Elixir v1.4.0 Application behaviour

A module for working with applications and defining application callbacks.

In Elixir (actually, in Erlang/OTP), an application is a component implementing some specific functionality, that can be started and stopped as a unit, and which can be re-used in other systems.

Applications are defined with an application file named APP.app where APP is the application name, usually in underscore_case. The application file must reside in the same ebin directory as the compiled modules of the application.

In Elixir, Mix is responsible for compiling your source code and generating your application .app file. Furthermore, Mix is also responsible for configuring, starting and stopping your application and its dependencies. For this reason, this documentation will focus on the remaining aspects of your application: the application environment and the application callback module.

You can learn more about Mix generation of .app files by typing mix help compile.app.

Application environment

Once an application is started, OTP provides an application environment that can be used to configure the application.

Assuming you are inside a Mix project, you can edit the application/0 function in the mix.exs file to the following:

def application do
  [env: [hello: :world]]
end

In the application function, we can define the default environment values for our application. By starting your application with iex -S mix, you can access the default value:

Application.get_env(:APP_NAME, :hello)
#=> :world

It is also possible to put and delete values from the application value, including new values that are not defined in the environment file (although this should be avoided).

Keep in mind that each application is responsible for its environment. Do not use the functions in this module for directly accessing or modifying the environment of other applications (as it may lead to inconsistent data in the application environment).

Application module callback

Often times, an application defines a supervision tree that must be started and stopped when the application starts and stops. For such, we need to define an application module callback. The first step is to define the module callback in the application definition in the mix.exs file:

def application do
  [mod: {MyApp, []}]
end

Our application now requires the MyApp module to provide an application callback. This can be done by invoking use Application in that module and defining a start/2 callback, for example:

defmodule MyApp do
  use Application

  def start(_type, _args) do
    MyApp.Supervisor.start_link()
  end
end

start/2 typically returns {:ok, pid} or {:ok, pid, state} where pid identifies the supervision tree and state is the application state. args is the second element of the tuple given to the :mod option.

The type argument passed to start/2 is usually :normal unless in a distributed setup where application takeovers and failovers are configured. This particular aspect of applications is explained in more detail in the OTP documentation:

A developer may also implement the stop/1 callback (automatically defined by use Application) which does any application cleanup. It receives the application state and can return any value. Note that shutting down the supervisor is automatically handled by the VM.

Summary

Functions

Gets the directory for app

Returns the given path inside app_dir/1

Deletes the key from the given app environment

Ensures the given app and its applications are started

Ensures the given app is started

Returns the value for key in app’s environment in a tuple

Returns the value for key in app’s environment

Formats the error reason returned by start/2, ensure_started/2, stop/1, load/1 and unload/1, returns a string

Returns all key-value pairs for app

Gets the application for the given module

Returns the value for key in app’s environment

Loads the given app

Returns a list with information about the applications which have been loaded

Puts the value in key for the given app

Returns the spec for app

Returns the value for key in app’s specification

Starts the given app

Returns a list with information about the applications which are currently running

Stops the given app

Unloads the given app

Callbacks

Called when an application is started

Called when an application is stopped

Types

app()
app() :: atom
key()
key() :: atom
start_type()
start_type() :: :permanent | :transient | :temporary
state()
state() :: term
value()
value() :: term

Functions

app_dir(app)
app_dir(app) :: String.t

Gets the directory for app.

This information is returned based on the code path. Here is an example:

File.mkdir_p!("foo/ebin")
Code.prepend_path("foo/ebin")
Application.app_dir(:foo)
#=> "foo"

Even though the directory is empty and there is no .app file it is considered the application directory based on the name “foo/ebin”. The name may contain a dash - which is considered to be the app version and it is removed for the lookup purposes:

File.mkdir_p!("bar-123/ebin")
Code.prepend_path("bar-123/ebin")
Application.app_dir(:bar)
#=> "bar-123"

For more information on code paths, check the Code module in Elixir and also Erlang’s :code module.

app_dir(app, path)
app_dir(app, String.t | [String.t]) :: String.t

Returns the given path inside app_dir/1.

delete_env(app, key, opts \\ [])
delete_env(app, key, [timeout: timeout, persistent: boolean]) :: :ok

Deletes the key from the given app environment.

See put_env/4 for a description of the options.

ensure_all_started(app, type \\ :temporary)
ensure_all_started(app, start_type) ::
  {:ok, [app]} |
  {:error, {app, term}}

Ensures the given app and its applications are started.

Same as start/2 but also starts the applications listed under :applications in the .app file in case they were not previously started.

ensure_started(app, type \\ :temporary)
ensure_started(app, start_type) :: :ok | {:error, term}

Ensures the given app is started.

Same as start/2 but returns :ok if the application was already started. This is useful in scripts and in test setup, where test applications need to be explicitly started:

:ok = Application.ensure_started(:my_test_dep)
fetch_env(app, key)
fetch_env(app, key) :: {:ok, value} | :error

Returns the value for key in app’s environment in a tuple.

If the configuration parameter does not exist, the function returns :error.

fetch_env!(app, key)
fetch_env!(app, key) :: value | no_return

Returns the value for key in app’s environment.

