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# Applications

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It is recommended to read this section alongside `app` and `application` in Kernel.

## Application Concept

After creating code to implement a specific functionality, you might consider transforming it into an application — a component that can be started and stopped as a unit, as well as reused in other systems.

The steps to create an application are as follows:

- Create an application callback module that describes how the application is to be started and stopped.

- Create an application specification and place it in an application resource file. Among other things, this file specifies which modules the application consists of and the name of the callback module.

If you use `systools`, the Erlang/OTP tools for packaging code (see Releases), the code for each application is placed in a separate directory following a pre-defined directory structure.

## Application Callback Module

How to start and stop the code for the application, including its supervision tree, is described by two callback functions:

    start(StartType, StartArgs) -> {ok, Pid} | {ok, Pid, State}
    stop(State)

- `start/2` is called when starting the application and is to create the supervision tree by starting the top supervisor. It is expected to return the pid of the top supervisor and an optional term, `State`, which defaults to `[]`. This term is passed as is to `stop/1`.
- `StartType` is usually the atom `normal`. It has other values only in the case of a takeover or failover; see Distributed Applications.
- `StartArgs` is defined by the key `mod` in the application resource file.
- `stop/1` is called after the application has been stopped and is to do any necessary cleaning up. The actual stopping of the application, that is, shutting down the supervision tree, is handled automatically as described in Starting and Stopping Applications.

Example of an application callback module for packaging the supervision tree from Supervisor Behaviour:

    -module(ch_app).
    -behaviour(application).

    -export([start/2, stop/1]).

    start(_Type, _Args) ->
        ch_sup:start_link().

    stop(_State) ->
        ok.

A library application that cannot be started or stopped does not need any application callback module.

## Application Resource File

To define an application, an application specification is created, which is put in an application resource file, or in short an `.app` file:

    {application, Application, [Opt1,...,OptN]}.

- `Application`, an atom, is the name of the application. The file must be named `Application.app`.
- Each `Opt` is a tuple `{Key,Value}`, which defines a certain property of the application. All keys are optional. Default values are used for any omitted keys.

The contents of a minimal `.app` file for a library application `libapp` looks as follows:

    {application, libapp, []}.

The contents of a minimal `.app` file `ch_app.app` for a supervision tree application like `ch_app` looks as follows:

    {application, ch_app,
     [{mod, {ch_app,[]}}]}.

The key `mod` defines the callback module and start argument of the application, in this case `ch_app` and `[]`, respectively. This means that the following is called when the application is to be started:

    ch_app:start(normal, [])

The following is called when the application is stopped:

    ch_app:stop([])

When using `systools`, the Erlang/OTP tools for packaging code (see Section Releases), the keys `description`, `vsn`, `modules`, `registered`, and `applications` are also to be specified:

    {application, ch_app,
     [{description, "Channel allocator"},
      {vsn, "1"},
      {modules, [ch_app, ch_sup, ch3]},
      {registered, [ch3]},
      {applications, [kernel, stdlib, sasl]},
      {mod, {ch_app,[]}}
     ]}.

- `description` - A short description, a string. Defaults to `""`.
- `vsn` - Version number, a string. Defaults to `""`.
- `modules` - All modules introduced by this application. `systools` uses this list when generating boot scripts and tar files. A module must only be included in one application. Defaults to `[]`.
- `registered` - All names of registered processes in the application. `systools` uses this list to detect name clashes between applications. Defaults to `[]`.
- `applications` - All applications that must be started before this application is started. `systools` uses this list to generate correct boot scripts. Defaults to `[]`. Notice that all applications have dependencies to at least Kernel and STDLIB.

#### Note

For details about the syntax and contents of the application resource file, see app in Kernel.

## Directory Structure

When packaging code using `systools`, the code for each application is placed in a separate directory, `lib/Application-Vsn`, where `Vsn` is the version number.

This can be useful to know, even if `systools` is not used, since Erlang/OTP is packaged according to the OTP principles and thus comes with a specific directory structure. The code server (see module `code` in Kernel) automatically uses code from the directory with the highest version number, if more than one version of an application is present.

### Directory Structure Guidelines for a Development Environment

Any directory structure for development will suffice as long as the released directory structure adheres to the description below, but it is encouraged that the same directory structure also be used in a development environment. The version number should be omitted from the application directory name since this is an artifact of the release step.

Some sub-directories are required. Some sub-directories are optional, meaning that it should only be used if the application itself requires it. Finally, some sub-directories are recommended, meaning it is encouraged that it is used and used as described here. For example, both documentation and tests are encouraged to exist in an application for it to be deemed a proper OTP application.

