": "HTTP",
                "UnhealthyThresholdCount": 5,
                "VpcId": {
                    "Ref": "VpcId"
                },
                "TargetGroupAttributes": [
                    {
                        "Key": "stickiness.enabled",
                        "Value": "true"
                    },
                    {
                        "Key": "stickiness.type",
                        "Value": "lb_cookie"
                    },
                    {
                        "Key": "stickiness.lb_cookie.duration_seconds",
                        "Value": "30"
                    }
                ]
            }
        }
    }
}
 

 
YAML Format
Parameters:
  VpcId:
    Type: 'AWS::EC2::VPC::Id'
    Description: VpcId of your existing Virtual Private Cloud (VPC)
    ConstraintDescription: must be the VPC Id of an existing Virtual Private Cloud.
Resources:
  ALBTargetGroup:
    Type: 'AWS::ElasticLoadBalancingV2::TargetGroup'
    Properties:
      HealthCheckPath: /wordpress/wp-admin/install.php
      HealthCheckIntervalSeconds: 10
      HealthCheckTimeoutSeconds: 5
      HealthyThresholdCount: 2
      Port: 80
      Protocol: HTTP
      UnhealthyThresholdCount: 5
      VpcId: !Ref VpcId
      TargetGroupAttributes:
        - Key: stickiness.enabled
          Value: 'true'
        - Key: stickiness.type
          Value: lb_cookie
        - Key: stickiness.lb_cookie.duration_seconds
          Value: '30'