Templates overview

This topic provides an introduction to the key parts of a template and how to use them.

You can use the sections on this page to learn the following about templates:

About templates

A CloudFormation template is a declaration of the Amazon resources that make up a stack. The template is a text file that follows specific formatting rules using either the JavaScript Object Notation (JSON) or YAML standard. Because templates are text files, you can create and edit them in any text editor and manage them in your source control system with the rest of your source code.

In the template, you declare the Amazon resources that you want to create. You define each resource as a set of properties and values using the syntax rules of the chosen format (JSON or YAML).

Templates have different sections, but the only required section is the Resources section, which must declare at least one resource.

For examples that show the complete structure of a template with all available sections, see Template formats. For a list of resources that you can create with CloudFormation, see Amazon resource and property types reference.

Resources: Hello Bucket!

The Resources section is a list of the resources you want to create. Each resource must have a Type attribute, which defines the kind of Amazon resource it is. The Type attribute has a special format:

AWS::ProductIdentifier::ResourceType

For example, the resource type for an Amazon S3 bucket is AWS::S3::Bucket.

Let's take a look at a basic template. The following template declares a single resource of type AWS::S3::Bucket: with the name HelloBucket.

{
    "Resources": {
        "HelloBucket": {
            "Type": "AWS::S3::Bucket"
        }
    }
}

Resources:
  HelloBucket:
    Type: 'AWS::S3::Bucket'

If you use this template to create a stack, CloudFormation will create an Amazon S3 bucket. Creating a bucket is simple, because CloudFormation can create a bucket with default settings. For other resources, such as an Amazon EC2 instance or Auto Scaling group, CloudFormation requires more information. Resource declarations use a Properties attribute to specify the information used to create a resource.

Depending on the resource type, some properties are required, such as the ImageId property for an AWS::EC2::Instance resource, and others are optional. Some properties have default values, such as the AccessControl property of the AWS::S3::Bucket resource, so specifying a value for those properties is optional. Other properties aren't required but may add functionality that you want, such as the WebsiteConfiguration property of the AWS::S3::Bucket resource. Specifying a value for such properties is entirely optional and based on your needs. In the example above, because the AWS::S3::Bucket resource has only optional properties and we didn't need any of the optional features, we could accept the defaults and omit the Properties attribute.

To view the properties for each resource type, see the topics in Amazon resource and property types reference.

Resource properties and using resources together

Usually, a property for a resource is simply a string value. For example, the following template specifies a canned ACL (PublicRead) for the AccessControl property of the bucket.

{
    "Resources": {
        "HelloBucket": {
            "Type": "AWS::S3::Bucket",
            "Properties": {
                "AccessControl": "PublicRead"
            }
        }
    }
}

Resources:
  HelloBucket:
    Type: 'AWS::S3::Bucket'
    Properties:
      AccessControl: PublicRead

Some resources can have multiple properties, and some properties can have one or more subproperties. For example, the AWS::S3::Bucket resource has two properties: AccessControl and WebsiteConfiguration. The WebsiteConfiguration property has two subproperties: IndexDocument and ErrorDocument. The following template shows our original bucket resource with the additional properties.

{
    "Resources": {
        "HelloBucket": {
            "Type": "AWS::S3::Bucket",
            "Properties": {
                "AccessControl": "PublicRead",
                "WebsiteConfiguration": {
                    "IndexDocument": "index.html",
                    "ErrorDocument": "error.html"
                }
            }
        }
    }
}

Resources:
  HelloBucket:
    Type: 'AWS::S3::Bucket'
    Properties:
      AccessControl: PublicRead
      WebsiteConfiguration:
        IndexDocument: index.html
        ErrorDocument: error.html

One of the greatest benefits of templates and CloudFormation is the ability to create a set of resources that work together to create an application or solution. The name used for a resource within the template is a logical name. When CloudFormation creates the resource, it generates a physical name that's based on the combination of the logical name, the stack name, and a unique ID.

