/Elixir 1.7

Application behaviour

A module for working with applications and defining application callbacks.

Applications are the idiomatic way to package software in Erlang/OTP. To get the idea, they are similar to the “library” concept common in other programming languages, but with some additional characteristics.

An application is a component implementing some specific functionality, with a standardized directory structure, configuration, and lifecycle. Applications are loaded, started, and stopped.

The application resource file

Applications are specified in their resource file, which is a file called APP.app, where APP is the application name. For example, the application resource file of the OTP application ex_unit is called ex_unit.app.

You’ll find the resource file of an application in its ebin directory, it is generated automatically by Mix. Some of its keys are taken from the keyword lists returned by the project/0 and application/0 functions defined in mix.exs, and others are generated by Mix itself.

You can learn more about the generation of application resource files in the documentation of Mix.Tasks.Compile.App, available as well by running mix help compile.app.

The application environment

The key env of an application resource file has a list of tuples that map atoms to terms, and its contents are known as the application environment. Note that this environment is unrelated to the operating system environment.

By default, the environment of an application is an empty list. In a Mix project you can set that key in application/0:

def application do
  [env: [redis_host: "localhost"]]

and the generated application resource file is going to have it included.

The environment is available after loading the application, which is a process explained later:

#=> :ok

Application.get_env(:APP_NAME, :redis_host)
#=> "localhost"

In Mix projects, the environment of the application and its dependencies can be overridden via the config/config.exs file. If you start the application with Mix, that configuration is available at compile time, and at runtime too, but take into account it is not included in the generated application resource file, and it is not available if you start the application without Mix.

For example, someone using your application can override its :redis_host environment variable as follows:

config :APP_NAME, redis_host: "redis.local"

The function put_env/3 allows dynamic configuration of the application environment, but as a rule of thumb each application is responsible for its own environment. Please do not use the functions in this module for directly accessing or modifying the environment of other applications.

The application environment can be overriden via the -config option of erl, as well as command-line flags, as we are going to see below.

The application callback module

The mod key of an application resource file configures an application callback module and start argument:

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

This key is optional, only needed for applications that start a supervision tree.

The MyApp module given to :mod needs to implement the Application behaviour. This can be done by putting use Application in that module and implementing the start/2 callback, for example:

defmodule MyApp do
  use Application

  def start(_type, _args) do
    children = []
    Supervisor.start_link(children, strategy: :one_for_one)

The start/2 callback has to spawn and link a supervisor and return {:ok, pid} or {:ok, pid, state}, where pid is the PID of the supervisor, and state is an optional 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. Distributed applications are beyond the scope of this documentation.

When an application is shutting down, its stop/1 callback is called after the supervision tree has been stopped by the runtime. This callback allows the application to do any final cleanup. The argument is the state returned by start/2, if it did, or [] otherwise. The return value of stop/1 is ignored.

By using Application, modules get a default implementation of stop/1 that ignores its argument and returns :ok, but it can be overridden.

Application callback modules may also implement the optional callback prep_stop/1. If present, prep_stop/1 is invoked before the supervision tree is terminated. Its argument is the state returned by start/2, if it did, or [] otherwise, and its return value is passed to stop/1.

The application lifecycle

Loading applications

Applications are loaded, which means that the runtime finds and processes their resource files:

#=> :ok

If an application has included applications, they are also loaded. And the procedure recurses if they in turn have included applications. Included applications are unrelated to applications in Mix umbrella projects, they are an Erlang/OTP concept that has to do with coordinated starts.

When an application is loaded, the environment specified in its resource file is merged with any overrides from config files passed to erl via the -config option. It is worth highlighting that releases pass sys.config this way. The resulting environment can still be overridden again via specific -Application flags passed to erl.

Loading an application does not load its modules.

In practice, you rarely load applications by hand because that is part of the start process, explained next.

Starting applications

Applications are also started:

#=> :ok

Once your application is compiled, running your system is a matter of starting your current application and its dependencies. Differently from other languages, Elixir does not have a main procedure that is responsible for starting your system. Instead, you start one or more applications, each with their own initialization and termination logic.

When an application is started, the runtime loads it if it hasn’t been loaded yet (in the technical sense described above). Then, it checks if the dependencies listed in the applications key of the resource file are already started. Having at least one dependency not started is an error condition, but when you start an application with mix run, Mix takes care of starting all the dependencies for you, so in practice you don’t need to worry about it unless you are starting applications manually with the API provided by this module.

If the application does not have a callback module configured, starting is done at this point. Otherwise, its start/2 callback if invoked. The PID of the top-level supervisor returned by this function is stored by the runtime for later use, and the returned application state is saved too, if any.

Stopping applications

Started applications are, finally, stopped:

#=> :ok

Stopping an application without a callback module is defined, but except for some system tracing, it is in practice a no-op.

Stopping an application with a callback module has three steps:

  1. If present, invoke the optional callback prep_stop/1.
  2. Terminate the top-level supervisor.
  3. Invoke the required callback stop/1.

The arguments passed to the callbacks are related to the state optionally returned by start/2, and are documented in the section about the callback module above.

