Constants¶

A constant is a value that you write once and use in many places. cfnrain replaces the constants and removes their section when it packages the template.

cfnrain has two ways to write constants. Both work. Pick one system per template, so readers know where to look.

cfnrain replaces both ${Rain::Name} and ${Const::Name} in !Sub strings. If a !Sub has no variables left after cfnrain inserts the constants, cfnrain replaces it with a plain string.

The Constants section¶

A template or a module can have a top-level Constants: section. A constant is a string or an object.

Constants:
  S3Arn: "arn:${AWS::Partition}:s3:::"
  BucketName: "${Name}-${AWS::Region}-${AWS::AccountId}"
  BucketArn: "${Const::S3Arn}${Const::BucketName}"
  Tags:
    Team: platform

Use a constant in two ways:

• ${Const::<Name>} inside a !Sub string inserts a string constant.
• !Ref Const::<Name> inserts an object constant. You can only insert the whole object, not a part of it.

A constant can use the constants above it with ${Const::<Name>}. cfnrain replaces the constants before it processes the modules.

Example¶

The parent template:

Constants:
  yyy: zzz
Metadata:
  test: !Sub ${Const::yyy}
Modules:
  Content:
    Source: ./constant-module.yaml

The module, constant-module.yaml:

Constants:
  foo: bar
  sub: "${Const::foo}-abc-${AWS::AccountId}"
  obj:
    TestObject:
      A: b

Resources:
  Bucket:
    Type: AWS::S3::Bucket
    Metadata:
      Test: !Sub ${Const::sub}-xyz
      TestObj: !Ref Const::obj
    Properties:
      BucketName: !Sub ${Const::foo}

The packaged template:

Metadata:
  test: zzz
Resources:
  ContentBucket:
    Type: AWS::S3::Bucket
    Metadata:
      Test:
        Fn::Sub: bar-abc-${AWS::AccountId}-xyz
      TestObj:
        TestObject:
          A: b
    Properties:
      BucketName: bar

The parent's ${Const::yyy} becomes the plain string zzz. In the module, ${Const::sub} keeps ${AWS::AccountId}, so it stays a Fn::Sub.

The Rain section¶

The Rain: section of a template can also hold constants:

Rain:
  Constants:
    App: billing

Use a constant in two ways:

• !Rain::Constant <Name> inserts the value. The value can be a string or an object.
• ${Rain::<Name>} inside a !Sub string inserts a string value.

A constant can use the constants above it, with ${Rain::<Name>} in a !Sub.

Example¶

The template:

Parameters:
  Env:
    Type: String

Rain:
  Constants:
    App: billing
    Prefix: !Sub ${Env}-${Rain::App}

Resources:
  Bucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: !Sub ${Env}-${Rain::App}-uploads
  Table:
    Type: AWS::DynamoDB::Table
    Properties:
      TableName: !Rain::Constant Prefix

The packaged template:

Parameters:
  Env:
    Type: String

Resources:
  Bucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: !Sub ${Env}-billing-uploads
  Table:
    Type: AWS::DynamoDB::Table
    Properties:
      TableName: !Sub ${Env}-billing

${Env} stays in the output, because it is a template parameter that CloudFormation resolves.

Known issue: a !Sub constant inside another !Sub¶

Rain:
  Constants:
    Prefix: !Sub ${Env}-billing
Resources:
  Table:
    Type: AWS::DynamoDB::Table
    Properties:
      TableName: !Sub ${Rain::Prefix}-orders   # packages to "-orders"