View Source Application behaviour (Elixir v1.14.1)
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
and config/runtime.exs
files. The
former is loaded at build-time, before your code compiles, and the latter at
runtime, just before your app starts. For example, someone using your application
can override its :db_host
environment variable as follows:
import Config
config :my_app, :db_host, "db.local"
See the "Configuration" section in the Mix
module for more information.
You can also change the application environment dynamically by using functions
such as put_env/3
and delete_env/2
.
Note: The config files
config/config.exs
andconfig/runtime.exs
are rarely used by libraries. Libraries typically define their environment in thedef application
function of theirmix.exs
. Configuration files are rather used by applications to configure their libraries.
Note: Each application is responsible for its own environment. Do not use the functions in this module for directly accessing or modifying the environment of other applications. Whenever you change the application environment, Elixir's build tool will only recompile the files that belong to that application. So if you read the application environment of another application, there is a chance you will be depending on outdated configuration, as your file won't be recompiled as it changes.
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. However, if you try to access
Application.fetch_env!/2
outside of a function:
defmodule MyApp.DBClient do
@db_host Application.fetch_env!(:my_app, :db_host)
def start_link() do
SomeLib.DBClient.start_link(host: @db_host)
end
end
You might see warnings and errors:
warning: Application.fetch_env!/2 is discouraged in the module body,
use Application.compile_env/3 instead
iex:3: MyApp.DBClient
** (ArgumentError) could not fetch application environment :db_host
for application :my_app because the application was not loaded nor
configured
This happens because, when defining modules, the application environment
is not yet available. Luckily, the warning tells us how to solve this
issue, by using Application.compile_env/3
instead:
defmodule MyApp.DBClient do
@db_host Application.compile_env(:my_app, :db_host, "db.local")
def start_link() do
SomeLib.DBClient.start_link(host: @db_host)
end
end
The difference here is that compile_env
expects the default value to be
given as an argument, instead of using the def application
function of
your mix.exs
. Furthermore, 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.
In any case, compile-time environments should be avoided. Whenever possible, reading the application environment at runtime should be the first choice.
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:
- If present, invoke the optional callback
prep_stop/1
. - Terminate the top-level supervisor.
- 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
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.
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 from a macro.
Reads the application environment at compilation time or raises.
Reads the application environment at compilation time from a macro 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
.
Link to this section Types
@type app() :: atom()
@type application_key() ::
:start_phases
| :mod
| :applications
| :optional_applications
| :included_applications
| :registered
| :maxT
| :maxP
| :modules
| :vsn
| :id
| :description
@type key() :: atom()
@type restart_type() :: :permanent | :transient | :temporary
@type state() :: term()
@type value() :: term()
Link to this section Callbacks
@callback 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.
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
.
@callback 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 nodenode
.{:failover, node}
- used if the application is distributed and is started on the current node because of a failover on nodenode
, 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.
@callback 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.
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
.
Link to this section Functions
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.
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"
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.
Reads the application environment at compilation time from a macro.
Typically, developers will use compile_env/3
. This function must
only be invoked from macros which aim to read the compilation environment
dynamically.
It expects a Macro.Env
as first argument, where the Macro.Env
is
typically the __CALLER__
in a macro. It raises if Macro.Env
comes
from a function.
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.
@spec compile_env!(Macro.Env.t(), app(), key() | list()) :: value()
Reads the application environment at compilation time from a macro or raises.
Typically, developers will use compile_env!/2
. This function must
only be invoked from macros which aim to read the compilation environment
dynamically.
It expects a Macro.Env
as first argument, where the Macro.Env
is
typically the __CALLER__
in a macro. It raises if Macro.Env
comes
from a function.
Deletes the key
from the given app
environment.
It receives the same options as put_env/4
. Returns :ok
.
@spec 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.
Ensures the given app
is loaded.
Same as load/1
but returns :ok
if the application was already
loaded.
@spec 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)
Returns the value for key
in app
's environment in a tuple.
If the configuration parameter does not exist, the function returns :error
.
Important: you must use this function to read only your own application environment. Do not read the environment of other applications.
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.
Returns the value for key
in app
's environment.
If the configuration parameter does not exist, raises ArgumentError
.
Important: you must use this function to read only your own application environment. Do not read the environment of other applications.
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.
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.
The application is located by analyzing the spec
of all loaded applications. Returns nil
if
the module is not listed in any application spec.
Returns the value for key
in app
's environment.
If the configuration parameter does not exist, the function returns the
default
value.
Important: you must use this function to read only your own application environment. Do not read the environment of other applications.
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
Application.get_env(:my_app, :my_app_databases, [])
to retrieve the list of
databases (specified by module names).
The engine can then traverse each repository in the list and call
Application.get_env(:my_app, Databases.RepoOne)
and so forth to retrieve the
configuration of each one. In this case, each configuration will be a keyword
list, so you can use the functions in the Keyword
module or even the Access
module to traverse it, for example:
config = Application.get_env(:my_app, Databases.RepoOne)
config[:ip]
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.
Returns a list with information about the applications which have been loaded.
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, it will raise.
It receives the same options as put_env/4
. Returns :ok
.
Puts the value
in key
for the given app
.
Options
:timeout
- the timeout for the change (defaults to5_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 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
:optional_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 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.
@spec 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
- ifapp
terminates, all other applications and the entire node are also terminated.:transient
- ifapp
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
- ifapp
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
.
Returns a list with information about the applications which are currently running.
Stops the given app
.
When stopped, the application is still loaded.
Unloads the given app
.
It will also unload all :included_applications
.
Note that the function does not purge the application modules.