You're probably wondering how you set properties on one resource based on the name or property of another resource. For example, you can create a CloudFront distribution backed by an S3 bucket or an EC2 instance that uses EC2 security groups, and all of these resources can be created in the same template. CloudFormation has a number of intrinsic functions that you can use to refer to other resources and their properties. You can use the Ref function to refer to an identifying property of a resource. Frequently, this is the physical name of the resource; however, sometimes it can be an identifier, such as the IP address for an AWS::EC2::EIP resource or an Amazon Resource Name (ARN) for an Amazon SNS topic. For a list of values returned by the Ref function, see Ref function. The following template contains an AWS::EC2::Instance resource. The resource's SecurityGroups property calls the Ref function to refer to the AWS::EC2::SecurityGroup resource InstanceSecurityGroup.

{
    "Resources": {
        "Ec2Instance": {
            "Type": "AWS::EC2::Instance",
            "Properties": {
                "SecurityGroups": [
                    {
                        "Ref": "InstanceSecurityGroup"
                    }
                ],
                "KeyName": "mykey",
                "ImageId": ""
            }
        },
        "InstanceSecurityGroup": {
            "Type": "AWS::EC2::SecurityGroup",
            "Properties": {
                "GroupDescription": "Enable SSH access via port 22",
                "SecurityGroupIngress": [
                    {
                        "IpProtocol": "tcp",
                        "FromPort": 22,
                        "ToPort": 22,
                        "CidrIp": "0.0.0.0/0"
                    }
                ]
            }
        }
    }
}

Resources:
  Ec2Instance:
    Type: 'AWS::EC2::Instance'
    Properties:
      SecurityGroups:
        - !Ref InstanceSecurityGroup
      KeyName: mykey
      ImageId: ''
  InstanceSecurityGroup:
    Type: 'AWS::EC2::SecurityGroup'
    Properties:
      GroupDescription: Enable SSH access via port 22
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: 22
          ToPort: 22
          CidrIp: 0.0.0.0/0

The SecurityGroups property is a list of security groups, and in the previous example we have only one item in the list. The following template has an additional item in the SecurityGroups property list.

{
    "Resources": {
        "Ec2Instance": {
            "Type": "AWS::EC2::Instance",
            "Properties": {
                "SecurityGroups": [
                    {
                        "Ref": "InstanceSecurityGroup"
                    },
                    "MyExistingSecurityGroup"
                ],
                "KeyName": "mykey",
                "ImageId": "ami-7a11e213"
            }
        },
        "InstanceSecurityGroup": {
            "Type": "AWS::EC2::SecurityGroup",
            "Properties": {
                "GroupDescription": "Enable SSH access via port 22",
                "SecurityGroupIngress": [
                    {
                        "IpProtocol": "tcp",
                        "FromPort": 22,
                        "ToPort": 22,
                        "CidrIp": "0.0.0.0/0"
                    }
                ]
            }
        }
    }
}

Resources:
  Ec2Instance:
    Type: 'AWS::EC2::Instance'
    Properties:
      SecurityGroups:
        - !Ref InstanceSecurityGroup
        - MyExistingSecurityGroup
      KeyName: mykey
      ImageId: ami-7a11e213
  InstanceSecurityGroup:
    Type: 'AWS::EC2::SecurityGroup'
    Properties:
      GroupDescription: Enable SSH access via port 22
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: 22
          ToPort: 22
          CidrIp: 0.0.0.0/0

MyExistingSecurityGroup is a string that refers to an existing EC2 security group instead of a security group declared in a template. You use literal strings to refer to existing Amazon resources.

In the example above, the KeyName property of the AWS::EC2::Instance is the literal string mykey. This means that a key pair with the name mykey must exist in the region where the stack is being created; otherwise, stack creation will fail because the key pair doesn't exist. The key pair you use can vary with the region where you are creating the stack, or you may want to share the template with someone else so that they can use it with their Amazon Web Services account. If so, you can use an input parameter so that the key pair name can be specified when the stack is created. The Ref function can refer to input parameters that are specified at stack creation time. The following template adds a Parameters section containing the KeyName parameter, which is used to specify the KeyName property for the AWS::EC2::Instance resource. The parameter type is AWS::EC2::KeyPair::KeyName, which ensures a user specifies a valid key pair name in his or her account and in the region where the stack is being created.

