New AWS CloudFormation YAML syntax and variable substitution in action

I’ve been using CloudFormation YAML syntax for a while now with Ansible and the serverless framework which would convert the YAML to JSON before uploading the template. That already gave me the YAML advantages of e.g. code comments, not having to care about commas etc.

A few days ago, AWS announced native YAML support for CloudFormation templates, in addition to the existing JSON format.

And along with that they added new shorthand syntax for several functions.

Let’s go through a template which I created not only in order to get used to the new syntax :)

Injecting “arguments” to inline Lambda functions

One of the real powers of Lambda and CloudFormation is that you can use Lambda to add almost any missing functionality to CloudFormation (e.g. custom resources), or to create small functions, without having to maintain another deployment workflow for the function (In this example I created an Lambda function which polls some web services and writes the result into a CloudWatch custom metric.)

The interesting part is how AccessPointName is injected into the Lambda function (in this example some Python code). We are making use of the new short substitution syntax here which allows us to replace CloudFormation references with their value:

   Type: AWS::Lambda::Function
       ZipFile: !Sub |
         def handler(event, context):  
           found_access_points = [access_point for access_point in api_result["allTheRouters"] if access_point["name"] == "${AccessPointName}"]

In this example the variable “AccessPointName“ gets then substituted by the value (in this particular case a stack parameter). Please also mind the “|” which is no special CloudFormation syntax but multi line YAML syntax.

Throughout the
you can find other usage examples of the new substitution syntax, for example a cron job with CloudWatch events which gets:

   Type: AWS::Events::Rule
     ScheduleExpression: !Sub rate(${CheckRateMinutes} minutes)
       - Arn:
           !GetAtt [CheckProgram, Arn]
         Id: InvokeLambda

Referencing with the !Ref and !GetAttr shortcuts

Another feature addition is a short hand syntax for Ref and GetAttr calls.

   Type: AWS::SNS::Topic
       - Endpoint: !Ref NotificationEmail
         Protocol: email

This example creates an SNS topic with an email subscription which is once again a CloudFormation template parameter.


With the new syntax it’s now possible to create YAML syntax, and we have nice shortcuts for commonly used functions. My personal highlight is the shorthand substitution syntax, esp. when using inline Lambda functions.

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