Writing a plugin

A plugin’s purpose is to transform input YAML into CloudFormation resources, outputs or mappings.

Getting Started

Before you start writing a plugin will need a working Go development environment, see Golang’s getting started.

In addition you need a version of kombustion built from source.

# Get the latest kombustion
$ go get github.com/KablamoOSS/kombustion

# Compile and install it
$ go install github.com/KablamoOSS/kombustion

# Verify you have a version that's BUILT_FROM_SOURCE
$ kombustion -v
> kombustion version BUILT_FROM_SOURCE

When kombustion is built from source a flag is made available that allows loading any arbirary plugin for the specific purpose of developing plugins. This flag does not exist for official releases of kombustion, preventing arbritrary plugins from being loaded.

You can load a plugin using --load-plugin path/to/plugin.so before your command. For example, we can load a plugin built in a different folder.

$ kombustion --load-plugin ../kombustion-plugin-example/kombustion-plugin-example.so generate stacks/MyDemoStack.yaml

The easiest way to get started is to start from a copy of the boilerplate example plugin. This repository has everything you need to get started writing a plugin, including default configuration, an example folder layout, and a build script.

$ go get github.com/KablamoOSS/kombustion-plugin-example
$ cp $GOPATH/src/github.com/KablamoOSS/kombustion-plugin-example \


You plugin needs to provide some configuration information to kombustion when it’s loaded. This includes information on the plugin itself, and the resources, mappings, and outputs the plugin provides.

The return types in plugin.go are all []byte, as communication between kombustion and plugins is sent as binary. This is a limitation/feature of Go’s plugin implementation. Marshalling/unmarshalling of binary is taken care of for you.

This is defined in plugin.go. To register the plugin we need to provide a Register function that returns api.RegisterPlugin with our config.

Just as with kombustion version is passed in at compile time (this is done in the build script below), and we have a fallback version BUILT_FROM_SOURCE. When not using --load-plugin an error will be thrown if the version defined in the plugin does not match the version expected.

The other key configuration value is Prefix. This should be set to a pattern of Organisation::Repository, so for the boilerplate example we would use Kablamo::Boilerplate.

The prefix is then added to the start of any resources you define, seperated with ::. So if we had a Log resource, the final resource export will be Kablamo::Example::Log

There is no enforcement on what a Prefix can be, excepting those in CloudFormation: AWS::* and Custom::*. If a user has two plugins that have clashing prefixes, they can use the Alias paramter in kombustion.yaml to add another name in front for example MyAlias::Kablamo::Example::Log

package main

import (
  // Import the plugin api functions
  // Import the plugin api types

var (
  version string

func init() {
  // Set a fallback version when not built from a tag
  if version == "" {
    version = "BUILT_FROM_SOURCE"

// Register plugin
func Register() []byte {
  return api.RegisterPlugin(types.Config{
    // Name should match the name of your repository
    // so github.com/KablamoOSS/kombustion-plugin-example
    // becomes kombustion-plugin-example
    Name: "kombustion-plugin-example",
    // Version is set at compile time, to the tag
    Version: version,
    // The prefix for all resources this plugin exports
    Prefix: "Kablamo::Example",
    Help: types.Help{
      Description: "An Example Plugin",
      Types: []types.TypeMapping{
          Name:        "Log",
          Description: "Creates a Log Group.",
          Config:      parsers.LogConfig{},

// You also need a main() function to be defined, but it will never be called
// This is required due to how Go's plugins work
func main() {}

Now our plugin is registered, we need to provide our parser functions.

// Parsers for this plugin
var Parsers = map[string]func(
  name string,
  data string,
) []byte{
  // resources.ParseLambdaFunction is explained in the next section
  "Log": api.RegisterParser(parser.ParseExampleLog),

Writing a Parser function

A Parser function takes in YAML as a string, and returns one or more of its type (either Resource, Mapping, Conditions, Metdata, Outputs, Parameters, Transforms).

They’re usually stored in ./parsers, and often with a seperate package per parser if they are complex enough.

To create a Resource in our function, as we did above, we need to import our resources package in plugin.go.

package main

import (
  // Import this plugins parser functions for resources

The process for a Parser function is the same for Resource, Mapping and Output. So in this example we’ll cover Resource.

In a new file at resources/lambdaFunction.go we start by importing them from kombustion core the CloudFormation parser functions, some core types, and a YAML library.

package resources

import (
  cfResources "github.com/KablamoOSS/kombustion/pkg/parsers/resources"
  kombustionTypes "github.com/KablamoOSS/kombustion/types"
  yaml "github.com/KablamoOSS/yaml"

First we need to make a struct defining the shape of the incoming YAML. This is what the user will put into their template.

In this example the YAML would look like:

# Parameters are show as an example here
    Type: String
    Type: String
  # MyLambda is passed to the parser function as the `name` argument
    # Type is derived from Prefix::ResourceName
    Type: Kablamo::Example::Log
    # Properties is the Config struct we are about to define, which is passed to the
    # parser function as `data`
      LogGroupName: ExampleLogGroup
      RetentionInDays: 5

The config struct uses tags to inform the YAML library how to unmarshall the yaml into the struct.

// LambdaFunctionConfig defines the shape of the input YAML this resource takes
type LogConfig struct {
	Properties struct {
		LogGroupName    interface{} `yaml:"LogGroupName,omitempty"`
		RetentionInDays interface{} `yaml:"RetentionInDays,omitempty"`
	} `yaml:"Properties"`

Now we have defined the shape of our YAML, we can write the parser function.

A parser function takes two arguments name and data both strings. Where name is the name of the object in the CloudFormation template, and data is the yaml of Properties of that object. See the yaml example above.