        ─ ${application}
          ├── doc
          │   ├── internal
          │   ├── examples
          │   └── src
          ├── include
          ├── priv
          ├── src
          │   └── ${application}.app.src
          └── test

- `src` - Required. Contains the Erlang source code, the source of the `.app` file and internal include files used by the application itself. Additional sub-directories within `src` can be used as namespaces to organize source files. These directories should never be deeper than one level.
- `priv` - Optional. Used for application specific files.
- `include` - Optional. Used for public include files that must be reachable from other applications.
- `doc` - Recommended. Any source documentation should be placed in sub-directories here.
- `doc/internal` - Recommended. Any documentation that describes implementation details about this application, not intended for publication, should be placed here.
- `doc/examples` - Recommended. Source code for examples on how to use this application should be placed here. It is encouraged that examples are sourced to the public documentation from this directory.
- `doc/src` - Recommended. All source files for documentation, such as Markdown, AsciiDoc, or XML-files, should be placed here.
- `test` - Recommended. All files regarding tests, such as test suites and test specifications, should be placed here.

Other directories in the development environment may be needed. If source code from languages other than Erlang is used, for instance C-code for NIFs, that code should be placed in a separate directory. By convention it is recommended to prefix such directories with the language name, for example `c_src` for C, `java_src` for Java or `go_src` for Go. Directories with `_src` suffix indicate that they are a part of the application and the compilation step. The final build artifacts should target the `priv/lib` or `priv/bin` directories.

The `priv` directory holds assets that the application needs during runtime. Executables should reside in `priv/bin` and dynamically-linked libraries should reside in `priv/lib`. Other assets are free to reside within the `priv` directory but it is recommended they do so in a structured manner.

Source files from other languages that generate Erlang code, such as ASN.1 or Mibs, should be placed in directories, at the top level or in `src`, with the same name as the source language, for example `asn1` and `mibs`. Build artifacts should be placed in their respective language directory, such as `src` for Erlang code or `java_src` for Java code.

In a development environment, it is acceptable that the `.app` file for the release resides in the `ebin` directory, but it is recommended that it is an artifact of the build step. By convention a `.app.src` located in the `src` directory is used. This file is nearly identical to the `.app` file, but certain fields, such as the application version, are replaced during the build step.

Directory names should not be capitalized.

It is encouraged to omit empty directories.

### Directory Structure for a Released System

A released application must follow a certain structure.

        ─ ${application}-${version}
          ├── bin
          ├── doc
          │   ├── html
          │   ├── man[1-9]
          │   ├── pdf
          │   ├── internal
          │   └── examples
          ├── ebin
          │   └── ${application}.app
          ├── include
          ├── priv
          │   ├── lib
          │   └── bin
          └── src

- `src` - Optional. Contains the Erlang source code and internal include files used by the application itself.
- `ebin` - Required. Contains the Erlang object code, the `.beam` files. The `.app` file must also be placed here.
- `priv` - Optional. Used for application specific files. `code:priv_dir/1` is to be used to access this directory.
- `priv/lib` - Recommended. Any shared-object files that are used by the application, such as NIFs or linked-in-drivers, should be placed here.
- `priv/bin` - Recommended. Any executable that is used by the application, such as port programs, should be placed here.
- `include` - Optional. Used for public include files that must be reachable from other applications.
- `bin` - Optional. Any executable that is a product of the application, such as escripts or shell scripts, should be placed here.
- `doc` - Optional. Any released documentation should be placed in sub-directories here.

The `src` directory could be useful to release for debugging purposes, but this is not required. The `include` directory should only be released if the application has public include files.

It is encouraged to omit empty directories.

## Application Controller

When an Erlang runtime system is started, a number of processes are started as part of the Kernel application. One of these processes is the application controller process, registered as `application_controller`.

All operations on applications are coordinated by the application controller. Use module `application` in Kernel to load, unload, start, and stop applications.

## Loading and Unloading Applications

Before an application can be started, it must be loaded. The application controller reads and stores the information from the `.app` file:

    1> application:load(ch_app).
    ok
    2> application:loaded_applications().
    [{kernel,"ERTS  CXC 138 10","2.8.1.3"},
     {stdlib,"ERTS  CXC 138 10","1.11.4.3"},
     {ch_app,"Channel allocator","1"}]

An application that has been stopped, or has never been started, can be unloaded. The information about the application is erased from the internal database of the application controller.

