Stratosphere: AWS CloudFormation in Haskell

AWS CloudFormation is a system that provisions and updates Amazon Web Services (AWS) resources based on declarative templates. Common criticisms of CloudFormation include the use of JSON as the template language and limited error-checking, often only available in the form of run-time errors and stack rollbacks. By wrapping templates in Haskell, it is possible to easily construct them and help ensure correctness.

The goals of stratosphere are to:

• Build a Haskell EDSL to specify CloudFormation templates. Since it is embedded in Haskell, it is type-checked and generally much easier to work with than raw JSON/YAML.
• Have a simple checking/linting system outside of the types that can find common errors in templates.

Funding / Sponsoring

This library is maintained by mbj and any pledge is greatly apprechiated.

Example

THIS SHOWS UNRELEASED API, to use it use a git source while 1.0 is under development old readme.

Here is an example of a Template that creates an EC2 instance, along with the JSON output:

module Main where

import Stratosphere

import qualified Data.ByteString.Lazy.Char8 as B

main :: IO ()
main = B.putStrLn $ encodeTemplate template

template :: Template
template
  = mkTemplate [ec2Instance]
  & set @"Description" "EC2 Example template"
  & set @"Parameters"  [keyName]

keyName :: Parameter
keyName
  = mkParameter "KeyName" "AWS::EC2::KeyPair::KeyName"
  & set @"Description"           "Name of an existing EC2 KeyPair to enable SSH access to the instance"
  & set @"ConstraintDescription" "Must be the name of an existing EC2 KeyPair."

ec2Instance :: Resource
ec2Instance
  = set @"DeletionPolicy" Retain
  . resource "EC2Instance"
  $ EC2.mkInstance
  & set @"ImageId" "ami-22111148"
  & set @"KeyName" (toRef keyName)

{
  "Description": "EC2 Example template",
  "Parameters": {
    "KeyName": {
      "Description": "Name of an existing EC2 KeyPair to enable SSH access to the instance",
      "ConstraintDescription": "Must be the name of an existing EC2 KeyPair.",
      "Type": "AWS::EC2::KeyPair::KeyName"
    }
  },
  "Resources": {
    "EC2Instance": {
      "DeletionPolicy": "Retain",
      "Properties": {
        "ImageId": "ami-22111148",
        "KeyName": {
          "Ref": "KeyName"
        }
      },
      "Type": "AWS::EC2::Instance"
    }
  }
}

Please see the examples directory for more in-depth examples (including this one). The stratosphere-example package produces a same named binary with a minimal CLI for exploration.

Its encouraged to use it as a playground while exploring this library.

STACK_YAML=stack-9.2.yaml stack build --copy-bins --test stratosphere-examples

Value Types

CloudFormation resource parameters can be literals (strings, integers, etc), references to another resource or a Parameter, or the result of some function call. We encapsulate all of these possibilities in the Value a type.

It is recommend using the OverloadedStrings and OverloadedLists extensions to reduce the number of Literals that have to be written.

Optional and required properties

Almost every CloudFormation resource has a handful of required arguments, and many more optional arguments. Each resource is represented as a record type with optional arguments wrapped in Maybe. Each resource also comes with a builder that accepts required resource properties as arguments. This allows the user to succinctly specify the resource properties they actually use without adding too much noise to their code.

To specify optional arguments, stratosphere exposes the set function that takes the type level symbol of the property to set and the value as argument. Its recommended to use the & function to chain these updates. See examples.

Auto-generation

All of the resources and resource properties are auto-generated from a JSON schema file and are placed in services/. The generator/ directory contains the auto-generator package stratosphere-generator code and the JSON model file. The services/ directory is included in git so the build process is simplified. To build stratosphere-generator from scratch and then build all of stratosphere, build the stratosphere-generator package via stack and execute the stratosphere-generator binary from the project root.

Contributing

Feel free to raise any issues, or even just make suggestions, by filing a Github issue.

Future Work

• Implement basic checker for things like undefined Refs and duplicate field names. This stuff would be too unwieldy to do in types, and performing a checking pass over a template should be pretty straightforward.

Development Build

# warning takes a while ;)
STACK_YAML=stack-9.2.yaml stack build --copy-bins --test stratosphere-generator