{
  "Parameters": {
    "KeyName": {
      "Description": "The EC2 Key Pair to allow SSH access to the instance",
      "Type": "AWS::EC2::KeyPair::KeyName"
    }
  },
  "Resources": {
    "Ec2Instance": {
      "Type": "AWS::EC2::Instance",
      "Properties": {
        "SecurityGroups": [
          {
            "Ref": "InstanceSecurityGroup"
          },
          "MyExistingSecurityGroup"
        ],
        "KeyName": {
          "Ref": "KeyName"
        },
        "ImageId": "ami-7a11e213"
      }
    },
    "InstanceSecurityGroup": {
      "Type": "AWS::EC2::SecurityGroup",
      "Properties": {
        "GroupDescription": "Enable SSH access via port 22",
        "SecurityGroupIngress": [
          {
            "IpProtocol": "tcp",
            "FromPort": 22,
            "ToPort": 22,
            "CidrIp": "0.0.0.0/0"
          }
        ]
      }
    }
  }
}

Parameters:
  KeyName:
    Description: The EC2 Key Pair to allow SSH access to the instance
    Type: 'AWS::EC2::KeyPair::KeyName'
Resources:
  Ec2Instance:
    Type: 'AWS::EC2::Instance'
    Properties:
      SecurityGroups:
        - !Ref InstanceSecurityGroup
        - MyExistingSecurityGroup
      KeyName: !Ref KeyName
      ImageId: ami-7a11e213
  InstanceSecurityGroup:
    Type: 'AWS::EC2::SecurityGroup'
    Properties:
      GroupDescription: Enable SSH access via port 22
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: 22
          ToPort: 22
          CidrIp: 0.0.0.0/0

The Ref function is handy if the parameter or the value returned for a resource is exactly what you want; however, you may need other attributes of a resource. For example, if you want to create a CloudFront distribution with an S3 origin, you need to specify the bucket location by using a DNS-style address. A number of resources have additional attributes whose values you can use in your template. To get these attributes, you use the Fn::GetAtt function. The following template creates a CloudFront distribution resource that specifies the DNS name of an S3 bucket resource using Fn::GetAtt function to get the bucket's DomainName attribute.

{
  "Resources": {
    "myBucket": {
      "Type": "AWS::S3::Bucket"
    },
    "myDistribution": {
      "Type": "AWS::CloudFront::Distribution",
      "Properties": {
        "DistributionConfig": {
          "Origins": [
            {
              "DomainName": {
                "Fn::GetAtt": [
                  "myBucket",
                  "DomainName"
                ]
              },
              "Id": "myS3Origin",
              "S3OriginConfig": {}
            }
          ],
          "Enabled": "true",
          "DefaultCacheBehavior": {
            "TargetOriginId": "myS3Origin",
            "ForwardedValues": {
              "QueryString": "false"
            },
            "ViewerProtocolPolicy": "allow-all"
          }
        }
      }
    }
  }
}

Resources:
  myBucket:
    Type: 'AWS::S3::Bucket'
  myDistribution:
    Type: 'AWS::CloudFront::Distribution'
    Properties:
      DistributionConfig:
        Origins:
          - DomainName: !GetAtt 
              - myBucket
              - DomainName
            Id: myS3Origin
            S3OriginConfig: {}
        Enabled: 'true'
        DefaultCacheBehavior:
          TargetOriginId: myS3Origin
          ForwardedValues:
            QueryString: 'false'
          ViewerProtocolPolicy: allow-all

The Fn::GetAtt function takes two parameters, the logical name of the resource and the name of the attribute to be retrieved. For a full list of available attributes for resources, see Fn::GetAtt. You'll notice that the Fn::GetAtt function lists its two parameters in an array. For functions that take multiple parameters, you use an array to specify their parameters.

Receiving user input using input parameters

So far, you've learned about resources and a little bit about how to use them together within a template. You've learned how to refer to input parameters, but we haven't gone deeply into how to define the input parameters themselves. Let's take a look at parameter declarations and how you can restrict and validate user input.

You declare parameters in a template's Parameters section. A parameter contains a list of attributes that define its value and constraints against its value. The only required attribute is Type, which can be String, Number, or an Amazon-specific type. You can also add a Description attribute that describes what kind of value you should specify. The parameter's name and description appear in the Specify Parameters page when you use the template in the Create Stack wizard.

The following template fragment is a Parameters section that declares the parameters used in the Specify Parameters page.