A parser function returns types.TemplateObject, and an error. When you return an error, it’s treated as fatal.

// ParseLambdaFunction converts our config into a cloudformation resource
func ParseExampleLog(
	name string,
	data string,
) (
	conditions kombustionTypes.TemplateObject,
	metadata kombustionTypes.TemplateObject,
	mappings kombustionTypes.TemplateObject,
	outputs kombustionTypes.TemplateObject,
	parameters kombustionTypes.TemplateObject,
	resources kombustionTypes.TemplateObject,
	transform kombustionTypes.TemplateObject,
	errors []error,
) {
  // Setup a variable to load the yaml into
  var config LambdaFunctionConfig

  // Attempt to unmarshall the yaml
  err := yaml.Unmarshal([]byte(data), &config)

	if err != nil {
    // Append the error to the errors array
		errors = append(errors, err)

  // validate the config to ensure we have required fields
  // and apply any other validation logic
  // We'll cover this shortly.
	validateErrs := config.Validate()

	// If there are any validation errors add them to our errors array and return
	if validateErrs != nil {
		errors = append(errors, validateErrs...)

  // Now we can create resources
  // To do this we need to call a create function from
  // github.com/KablamoOSS/kombustion/pkg/parsers/resources
  // which we import as cfResource

  // We're deriving a name from the source `name` value, to ensure the outputs
  // of this function are unique to the inputs.
  // This plugin may be used twice in the same stack, and should not collide
  logGroupName := fmt.Sprintf("%s%s", name, config.Properties.LogGroupName)

	// Create a new resource and add it to cf
	resources = kombustionTypes.TemplateObject{
		// (name) lets us create a new resource with the same name as the input type
		(name): cfResources.NewLogsLogGroup(
				LogGroupName:    fmt.Sprintf("!Join [ \"-\", [ !Ref %s, !Ref Environment ] ]", logGroupName),
				RetentionInDays: config.Properties.RetentionInDays,

  // Here we're adding a Parameter
	parameters = kombustionTypes.TemplateObject{
		(logGroupName): map[string]string{
			"Type": "String",

  // And this is an example of how to add metadata
	metadata = kombustionTypes.TemplateObject{
		(logGroupName): map[string]string{
			"Source": "kablamo-plugin-example",


Returning Errors

You must return all errors in the errors []error array. These are then printed to the user, with information about the plugin, and the block in the template that caused it.

Don’t print errors to stdout, as the user won’t know where they’re coming from.

If errors []error contains any errors, the task will fail.

And example of how an error is printed:

✖  Error: Missing field 'LogGroupName'
☞  Resolution:
   ├─ Name:    MyNetwork
   ├─ Plugin:  kombustion-plugin-example
   └─ Type:    Kablamo::Example::Log


In this example our validation function is only ensuring requried fields are provided. However, you can use any logic you want here to validate the input YAML matches what you require.

// Validate - input Config validation
func (config LogConfig) Validate() (errors []error) {
	props := config.Properties

	// Ensure LogGrouName has a value
	if props.LogGroupName == nil {
		errors = append(errors, fmt.Errorf("Missing required field 'LogGroupName'"))

	// Ensure RetentionInDays has a value
	if props.RetentionInDays == nil {
		errors = append(errors, fmt.Errorf("Missing required field 'RetentionInDays'"))


Build process

When building locally all you need is to add the plugin buildmode to your normal build command:

$ go build --buildmode=plugin

This will create a folderName.so file, which you can then load with --load-plugin path/to/folderName.so.

Behind the scenes the plugin and core have been compiled with the C compiler on your machine. Becuase these match it sould just work.


For release, you need to ensure you have a build for every operating system and architecture kombustion supports.

We use a tool called xgo which uses Docker to compile for all possible versions, using the correct C compiler.

The following is a working .travis.yml configuration that will compile and attach a release to your Github repository.

xgo has a consistent naming convention, which kombustion relies on when downloading a plugin to determine what operating system and architecture the plugin was compiled for.

To make this build script work, you need to add it to your root directory as .travis.yml in a public Github repository.

Then you need to setup a token to allow TravisCI to publish a release on your behalf.

From the root directory of your repository run:

$ travis setup releases

It will then connect to your Github account, create and encrypt a token to allow publishing releases.

language: go
- linux
- 1.10.1
sudo: required
- go get -t ./...
- go generate
- go test ./...
- go get github.com/karalabe/xgo
- |
  # Get the full go repo url
  REPO=$(pwd |  rev | cut -d'/' -f-3 | rev)

  # Get the name of the app

  # Get this tag as the version
  VERSION=$(git describe --abbrev=0 --tags)

  # Ensure a fresh build folder
  rm -rf build && mkdir build
  # Compile
  xgo \
    -dest build/ \
    -buildmode=plugin  \
    --targets=darwin/amd64,freebsd/386,freebsd/amd64,freebsd/arm,linux/386,linux/amd64,linux/arm64  \
    --ldflags "-X plugin.version=${VERSION}" \

  # Package
  cd build
  # For each compiled binary, we're repackaging it in a zip with the architecture name, and
  # renaming the binary to the app name
  for FILE in $(ls .); do
    mv $FILE $APP.so
    tar cvzf ${FILE}.tgz $APP.so
    rm -f $APP.so
  cd ..

# Deploy to Github release on tags
  provider: releases
    secure: XXXXX...YourSecretHere
  file_glob: true
  file: "build/*"
  skip_cleanup: true
    tags: true

Creating a release

Finally to create a release, you need to create a tag, and push both the commit and tag up.

# Create a tag
$ git tag v0.1.0

# Ensure the tagged commit has been pushed
$ git push

# Now push your tags
$ git push --tags