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:

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.

Michael Wittig

I’ve been building on AWS since 2012 together with my brother Andreas. We are sharing our insights into all things AWS on cloudonaut and have written the book AWS in Action. Besides that, we’re currently working on bucketAV and marbot.

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Optional Parameter in CloudFormation

Michael Wittig

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