  "Parameters": {
    "KeyName": {
      "Description" : "Name of an existing EC2 KeyPair to enable SSH access into the WordPress web server",
      "Type": "AWS::EC2::KeyPair::KeyName"
    },
    "WordPressUser": {
      "Default": "admin",
      "NoEcho": "true",
      "Description" : "The WordPress database admin account user name",
      "Type": "String",
      "MinLength": "1",
      "MaxLength": "16",
      "AllowedPattern" : "[a-zA-Z][a-zA-Z0-9]*"
    },
    "WebServerPort": {
      "Default": "8888",
      "Description" : "TCP/IP port for the WordPress web server",
      "Type": "Number",
      "MinValue": "1",
      "MaxValue": "65535"
    }
  }

Parameters:
  KeyName:
    Description: Name of an existing EC2 KeyPair to enable SSH access into the WordPress web server
    Type: AWS::EC2::KeyPair::KeyName
  WordPressUser:
    Default: admin
    NoEcho: true
    Description: The WordPress database admin account user name
    Type: String
    MinLength: 1
    MaxLength: 16
    AllowedPattern: "[a-zA-Z][a-zA-Z0-9]*"
  WebServerPort:
    Default: 8888
    Description: TCP/IP port for the WordPress web server
    Type: Number
    MinValue: 1
    MaxValue: 65535

For parameters with default values, CloudFormation uses the default values unless users specify another value. If you omit the default attribute, users are required to specify a value for that parameter; however, requiring the user to input a value does not ensure that the value is valid. To validate the value of a parameter, you can declare constraints or specify an Amazon-specific parameter type.

You'll notice that the KeyName parameter has no Default attribute and the other parameters do. For example, the WordPressUser parameter has the attribute Default: admin, but the KeyName parameter has none. Users must specify a key name value at stack creation. If they don’t, CloudFormation fails to create the stack and throws an exception:

Parameters: [KeyName] must have values

For Amazon-specific parameter types, CloudFormation validates input values against existing values in the user's Amazon Web Services account and in the region where they're creating the stack before creating any stack resources. In the sample template, the KeyName parameter is an Amazon-specific parameter type of AWS::EC2::KeyPair::KeyName. CloudFormation checks that users specify a valid EC2 key pair name before creating the stack. Another example of an Amazon-specific parameter type is AWS::EC2::VPC::Id, which requires users to specify a valid VPC ID. In addition to upfront validation, the Amazon console shows a drop down list of valid values for Amazon-specific parameter types, such as valid EC2 key pair names or VPC IDs, when users use the Create Stack wizard.

For the String type, you can use the following attributes to declare constraints: MinLength, MaxLength, Default, AllowedValues, and AllowedPattern. In the example above, the WordPressUser parameter has three constraints: the parameter value must be 1 to 16 character long (MinLength, MaxLength) and must begin with a letter followed by any combination of letters and numbers (AllowedPattern).

For the Number type, you can declare the following constraints: MinValue, MaxValue, Default, and AllowedValues. A number can be an integer or a float value. In the example above, the WebServerPort parameter must be a number between 1 and 65535 inclusive (MinValue, MaxValue).

Earlier in this section, we mentioned that parameters are a good way to specify sensitive or implementation-specific data, such as passwords or user names, that you need to use but don't want to embed in the template itself. If you set the NoEcho attribute to true, CloudFormation returns the parameter value masked as asterisks (*****) for any calls that describe the stack or stack events, except for information stored in the locations specified below. In the example above, the WordPressUser parameter value isn't visible to anyone viewing the stack's settings, and its value is returned as asterisks.

Specifying conditional values using mappings

Parameters are a great way to specify unique or sensitive values for use in the properties of stack resources; however, there may be settings that are region dependent or are somewhat complex for users to figure out because of other conditions or dependencies. In these cases, you would want to put some logic in the template itself so that users can specify simpler values (or none at all) to get the results that they want.

For example, you might need a way to specify the right instance type based on a conditional input (in this example, the region where the stack is created). There are two template features that can help, the Mappings section and the AWS::Region pseudo parameter.

