Booter allows modules to define a list of boot steps using module attributes. Each step define what to call, what it requires and enables. A directed acyclic graph is then created from theses steps and progressively executed.

Why? Complicated applications can be composed of multiple subsystems or groups or processes, independants or dependants of each others. And starting theses subsystems is not easy as :application.start/2 or a supervisor child spec.

Inspired/adapted to Elixir by RabbitMQ's boot process implemented in rabbit.erl and rabbit_misc.erl. For an in-depth explaination, read Alvaro Videla's article and slides.

Usage

Defining boot steps

Using boot_step/3 macro (recommended):

defmodule MyModule do
  use Booter

  # without name (__MODULE__ is assumed)
  boot_step mfa: {mod, fun, args}, requires: :required_step, enables: :another_step

  # with name
  boot_step :awesome_name, mfa: {mod, fun, args}, requires: :required_step, enables: :another_step

  # With name and description
  boot_step :awesome_name, "Unicorn generator", mfa: {mod,fun,args}, requires: :rainbow_server, enables: :magic
end

Or without:

defmodule MyModule do
  Module.register_attribute __MODULE__, :boot_step, accumulate: true, persist: true

  @boot_step %Booter.Step{name: :awesome_step, description: "Awesome things",
                          mfa: { :application, :start, [:awesome] },
                          requires: awesome_step}
end

Start boot

Just call Booter.boot!. Can raise exceptions.

Usually called in start/2 of Application behaviour.

  defmodule MyApp do
    use Application

    def start(_) do
      # Start your main supervisor
      { :ok, pid } = Supervisor.start_link()
      # Boot the rest
      Booter.boot!
      { :ok, pid }
    end
  end