CloudFormation Intrinsic Functions¶

Some of the intrinsic functions: - Ref - Fn::GetAtt - Fn::FindInMap - Fn::ImportValue - Fn::Join - Fn::Sub - Condition Functions (Fn::If, Fn::Not, Fn::Equals, etc)

Fn::Ref¶

• The Fn::Ref function can be leveraged to reference
  • parameters -> returns the value of the parameter
  • resources -> returns the physical ID of the underlying resource
• The shorthand for this in YAML is !Ref

DbSubnet1:
    Type: AWS::EC2::Subnet
    Properties:
        VpcId: !Ref MyVPC

Fn::GetAtt¶

• Attributes are attached to any resource you create
• To know the attributes of your resources, the best place to look at is the documentation

Resources:
    EC2Instance:
        Type: AWS::EC2::Instance
        Properties:
            ImageId: ami-1234567
            InstanceType: t2.micro
    NewVolume:
        Type: AWS::EC2::Volume
        Condition: CreateProdResources
        Properties:
            Size: 100
            AvailabilityZone:
                !GetAtt EC2Instance.AvailabilityZone

Fn::FindInMap¶

• We use Fn::FindInMap to return a named value from a specific key

RegionMap:
    us-east-1:
        "32": "ami-6411e20d"
        "64": "ami-7a11e213"
    us-west-1:
        "32": "ami-c9c7978c"
        "64": "ami-cfc7978a"
    eu-west-1:
        "32": "ami-37c2f643"
        "64": "ami-31c2f645"
Resources:
    myEc2Instance:
        Type: AWS::EC2::Instance
        Properties:
            ImageId: !FindInMap [RegionMap, !Ref AWS::Region, 32]
        InstanceType: m1.small        

You can reference the map using

!FindInMap [ MapName, TopLevelKey, SecondLevelKey ]

Fn::ImportValue¶

• Import values that are exported in other templates

Resources:
    MySecureInstance:
        Type: AWS::EC2::Instance
    Properties:
        AvailabilityZone: us-east-1a
        ImageId: ami-a4c7edb2
        InstanceType: t2.micro
        SecurityGroups:
            - !ImportValue SSHSecurityGroup

Fn::join¶

• Join values with a delimiter

!Join [ delimiter, [ comma-delimited list of values ] ]

This creates "a:b:c"

!Join [ ":", [ a, b, c ] ]

Fn::Sub¶

• Sn::Sub or !Sub as a shorthand is used to substitute variables from a text. It's a very handy function that will allow you to fully customize your templates.
• For example you can combine Fn::Sub with references or AWS Pseudo variables.
• String must contain ${variableName} and will substitute them

!Sub
    - String
    - {Var1Name: Var1Value, Var2Name: Var2Value }

!Sub String

Condition Functions¶

Conditions:
    CreateProdResources: !Equal [ !Ref EnvType, prod ]

• The logical ID is for you yo choose. It's how you name condition
• The intrinsic function (logical) can be any of the following:
  • Fn::And
  • Fn::Equals
  • Fn::If
  • Fn::Not
  • Fn::Or