The AWS::Region pseudo parameter is a value that CloudFormation resolves as the region where the stack is created. Pseudo parameters are resolved by CloudFormation when you create the stack. With mappings, you can use an input value as a condition that determines another value. Similar to a switch statement, a mapping associates one set of values with another. Using the AWS::Region parameter together with a mapping, you can ensure that an instance type that's available in the region is specified. The following template contains a Mappings section with a mapping named RegionMap that's used to map an instance type to the appropriate region.

The following template also declares the AWS::SSM::Parameter::Value<AWS::EC2::Image::Id> parameter type. It provides the Systems Manager parameter alias (/aws/service/ami-amazon-linux-latest/amzn2-ami-hvm-x86_64-gp2) as the default value for the ImageId property of the EC2 instance. This is a value that CloudFormation resolves as the AMI ID value for the latest Amazon Linux 2 AMI in the region where the stack is created.

{
    "Parameters": {
        "LatestAmiId": {
            "Description": "The latest Amazon Linux 2 AMI from the Parameter Store",
            "Type": "AWS::SSM::Parameter::Value<AWS::EC2::Image::Id>",
            "Default": "/aws/service/ami-amazon-linux-latest/amzn2-ami-hvm-x86_64-gp2"
        }
    },
    "Mappings": {
        "RegionMap": {
            "us-east-1": {
                "InstanceType": "t2.micro"
            },
            "us-west-1": {
                "InstanceType": "t2.micro"
            },
            "eu-west-1": {
                "InstanceType": "t2.micro"
            },
            "eu-north-1": {
                "InstanceType": "t3.micro"
            },
            "me-south-1": {
                "InstanceType": "t3.micro"
            }
        }
    },
    "Resources": {
        "Ec2Instance": {
            "Type": "AWS::EC2::Instance",
            "Properties": {
                "ImageId": {
                    "Ref": "LatestAmiId"
                },
                "InstanceType": {
                    "Fn::FindInMap": [
                        "RegionMap",
                        {
                            "Ref": "AWS::Region"
                        },
                        "InstanceType"
                    ]
                }
            }
        }
    }
}

Parameters:
  LatestAmiId:
    Description: The latest Amazon Linux 2 AMI from the Parameter Store
    Type: 'AWS::SSM::Parameter::Value<AWS::EC2::Image::Id>'
    Default: '/aws/service/ami-amazon-linux-latest/amzn2-ami-hvm-x86_64-gp2'
Mappings:
  RegionMap:
    us-east-1:
      InstanceType: t2.micro
    us-west-1:
      InstanceType: t2.micro
    eu-west-1:
      InstanceType: t2.micro
    eu-north-1:
      InstanceType: t3.micro
    me-south-1:
      InstanceType: t3.micro
Resources:
  Ec2Instance:
    Type: 'AWS::EC2::Instance'
    Properties:
      ImageId: !Ref LatestAmiId
      InstanceType: !FindInMap
        - RegionMap
        - !Ref 'AWS::Region'
        - InstanceType

In the RegionMap, each region is mapped to a name-value pair. The name-value pair is a label, and the value to map. In the RegionMap, InstanceType is the label and the instance type is the value. To use a map to return a value, you use the Fn::FindInMap function, passing the name of the map, the value used to find the mapped value, and the label of the mapped value you want to return. In the example above, the InstanceType property of the resource Ec2Instance uses the Fn::FindInMap function to determine its value by specifying RegionMap as the map to use, AWS::Region as the input value to map from, and InstanceType as the label to identify the value to map to. For example, if this template were used to create a stack in the US West (N. California) Region, InstanceType would be set to t2.micro.

Constructed values and output values

Parameters and mappings are an excellent way to pass or determine specific values at stack creation time, but there can be situations where a value from a parameter or other resource attribute is only part of the value you need. For example, in the following WordPress template fragment, the Fn::Join function constructs the Target subproperty of the HealthCheck property for the ElasticLoadBalancer resource by concatenating the WebServerPort parameter with other literal strings to form the value needed.

