Modules¶

A module is a template file that another template includes. cfnrain inserts the module's resources into the parent template when it packages it. Modules let you split a large template into files and reuse a group of resources in many stacks.

Modules are a client-side feature. CloudFormation only sees the packaged template, so run cfnrain pkg to see the result. Run linters such as cfn-lint on the packaged template, because they do not know the Modules section.

Example¶

The module, bucket-module.yaml:

Parameters:
  Name:
    Type: String

Resources:
  Bucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: !Ref Name
      VersioningConfiguration:
        Status: Enabled

Outputs:
  Arn:
    Value: !GetAtt Bucket.Arn

The parent template:

Modules:
  Logs:
    Source: ./bucket-module.yaml
    Properties:
      Name: billing-logs
    Overrides:
      Bucket:
        DeletionPolicy: Retain
        Properties:
          VersioningConfiguration:
            Status: Suspended

Outputs:
  LogsArn:
    Value: !GetAtt Logs.Arn

The packaged template:

Resources:
  LogsBucket:
    DeletionPolicy: Retain
    Type: AWS::S3::Bucket
    Properties:
      BucketName: billing-logs
      VersioningConfiguration:
        Status: Suspended

Outputs:
  LogsArn:
    Value: !GetAtt LogsBucket.Arn

The Modules section¶

Each entry in Modules has a name and a configuration:

Key	What it does
`Source`	Where the module file is. See Sources.
`Properties`	Values for the module's `Parameters`
`Overrides`	Changes to the module's resources. See Overrides.
`ForEach`	Make one copy of the module for each item in a list. See ForEach.

Experimental status¶

The Modules section is not experimental and needs no flag. Only the older !Rain::Module directive needs --experimental. See Experimental features.

The feature is still young. The format can change in a minor release.

Next steps¶

Writing modules: parameters, outputs, logical IDs, overrides and sources
ForEach: one copy of a module for each item in a list
IfParam and conditions: leave out resources based on the properties
Intrinsic functions: Fn::Merge, Fn::InsertFile and Fn::Invoke
Packages: share modules as zip files
Constants: values that you write once