It is important to highlight that step 2 is a blocking one. Termination of a supervisor triggers a recursive chain of children terminations, therefore orderly shutting down all descendant processes. The stop/1 callback is invoked only after termination of the whole supervision tree.

Shutting down a live system cleanly can be done by calling System.stop/1. It will shut down every application in the opposite order they had been started.

From Erlang/OTP 19.1, a SIGTERM from the operating system will automatically translate to System.stop/0. Erlang/OTP 20 gives user more explicit control over OS signals via the :os.set_signal/2 function.


The Mix build tool can also be used to start your applications. For example, mix test automatically starts your application dependencies and your application itself before your test runs. mix run --no-halt boots your current project and can be used to start a long running system. See mix help run.

Developers can also use tools like Distillery that build releases. Releases are able to package all of your source code as well as the Erlang VM into a single directory. Releases also give you explicit control over how each application is started and in which order. They also provide a more streamlined mechanism for starting and stopping systems, debugging, logging, as well as system monitoring.

Finally, Elixir provides tools such as escripts and archives, which are different mechanisms for packaging your application. Those are typically used when tools must be shared between developers and not as deployment options. See mix help archive.build and mix help escript.build for more detail.

Further information

For further details on applications please check the documentation of the application Erlang module, and the Applications section of the OTP Design Principles User’s Guide.






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

fetch_env(app, key)

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


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

get_env(app, key, default \\ nil)

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

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

Puts the value in key for the given app


Returns the spec for app

spec(app, key)

Returns the value for key in app’s specification

start(app, type \\ :temporary)

Starts the given app

started_applications(timeout \\ 5000)

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


Stops the given app


Unloads the given app



Called before stopping the application

start(start_type, start_args)

Called when an application is started

start_phase(phase, start_type, phase_args)

Start an application in synchronous phases


Called after an application has been stopped



app() :: atom()


key() :: atom()


restart_type() :: :permanent | :transient | :temporary


start_type() :: :normal | {:takeover, node()} | {:failover, node()}


state() :: term()


value() :: term()



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

Gets the directory for app.

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

#=> "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:

#=> "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.

If path is a string, then it will be used as the path inside app_dir/1. If path is a list of strings, it will be joined (see Path.join/1) and the result will be used as the path inside app_dir/1.



Application.app_dir(:foo, "my_path")
#=> "foo/my_path"

Application.app_dir(:foo, ["my", "nested", "path"])
#=> "foo/my/nested/path"

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(), restart_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(), restart_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()) :: value() | no_return()

Returns the value for key in app’s environment.

If the configuration parameter does not exist, raises ArgumentError.

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.


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

Returns all key-value pairs for app.


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.


get_env/3 is commonly used to read the configuration of your OTP applications. Since Mix configurations are commonly used to configure applications, we will use this as a point of illustration.

Consider a new application :my_app. :my_app contains a database engine which supports a pool of databases. The database engine needs to know the configuration for each of those databases, and that configuration is supplied by key-value pairs in environment of :my_app.

config :my_app, Databases.RepoOne,
  # A database configuration
  ip: "localhost"
  port: 5433

config :my_app, Databases.RepoTwo,
  # Another database configuration (for the same OTP app)
  ip: "localhost"
  port: 20717

config :my_app, my_app_databases: [Databases.RepoOne, Databases.RepoTwo]

Our database engine used by :my_app needs to know what databases exist, and what the database configurations are. The database engine can make a call to get_env(:my_app, :my_app_databases) to retrieve the list of databases (specified by module names). Our database engine can then traverse each repository in the list and then call get_env(:my_app, Databases.RepoOne) and so forth to retrieve the configuration of each one.

Important: if you are writing a library to be used by other developers, it is generally recommended to avoid the application environment, as the application environment is effectively a global storage. For more information, read our library guidelines.


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() :: [
  {app(), description :: charlist(), vsn :: charlist()}

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()) ::

Puts the value in key for the given app.


  • :timeout - the timeout for the change (defaults to 5_000 milliseconds)
  • :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()) :: [{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(), restart_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()) :: [
  {app(), description :: charlist(), vsn :: charlist()}

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


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

Stops the given app.

When stopped, the application is still loaded.


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.


prep_stop(state) (optional)

prep_stop(state()) :: state()

Called before stopping the application.

This function is called before the top-level supervisor is terminated. It receives the state returned by start/2, if it did, or [] otherwise. The return value is later passed to stop/1.

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 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 node 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.

start_phase(phase, start_type, phase_args) (optional)

start_phase(phase :: term(), start_type(), phase_args :: term()) ::
  :ok | {:error, reason :: term()}

Start an application in synchronous phases.

This function is called after start/2 finishes but before Application.start/2 returns. It will be called once for every start phase defined in the application’s (and any included applications’) specification, in the order they are listed in.


stop(state()) :: term()

Called after an application has been stopped.

This function is called after an application has been stopped, i.e., after 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 state returned by start/2, if it did, or [] otherwise. If the optional callback prep_stop/1 is present, state is its return value instead.

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

© 2012 Plataformatec
Licensed under the Apache License, Version 2.0.