{
  "Resources": {
    "ElasticLoadBalancer": {
      "Type": "AWS::ElasticLoadBalancing::LoadBalancer",
      "Properties": {
        "AvailabilityZones": {
          "Fn::GetAZs": ""
        },
        "Instances": [
          {
            "Ref": "Ec2Instance1"
          },
          {
            "Ref": "Ec2Instance2"
          }
        ],
        "Listeners": [
          {
            "LoadBalancerPort": "80",
            "InstancePort": {
              "Ref": "WebServerPort"
            },
            "Protocol": "HTTP"
          }
        ],
        "HealthCheck": {
          "Target": {
            "Fn::Join": [
              "",
              [
                "HTTP:",
                {
                  "Ref": "WebServerPort"
                },
                "/"
              ]
            ]
          },
          "HealthyThreshold": "3",
          "UnhealthyThreshold": "5",
          "Interval": "30",
          "Timeout": "5"
        }
      }
    }
  }
}

Resources:
  ElasticLoadBalancer:
    Type: 'AWS::ElasticLoadBalancing::LoadBalancer'
    Properties:
      AvailabilityZones: !GetAZs ''
      Instances:
        - !Ref Ec2Instance1
        - !Ref Ec2Instance2
      Listeners:
        - LoadBalancerPort: '80'
          InstancePort: !Ref WebServerPort
          Protocol: HTTP
      HealthCheck:
        Target: !Join 
          - ''
          - - 'HTTP:'
            - !Ref WebServerPort
            - /
        HealthyThreshold: '3'
        UnhealthyThreshold: '5'
        Interval: '30'
        Timeout: '5'

The Fn::Join function takes two parameters, a delimiter that separates the values you want to concatenate and an array of values in the order that you want them to appear. In the example above, the Fn::Join function specifies an empty string as the delimiter and HTTP:, the value of the WebServerPort parameter, and a / character as the values to concatenate. If WebServerPort had a value of 8888, the Target property would be set to the following value:

HTTP:8888/

For more information about the Fn::Join function, see Fn::Join.

The Fn::Join function is also useful for declaring output values for the stack. The Outputs section in the template contains declarations for the values that you want to have available after the stack is created. An output is a convenient way to capture important information about your resources or input parameters. For example, we might declare the following outputs for a template that creates a WordPress site.

{
    "Outputs": {
        "InstallURL": {
            "Value": {
                "Fn::Join": [
                    "",
                    [
                        "http://",
                        {
                            "Fn::GetAtt": [
                                "ElasticLoadBalancer",
                                "DNSName"
                            ]
                        },
                        "/wp-admin/install.php"
                    ]
                ]
            },
            "Description": "Installation URL of the WordPress website"
        },
        "WebsiteURL": {
            "Value": {
                "Fn::Join": [
                    "",
                    [
                        "http://",
                        {
                            "Fn::GetAtt": [
                                "ElasticLoadBalancer",
                                "DNSName"
                            ]
                        }
                    ]
                ]
            }
        }
    }
}

Outputs:
  InstallURL:
    Value: !Join 
      - ''
      - - 'http://'
        - !GetAtt 
          - ElasticLoadBalancer
          - DNSName
        - /wp-admin/install.php
    Description: Installation URL of the WordPress website
  WebsiteURL:
    Value: !Join 
      - ''
      - - 'http://'
        - !GetAtt 
          - ElasticLoadBalancer
          - DNSName

Each output value has a name, a Value attribute that contains declaration of the value returned as the output value, and optionally a description of the value. In the previous example, InstallURL is the string returned by a Fn::Join function call that concatenates http://, the DNS name of the resource ElasticLoadBalancer, and /wp-admin/install.php. The output value would be similar to the following:

http://mywptests-elasticl-1gb51l6sl8y5v-206169572.aws-region.elb.amazonaws.com.cn/wp-admin/install.php

After creating our stack, we might use this link to go to the installation page for the WordPress blog that we created. CloudFormation generates the output values after it finishes creating the stack. You can view output values in the Outputs tab of the CloudFormation console or by using the describe-stacks CLI command.

Learn more

We just walked through the basic parts of a template and how to use them.

We didn't cover two top level sections in a template: AWSTemplateFormatVersion and Description.

AWSTemplateFormatVersion is simply the version of the template format. If you don't specify it, CloudFormation will use the latest version. For more information, see Format version.

The Description is any valid JSON or YAML string. This description appears in the Specify Parameters page of the Create Stack wizard. For more information, see Description.

Of course, there are more advanced template and stack features. Here is a list of a few important ones that you'll want to learn more about: