CloudFormation Templates in YAML

AWS recently announced support for authoring CloudFormation templates in YAML instead of JSON. This is a big deal for one simple reason: YAML supports the use of comments, which has been a major gap in JSON templating.

YAML is a ubiquitous data serialization language and is used a lot for configuration file syntax as well as an alternative to JSON and XML. It has a smallish learning curve because of non-intuitive features like the syntactical importance of indentation. Nevertheless, it offers a strong alternative to authoring files in JSON because of its readability and relative lack of delimiter collision.

If you have existing JSON CloudFormation templates, you can convert them to YAML via the most excellent Python package “json2yaml“. Installing the package is as simple as:

pip install json2yaml

Once installed, you can try converting a template as follows:

cd /path/to/templates
json2yaml ./mytemplate.json ./mytemplate.yml

If you do not specify the 2nd parameter for the YAML output file, json2yaml will stream the converted file content to STDOUT.

I used json2yaml to convert a relatively sophisticated JSON-based CloudFormation template for deploying a CodeCommit repository and then used the YAML output version to create a new CF stack and it worked flawlessly.

To learn more about YAML, I recommend reading the Wikipedia page about it along with using this handy reference sheet from

Now, go forth and create stacks with all the comments you have ever wanted to include!


Confusing syntax error with AWS CLI validation of CF templates

When using awscli to create CloudFormation stacks, there’s a pre-create step of template validation that checks for template syntax issues:

aws cloudformation validate-template --template-body file:///path/to/template.json

The URI prefix file:// indicates that we are using local templates while templates at web-accessible locations like your S3 bucket are accessed using the –template-url option. For more information see the awscli docs or CLI help:

aws cloudformation validate-template help

For local templates, make sure you don’t forget the file:// URI and try to refer to the template via normal filesystem paths, otherwise you’ll get a confusing syntax error.

Without file:// URI:

aws cloudformation validate-template --template-body /Users/foo/git-repos/aws-mojo/cloudformation/aws-deploy-codecommit-repo.yml

An error occurred (ValidationError) when calling the ValidateTemplate operation: Template format error: unsupported structure.

That’s a rather unhelpful error message that makes me think I’ve got some sort of template content error.

When we run the same command with file:// URI, we get the expected output:

aws cloudformation validate-template --template-body file:///Users/foo/git-repos/aws-mojo/cloudformation/aws-deploy-codecommit-repo.yml
   "Description": "CloudFormation template for creating a CodeCommit repository along with a SNS topic for repo activity trigger notifications",
   "Parameters": [
   "NoEcho": false,
   "Description": "Email address for SNS notifications on repo events",
   "ParameterKey": "Email"
   "NoEcho": false,
   "Description": "A unique name for the CodeCommit repository",
   "ParameterKey": "RepoName"
   "DefaultValue": "Dev",
   "NoEcho": false,
   "Description": "Environment type (can be used for tagging)",
   "ParameterKey": "Environment"
   "NoEcho": false,
   "Description": "A description of the CodeCommit repository",
   "ParameterKey": "RepoDescription"