Elixir v1.0.4 Application

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 as well.

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

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 compilation 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 applications.

Assuming you are inside a Mix project, you can edit your application 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)
#=> {:ok, :hello}

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 those should be avoided).

In the future, we plan to support configuration files which allows developers to configure the environment of their dependencies.

Keep in mind that each application is responsible for its environment. Do not use the functions in this module for directly access or modify the environment of other application (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 most commonly returns {:ok, pid} or {:ok, pid, state} where pid identifies the supervision tree and the state is the application state. args is second element of the tuple given to the :mod option.

The type passed into start/2 is usually :normal unless in a distributed setup where applications takeover and failovers are configured. This particular aspect of applications can be read with 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. Notice that shutting down the supervisor is automatically handled by the VM;

Summary

Types

app()

key()

start_type()

value()

Functions

app_dir(app)

Gets the directory for app

app_dir(app, path)

Returns the given path inside app_dir/1

delete_env(app, key, opts \\ [])

Deletes the key from the given app environment

ensure_all_started(app, type \\ :temporary)

Ensures the given app and its applications are started

ensure_started(app, type \\ :temporary)

Ensures the given app is started

fetch_env(app, key)

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

format_error(reason)

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)

Returns all key-value pairs for app

get_env(app, key, default \\ nil)

Returns the value for key in app’s environment

load(app)

Loads the given app

put_env(app, key, value, opts \\ [])

Puts the value in key for the given app

start(app, type \\ :temporary)

Starts the given app

stop(app)

Stops the given app

unload(app)

Unloads the given app

Types

app :: atom

key :: atom

start_type :: :permanent | :transient | :temporary

value :: term

Functions

app_dir(app)

Specs

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)

Specs

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

Returns the given path inside app_dir/1.

delete_env(app, key, opts \\ [])

Specs

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)

Specs

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)

Specs

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)

Specs

fetch_env(app, key) :: {:ok, value} | :error

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

If the specified application is not loaded, or the configuration parameter does not exist, the function returns :error.

format_error(reason)

Specs

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)

Specs

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

Returns all key-value pairs for app.

get_env(app, key, default \\ nil)

Specs

get_env(app, key, value) :: value

Returns the value for key in app’s environment.

If the specified application is not loaded, or the configuration parameter does not exist, the function returns the default value.

load(app)

Specs

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.

put_env(app, key, value, opts \\ [])

Specs

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.

start(app, type \\ :temporary)

Specs

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.

stop(app)

Specs

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

Stops the given app.

When stopped, the application is still loaded.

unload(app)

Specs

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.