Application.Behaviour

Default callbacks for applications.

In Erlang/OTP, an application is a component that can be started and stopped as a unit, and which can be reused in other systems.

The first step in creating an application is to define an application specification. For example, if your application is named :my_app, an app specification should exist at ebin/my_app.app. This file is usually defined by build tools like Mix.

With the app specification in hand, we must define application module callbacks that control how to start and stop instances of the application. This module is about defining such callbacks.

There are two callbacks which must be implemented:

1. start(type, args) - must return { :ok, pid } or { :ok, pid, state }, where pid is the process identifier of the supervisor tree root and state is application defined state information;

2. stop(state) - receives the state returned by start and should do any necessary clean up. Notice that shutting down the supervisor is automatically handled by the VM;

When using this module, it tags the module behaviour as :application and provides a default stop/1 callback. The start/2 callback still needs to be implemented by the user.

You can learn more about the :application module, the application specification and the application module callbacks from these sources:

• http://www.erlang.org/doc/man/application.html
• http://www.erlang.org/doc/design_principles/applications.html
• http://learnyousomeerlang.com/building-otp-applications

Example

defmodule MyApp do
  use Application.Behaviour

  def start(_type, args) do
    MyApp.Sup.start_link(args)
  end
end