Application behaviour (Elixir v1.12.0-dev) View Source

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 life cycle. Applications are loaded, started, and stopped. Each application also has its own environment, which provides a unified API for configuring each application.

Developers typically interact with the application environment and its callback module. Therefore those will be the topics we will cover first before jumping into details about the application resource file and life-cycle.

The application environment

Each application has its own environment. The environment is a keyword list that maps atoms to terms. 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's mix.exs file, you can set the :env key in application/0:

def application do
  [env: [db_host: "localhost"]]
end

Now, in your application, you can read this environment by using functions such as fetch_env!/2 and friends:

defmodule MyApp.DBClient do
  def start_link() do
    SomeLib.DBClient.start_link(host: db_host())
  end

  defp db_host do
    Application.fetch_env!(:my_app, :db_host)
  end
end

In Mix projects, the environment of the application and its dependencies can be overridden via the config/config.exs file. For example, someone using your application can override its :db_host environment variable as follows:

import Config
config :my_app, :db_host, "db.local"

You can also change the application environment dynamically by using functions such as put_env/3 and delete_env/2. However, 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.

Compile-time environment

In the previous example, we read the application environment at runtime:

defmodule MyApp.DBClient do
  def start_link() do
    SomeLib.DBClient.start_link(host: db_host())
  end

  defp db_host do
    Application.fetch_env!(:my_app, :db_host)
  end
end

In other words, the environment key :db_host for application :my_app will only be read when MyApp.DBClient effectively starts. While reading the application environment at runtime is the preferred approach, in some rare occasions you may want to use the application environment to configure the compilation of a certain project. This is often done by calling get_env/3 outside of a function:

defmodule MyApp.DBClient do
  @db_host Application.get_env(:my_app, :db_host, "db.local")

  def start_link() do
    SomeLib.DBClient.start_link(host: @db_host)
  end
end

This approach has one big limitation: if you change the value of the application environment after the code is compiled, the value used at runtime is not going to change! For example, if your config/runtime.exs has:

config :my_app, :db_host, "db.production"

This value will have no effect as the code was compiled to connect to "db.local", which is mostly likely unavailable in the production environment.

For those reasons, reading the application environment at runtime should be the first choice. However, if you really have to read the application environment during compilation, we recommend you to use compile_env/3 instead:

@db_host Application.compile_env(:my_app, :db_host, "db.local")

By using compile_env/3, tools like Mix will store the values used during compilation and compare the compilation values with the runtime values whenever your system starts, raising an error in case they differ.

The application callback module

Applications can be loaded, started, and stopped. Generally, build tools like Mix take care of starting an application and all of its dependencies for you, but you can also do it manually by calling:

{:ok, _} = Application.ensure_all_started(:some_app)

When an application starts, developers may configure a callback module that executes custom code. Developers use this callback to start the application supervision tree.

The first step to do so is to add a :mod key to the application/0 definition in your mix.exs file. It expects a tuple, with the application callback module and start argument (commonly an empty list):

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

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)
  end
end

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 resource file

In the sections above, we have configured an application in the application/0 section of the mix.exs file. Ultimately, Mix will use this configuration to create an application resource file, which is a file called APP_NAME.app. For example, the application resource file of the OTP application ex_unit is called ex_unit.app.

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 life cycle

Loading applications

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

Application.load(:ex_unit)
#=> :ok

When an application is loaded, the environment specified in its resource file is merged with any overrides from config files.

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:

Application.start(:ex_unit)
#=> :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 Application.load/1 is automatically invoked if it hasn't been done yet. 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. Functions like ensure_all_started/1 takes care of starting an application and all of its dependencies for you.

If the application does not have a callback module configured, starting is done at this point. Otherwise, its start/2 callback is 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:

Application.stop(:ex_unit)
#=> :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.

By default, a SIGTERM from the operating system will automatically translate to System.stop/0. You can also have more explicit control over operating system signals via the :os.set_signal/2 function.

Tooling

The Mix build tool automates most of the application management tasks. 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 mix release to 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.

Link to this section Summary

Functions

Gets the directory for app.

Returns the given path inside app_dir/1.

Reads the application environment at compilation time.

Reads the application environment at compilation time or raises.

Deletes the key from the given app environment.

Ensures the given app and its applications are started.

Ensures the given app is loaded.

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 environment for multiple apps at the same time.

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

Callback invoked after code upgrade, if the application environment has changed.

Called before stopping the application.

Called when an application is started.

Starts an application in synchronous phases.

Called after an application has been stopped.

Link to this section Types

Specs

app() :: atom()

Specs

application_key() ::
  :start_phases
  | :mod
  | :applications
  | :included_applications
  | :registered
  | :maxT
  | :maxP
  | :modules
  | :vsn
  | :id
  | :description

Specs

key() :: atom()

Specs

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

Specs

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

Specs

state() :: term()

Specs

value() :: term()

Link to this section Functions

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.