    3> application:unload(ch_app).
    ok
    4> application:loaded_applications().
    [{kernel,"ERTS  CXC 138 10","2.8.1.3"},
     {stdlib,"ERTS  CXC 138 10","1.11.4.3"}]

#### Note

Loading/unloading an application does not load/unload the code used by the application. Code loading is handled in the usual way by the code server.

## Starting and Stopping Applications

An application is started by calling:

    5> application:start(ch_app).
    ok
    6> application:which_applications().
    [{kernel,"ERTS  CXC 138 10","2.8.1.3"},
     {stdlib,"ERTS  CXC 138 10","1.11.4.3"},
     {ch_app,"Channel allocator","1"}]

If the application is not already loaded, the application controller first loads it using `application:load/1`. It checks the value of the `applications` key to ensure that all applications that are to be started before this application are running.

Following that, the application controller creates an application master for the application.

The application master establishes itself as the group leader of all processes in the application and will forward I/O to the previous group leader.

#### Note

The purpose of the application master being the group leader is to easily keep track of which processes that belong to the application. That is needed to support the `application:get_application/0` and `application:get_env/1` functions, and also when stopping an application to ensure that all processes belonging to the application are terminated.

The application master starts the application by calling the application callback function `start/2` in the module with the start argument defined by the `mod` key in the `.app` file.

An application is stopped, but not unloaded, by calling:

    7> application:stop(ch_app).
    ok

The application master stops the application by telling the top supervisor to shut down. The top supervisor tells all its child processes to shut down, and so on; the entire tree is terminated in reverse start order. The application master then calls the application callback function `stop/1` in the module defined by the `mod` key.

## Configuring an Application

An application can be configured using configuration parameters. These are a list of `{Par,Val}` tuples specified by a key `env` in the `.app` file:

    {application, ch_app,
     [{description, "Channel allocator"},
      {vsn, "1"},
      {modules, [ch_app, ch_sup, ch3]},
      {registered, [ch3]},
      {applications, [kernel, stdlib, sasl]},
      {mod, {ch_app,[]}},
      {env, [{file, "/usr/local/log"}]}
     ]}.

`Par` is to be an atom. `Val` is any term. The application can retrieve the value of a configuration parameter by calling `application:get_env(App, Par)` or a number of similar functions. For more information, see module `application` in Kernel.

Example:

    % erl
    Erlang (BEAM) emulator version 5.2.3.6 [hipe] [threads:0]

    Eshell V5.2.3.6  (abort with ^G)
    1> application:start(ch_app).
    ok
    2> application:get_env(ch_app, file).
    {ok,"/usr/local/log"}

The values in the `.app` file can be overridden by values in a system configuration file. This is a file that contains configuration parameters for relevant applications:

    [{Application1, [{Par11,Val11},...]},
     ...,
     {ApplicationN, [{ParN1,ValN1},...]}].

The system configuration is to be called `Name.config` and Erlang is to be started with the command-line argument `-config Name`. For details, see `config` in Kernel.

Example:

A file `test.config` is created with the following contents:

    [{ch_app, [{file, "testlog"}]}].

The value of `file` overrides the value of `file` as defined in the `.app` file:

    % erl -config test
    Erlang (BEAM) emulator version 5.2.3.6 [hipe] [threads:0]

    Eshell V5.2.3.6  (abort with ^G)
    1> application:start(ch_app).
    ok
    2> application:get_env(ch_app, file).
    {ok,"testlog"}

If release handling is used, exactly one system configuration file is to be used and that file is to be called `sys.config`.

The values in the `.app` file and the values in a system configuration file can be overridden directly from the command line:

    % erl -ApplName Par1 Val1 ... ParN ValN

Example:

    % erl -ch_app file '"testlog"'
    Erlang (BEAM) emulator version 5.2.3.6 [hipe] [threads:0]

    Eshell V5.2.3.6  (abort with ^G)
    1> application:start(ch_app).
    ok
    2> application:get_env(ch_app, file).
    {ok,"testlog"}

## Application Start Types

A start type is defined when starting the application:

    application:start(Application, Type)

`application:start(Application)` is the same as calling `application:start(Application, temporary)`. The type can also be `permanent` or `transient`:

- If a permanent application terminates, all other applications and the runtime system are also terminated.
- If a transient application terminates with reason `normal`, this is reported but no other applications are terminated. If a transient application terminates abnormally, that is with any other reason than `normal`, all other applications and the runtime system are also terminated.
- If a temporary application terminates, this is reported but no other applications are terminated.

An application can always be stopped explicitly by calling `application:stop/1`. Regardless of the mode, no other applications are affected.

The transient mode is of little practical use, since when a supervision tree terminates, the reason is set to `shutdown`, not `normal`.

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