Elixir v1.4.5 Application behaviour View Source
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.
Link to this section 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
Link to this section Types
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
.
Deletes the key
from the given app
environment.
See put_env/4
for a description of the options.
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, 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)
Returns the value for key
in app
’s environment in a tuple.
If the configuration parameter does not exist, the function returns :error
.
Returns the value for key
in app
’s environment.
If the configuration parameter does not exist, raises ArgumentError
.
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.
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 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.
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.
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, 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
- 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
.
started_applications(timeout) :: [tuple]
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.
Link to this section Callbacks
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 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 (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.
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
.