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Optional Parameter in CloudFormation

Michael Wittig – 23 Mar 2016

Sometimes you want a CloudFormation Parameter to be optional. Unfortunately a blank Parameter contains an empty string. If you pass this empty string to e.g. the KeyName Property of an EC2 Instance or Launch Configuration you end up with a validation error.

Imagine the following CloudFormation template:

{
  "AWSTemplateFormatVersion": "2010-09-09",
  "Description": "Optional Parameters in CloudFormation: Problem",
  "Parameters": {
    "KeyName": {
      "Description": "Provides the name of the EC2 key pair",
      "Type": "String"
    }
  },
  "Resources": {
    "LaunchConfiguration": {
      "Type": "AWS::AutoScaling::LaunchConfiguration",
      "Properties": {
        "KeyName": {"Ref": "KeyName"}
        [...]
      }
    }
  }
}

If you leave the KeyName blank you will get the following error:

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Parameter validation failed: parameter value for parameter name KeyName does not exist. Rollback requested by user.

The solution is the make use of CloudFormation Conditions, the Condition Function Fn::If and the Pseudo Parameter AWS::NoValue.

{
  "AWSTemplateFormatVersion": "2010-09-09",
  "Description": "Optional Parameters in CloudFormation: Solution",
  "Parameters": {
    "KeyName": {
      "Description": "Provides the name of the EC2 key pair",
      "Type": "String"
    }
  },
  "Conditions": {
    "HasKeyName": {"Fn::Not": [{"Fn::Equals": ["", {"Ref": "KeyName"}]}]}
  },
  "Resources": {
    "LaunchConfiguration": {
      "Type": "AWS::AutoScaling::LaunchConfiguration",
      "Properties": {
        "KeyName": {"Fn::If": ["HasKeyName", {"Ref": "KeyName"}, {"Ref": "AWS::NoValue"}]}
        [...]
      }
    }
  }
}

Now you can leave the KeyName parameter blank to not add a EC2 KeyPair to the LaunchConfiguration.

Published on

Tags: aws cloudformation parameters conditions
Michael Wittig

Michael Wittig

I'm an independent consultant, technical writer, and programming founder. All these activities have to do with AWS. I'm writing this blog and all other projects together with my brother Andreas.

In 2009, we joined the same company as software developers. Three years later, we were looking for a way to deploy our software—an online banking platform—in an agile way. We got excited about the possibilities in the cloud and the DevOps movement. It’s no wonder we ended up migrating the whole infrastructure of Tullius Walden Bank to AWS. This was a first in the finance industry, at least in Germany! Since 2015, we have accelerated the cloud journeys of startups, mid-sized companies, and enterprises. We have penned books like Amazon Web Services in Action and Rapid Docker on AWS, we regularly update our blog, and we are contributing to the Open Source community. Besides running a 2-headed consultancy, we are entrepreneurs building Software-as-a-Service products.

We are available for projects.

You can contact me via Email, Twitter, and LinkedIn.

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