Cloudformation Transform::Include - YAML/JSON malformed

Good day y’all,

You’re here because you probably had issues using the Transform::Include feature of Cloudformation.

Note: by the time you get to this blog, AWS might have fixed this bug.

Let me tell you .. it’s awesome!

Most people I’ve worked with have been looking for this for a while! To achieve it, they were using Nested stacks.
But we all know that it’s very different.

I was personally very excited to see that announcement. But really … I only had an opportunity to start using it yesterday.
And it’s awesome!

Now there’s a few gotchas … that might get fixed in the future, but for now there’s a few issue.

Let’s consider the following cloudformation template:

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AWSTemplateFormatVersion: '2010-09-09'
Description: Transform Include example
Parameters:
  ArtifactBucket:
    Type: String
  BucketName:
    Type: String
Resources:
  'Fn::Transform':
    Name: 'AWS::Include'
    Parameters:
      Location: !Sub "s3://${ArtifactBucket}/s3.yaml"

This cloudformation teamplte basically is just gonna to create an s3 bucket.
The definition of the S3 resource would be done in the s3.yaml file, hosted in our artifact bucket.

The s3.yaml would have the following content:

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AppBucket:
  Type: "AWS::S3::Bucket"
  Properties:
    BucketName: !Ref BucketName

So far, easy hey!

Note: Before you try to deploy the template, make sure you’ve uploaded the s3.yaml snippet in the right location first

Now, let’s try to deploy that cloudformation template:

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~ $ aws cloudformation deploy --stack-name transform-include-example --template-file file://cloudformation.yaml
Waiting for changeset to be created..
Failed to create the changeset: Waiter ChangeSetCreateComplete failed: Waiter encountered a terminal failure state Status: FAILED. Reason: Transform AWS::Include failed with: The specified S3 object's content should be valid Yaml/JSON

Wait … what ?! Is that telling me that the s3.yaml file is not valid?? But .. when I lint my yaml file, it says it’s valid!!

Well well… actually, not sure if it’s a bug, or not (although it smells like one), but basically you cannot use the short notation of the Cloudformation intrinsic functions.

You can’t use !Ref BucketName in this case.

Instead the correct snippet would be:

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AppBucket:
  Type: "AWS::S3::Bucket"
  Properties:
    BucketName:
      Ref: BucketName

Try that, and let me know on Twitter!