If the configuration parameter does not exist, raises ArgumentError.

format_error(reason)
format_error(any) :: String.t

Formats the error reason returned by start/2, ensure_started/2, stop/1, load/1 and unload/1, returns a string.

get_all_env(app)
get_all_env(app) :: [{key, value}]

Returns all key-value pairs for app.

get_application(module)
get_application(atom) :: atom | nil

Gets the application for the given module.

The application is located by analyzing the spec of all loaded applications. Returns nil if the module is not listed in any application spec.

get_env(app, key, default \\ nil)
get_env(app, key, value) :: value

Returns the value for key in app’s environment.

If the configuration parameter does not exist, the function returns the default value.

load(app)
load(app) :: :ok | {:error, term}

Loads the given app.

In order to be loaded, an .app file must be in the load paths. All :included_applications will also be loaded.

Loading the application does not start it nor load its modules, but it does load its environment.

loaded_applications()
loaded_applications() :: [tuple]

Returns a list with information about the applications which have been loaded.

put_env(app, key, value, opts \\ [])
put_env(app, key, value, [timeout: timeout, persistent: boolean]) :: :ok

Puts the value in key for the given app.

Options

  • :timeout - the timeout for the change (defaults to 5000ms)
  • :persistent - persists the given value on application load and reloads

If put_env/4 is called before the application is loaded, the application environment values specified in the .app file will override the ones previously set.

The persistent option can be set to true when there is a need to guarantee parameters set with this function will not be overridden by the ones defined in the application resource file on load. This means persistent values will stick after the application is loaded and also on application reload.

spec(app)
spec(app) :: [{key, value}] | nil

Returns the spec for app.

The following keys are returned:

  • :description
  • :id
  • :vsn
  • :modules
  • :maxP
  • :maxT
  • :registered
  • :included_applications
  • :applications
  • :mod
  • :start_phases

Note the environment is not returned as it can be accessed via fetch_env/2. Returns nil if the application is not loaded.

spec(app, key)
spec(app, key) :: value | nil

Returns the value for key in app’s specification.

See spec/1 for the supported keys. If the given specification parameter does not exist, this function will raise. Returns nil if the application is not loaded.

start(app, type \\ :temporary)
start(app, start_type) :: :ok | {:error, term}

Starts the given app.

If the app is not loaded, the application will first be loaded using load/1. Any included application, defined in the :included_applications key of the .app file will also be loaded, but they won’t be started.

Furthermore, all applications listed in the :applications key must be explicitly started before this application is. If not, {:error, {:not_started, app}} is returned, where app is the name of the missing application.

In case you want to automatically load and start all of app’s dependencies, see ensure_all_started/2.

The type argument specifies the type of the application:

  • :permanent - if app terminates, all other applications and the entire node are also terminated.

  • :transient - if app terminates with :normal reason, it is reported but no other applications are terminated. If a transient application terminates abnormally, all other applications and the entire node are also terminated.

  • :temporary - if app terminates, it is reported but no other applications are terminated (the default).

Note that it is always possible to stop an application explicitly by calling stop/1. Regardless of the type of the application, no other applications will be affected.

Note also that the :transient type is of little practical use, since when a supervision tree terminates, the reason is set to :shutdown, not :normal.

started_applications(timeout \\ 5000)
started_applications(timeout) :: [tuple]

Returns a list with information about the applications which are currently running.

stop(app)
stop(app) :: :ok | {:error, term}

Stops the given app.

When stopped, the application is still loaded.

unload(app)
unload(app) :: :ok | {:error, term}

Unloads the given app.

It will also unload all :included_applications. Note that the function does not purge the application modules.

Callbacks

start(start_type, start_args)
start(start_type, start_args :: term) ::
  {:ok, pid} |
  {:ok, pid, state} |
  {:error, reason :: term}

Called when an application is started.

This function is called when an the application is started using Application.start/2 (and functions on top of that, such as Application.ensure_started/2. This function should start the top-level process of the application (which should be the top supervisor of the application’s supervision tree if the application follows the OTP design principles around supervision).

start_type defines how the application is started:

  • :normal - used if the startup is a normal startup or if the application is distributed and is started on the current node because of a failover from another mode and the application specification key :start_phases is :undefined.
  • {:takeover, node} - used if the application is distributed and is started on the current node because of a failover on the node node.
  • {:failover, node} - used if the application is distributed and is started on the current node because of a failover on node node, and the application specification key :start_phases is not :undefined.

start_args are the arguments passed to the application in the :mod specification key (e.g., mod: {MyApp, [:my_args]}).

This function should either return {:ok, pid} or {:ok, pid, state} if startup is successful. pid should be the PID of the top supervisor. state can be an arbitrary term, and if omitted will default to []; if the application is later stopped, state is passed to the stop/1 callback (see the documentation for the stop/1 callback for more information).

use Application provides no default implementation for the start/2 callback.

stop(state)
stop(state) :: term

Called when an application is stopped.

This function is called when an application has stopped, i.e., when its supervision tree has been stopped. It should do the opposite of what the start/2 callback did, and should perform any necessary cleanup. The return value of this callback is ignored.

state is the return value of the start/2 callback or the return value of the prep_stop/1 function if the application module defines such a function.

use Application defines a default implementation of this function which does nothing and just returns :ok.