Specs

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.

Examples

File.mkdir_p!("foo/ebin")
Code.prepend_path("foo/ebin")

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

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

compile_env(app, key_or_path, default \\ nil)

View Source (macro) (since 1.10.0)

Specs

compile_env(app(), key() | list(), value()) :: value()

Reads the application environment at compilation time.

Similar to get_env/3, except it must be used to read values at compile time. This allows Elixir to track when configuration values change between compile time and runtime.

The first argument is the application name. The second argument key_or_path is either an atom key or a path to traverse in search of the configuration, starting with an atom key.

For example, imagine the following configuration:

config :my_app, :key, [foo: [bar: :baz]]

We can access it during compile time as:

Application.compile_env(:my_app, :key)
#=> [foo: [bar: :baz]]

Application.compile_env(:my_app, [:key, :foo])
#=> [bar: :baz]

Application.compile_env(:my_app, [:key, :foo, :bar])
#=> :baz

A default value can also be given as third argument. If any of the keys in the path along the way is missing, the default value is used:

Application.compile_env(:my_app, [:unknown, :foo, :bar], :default)
#=> :default

Application.compile_env(:my_app, [:key, :unknown, :bar], :default)
#=> :default

Application.compile_env(:my_app, [:key, :foo, :unknown], :default)
#=> :default

Giving a path is useful to let Elixir know that only certain paths in a large configuration are compile time dependent.

Link to this macro

compile_env!(app, key_or_path)

View Source (macro) (since 1.10.0)

Specs

compile_env!(app(), key() | list()) :: value()

Reads the application environment at compilation time or raises.

This is the same as compile_env/3 but it raises an ArgumentError if the configuration is not available.

Link to this function

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

View Source

Specs

delete_env(app(), key(), timeout: timeout(), persistent: boolean()) :: :ok

Deletes the key from the given app environment.

It receives the same options as put_env/4. Returns :ok.

Link to this function

ensure_all_started(app, type \\ :temporary)

View Source

Specs

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.

Link to this function

ensure_loaded(app)

View Source (since 1.10.0)

Specs

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

Ensures the given app is loaded.

Same as load/2 but returns :ok if the application was already loaded.

Link to this function

ensure_started(app, type \\ :temporary)

View Source

Specs

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)

Specs

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.

Specs

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

Returns the value for key in app's environment.

If the configuration parameter does not exist, raises ArgumentError.

Important: if you are reading the application environment at compilation time, for example, inside the module definition instead of inside of a function, see compile_env!/2 instead.

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.

Specs

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

Returns all key-value pairs for app.

Specs

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.

Link to this function

get_env(app, key, default \\ nil)

View Source

Specs

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.

Important: if you are reading the application environment at compilation time, for example, inside the module definition instead of inside of a function, see compile_env/3 instead.

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.

Examples

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.

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.

Specs

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

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

Link to this function

put_all_env(config, opts \\ [])

View Source (since 1.9.0)

Specs

put_all_env([{app(), [{key(), value()}]}],
  timeout: timeout(),
  persistent: boolean()
) :: :ok

Puts the environment for multiple apps at the same time.

The given config should not:

  • have the same application listed more than once
  • have the same key inside the same application listed more than once

If those conditions are not met, the behaviour is undefined (on Erlang/OTP 21 and earlier) or will raise (on Erlang/OTP 22 and later).

It receives the same options as put_env/4. Returns :ok.

Link to this function

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

View Source

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 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.

Specs

spec(app()) :: [{application_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.

Specs

spec(app(), application_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.

Link to this function

start(app, type \\ :temporary)

View Source

Specs

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.

Link to this function

started_applications(timeout \\ 5000)

View Source

Specs

started_applications(timeout()) :: [
  {app(), description :: charlist(), vsn :: charlist()}
]

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

Specs

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

Stops the given app.

When stopped, the application is still loaded.

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.

Link to this section Callbacks

Link to this callback

config_change(changed, new, removed)

View Source (optional)

Specs

config_change(changed, new, removed) :: :ok
when changed: keyword(), new: keyword(), removed: [atom()]

Callback invoked after code upgrade, if the application environment has changed.

changed is a keyword list of keys and their changed values in the application environment. new is a keyword list with all new keys and their values. removed is a list with all removed keys.

Link to this callback

prep_stop(state)

View Source (optional)

Specs

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.

Link to this callback

start(start_type, start_args)

View Source

Specs

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 (for example, 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.

Link to this callback

start_phase(phase, start_type, phase_args)

View Source (optional)

Specs

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

Starts 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.

Specs

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.