cfndsl

AWS Cloudformation templates are an incredibly powerful way to build sets of resources in Amazon's AWS environment. Unfortunately, because they are specified in JSON, they are also difficult to write and maintain:

• JSON does not allow comments

• All structures are JSON, so it is sometimes easy for a person reading a template to get lost.

• References and internal functions have a particularly unpleasant syntax.

The cnfdsl gem provides a simple DSL that allows you to write equivalent templates in a more friendly language and generate the correct json templates by running ruby.

Getting Started

sudo gem install cfndsl

Now write a template in the dsl

CloudFormation {
  Description "Test"

  Parameter("One") {
    String
    Default "Test"
	MaxLength 15
  }

  Output(:One,FnBase64( Ref("One")))

  EC2_Instance(:MyInstance) {
    ImageId "ami-12345678"
  }

}

Then run cfndsl on the file

chris@raspberrypi:~/git/cfndsl$ cfndsl test.rb | json_pp
{
   "Parameters" : {
      "One" : {
         "Type" : "String",
         "Default" : "Test",
         "MaxLength" : 15
      }
   },
   "Resources" : {
      "MyInstance" : {
         "Type" : "AWS::EC2::Instance",
         "Properties" : {
            "ImageId" : "ami-12345678"
         }
      }
   },
   "AWSTemplateFormatVersion" : "2010-09-09",
   "Outputs" : {
      "One" : {
         "Value" : {
            "Fn::Base64" : {
               "Ref" : "One"
            }
         }
      }
   },
   "Description" : "Test"
}

Aside: that is correct - a significant amount of the development for this gem was done on a Raspberry Pi.

Samples

There is a more detailed example in the samples directory. The file "autoscale.template" is one of the standard Amazon sample templates. "autoscale.rb" generates an equivalent template file.

Command Line Options

The cfndsl command line program now accepts some command line options.

Usage: cfndsl [options] FILE
    -o, --output FILE                Write output to file
    -y, --yaml FILE                  Import yaml file as local variables
    -r, --ruby FILE                  Evaluate ruby file before template
    -j, --json FILE                  Import json file as local variables
    -p, --pretty                     Pretty-format output JSON
    -D, --define "VARIABLE=VALUE"    Directly set local VARIABLE as VALUE
    -v, --verbose                    Turn on verbose ouptut
    -h, --help                       Display this screen

By default, cfndsl will attempt to evaluate FILE as cfndsl template and print the resulting cloudformation json template to stdout. With the -o option, you can instead have it write the resulting json template to a given file. The -v option prints out additional information (to stderr) about what is happening in the model generation process.

The -y, -j, -r and -D options can be used to control some things about the environment that the template code gets evaluate in. For instance, the -D option allows you to set a variable at the command line that can then be referred to within the template itself.

This is best illustrated with a example. Consider the following cfndsl template

# cfndsl template t1.rb
CloudFormation {

  DESCRIPTION ||= "default description"
  MACHINES ||= 1

  Description DESCRIPTION

  (1..MACHINES).each do |i|
    name = "machine#{i}"
    EC2_Instance(name) {
      ImageId "ami-12345678"
      Type "t1.micro"
    }
  end

}

Note the two variables "DESCRIPTION" and "MACHINES". The template sets these to some reasonable default values, and if you run cfndsl on it without changing them in any way you get the following cloudformation template:

{
  "Resources": {
    "machine1": {
      "Type": "AWS::EC2::Instance",
      "Properties": {
        "ImageId": "ami-12345678"
      }
    }
  },
  "Description": "default description",
  "AWSTemplateFormatVersion": "2010-09-09"
}

However if you run the command

$ cfndsl t1.rb -D 'DESCRIPTION="3 machine cluster"' -D 'MACHINES=3'

you get the following generated template.

{
  "Resources": {
    "machine3": {
      "Type": "AWS::EC2::Instance",
      "Properties": {
        "ImageId": "ami-12345678"
      }
    },
    "machine2": {
      "Type": "AWS::EC2::Instance",
      "Properties": {
        "ImageId": "ami-12345678"
      }
    },
    "machine1": {
      "Type": "AWS::EC2::Instance",
      "Properties": {
        "ImageId": "ami-12345678"
      }
    }
  },
  "Description": "3 machine cluster",
  "AWSTemplateFormatVersion": "2010-09-09"
}

The -y and -j options allow you to group several variable definitions into a single file (formated as either yaml or ruby respectively). If you had a file called 't1.yaml' that contained the following,

# t1.yaml
DESCRIPTION: 5 machine cluster
MACHINES: 5

the command

$ cfndsl t1.rb -y t1.yaml

would generate a template with 5 instances declared.

Finally, the -r option gives you the opportunity to execute some arbitrary ruby code in the evaluation context before the cloudformation template is evaluated.

Rake task

Simply add the following to your Rakefile:

require 'cfndsl/rake_task'

CfnDsl::RakeTask.new do |t|
  t.cfndsl_opts = {
    verbose: true,
    files: [{
      filename: 'templates/application.rb',
      output: 'application.json'
    }],
    extras: [
      [ :yaml, 'templates/default_params.yml' ]
    ]
  }
end

And then use rake to generate the cloudformation:

$ bin/rake generate