path: root/system/doc/design_principles/applications.xml

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<chapter>
  <header>
    <copyright>
      <year>1997</year><year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.
    
    </legalnotice>

    <title>Applications</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>applications.xml</file>
  </header>
  <marker id="appl"></marker>
  <p>This section is to be read with the <c>app(4)</c> and
  <c>application(3)</c> manual pages in Kernel.</p>

  <section>
    <title>Application Concept</title>
    <p>When you have written code implementing some specific functionality
      you might want to make the code into an <em>application</em>,
      that is, a component that can be started and stopped as a unit,
      and which can also be reused in other systems.</p>
    <p>To do this, create an
      <seealso marker="#callback_module">application callback module</seealso>,
      and describe how the application is to be started and stopped.</p>
    <p>Then, an <em>application specification</em> is needed, which is
      put in an
      <seealso marker="#appl_res_file">application resource file</seealso>.
      Among other things, this file specifies which modules the application
      consists of and the name of the callback module.</p>
    <p>If you use <c>systools</c>, the Erlang/OTP tools for packaging code
      (see <seealso marker="release_structure">Releases</seealso>),
      the code for each application is placed in a
      separate directory following a pre-defined
    <seealso marker="#app_dir">directory structure</seealso>.</p>
  </section>

  <section>
    <marker id="callback_module"></marker>
    <title>Application Callback Module</title>
    <p>How to start and stop the code for the application, that is,
      the supervision tree, is described by two callback functions:</p>
    <code type="none">
start(StartType, StartArgs) -> {ok, Pid} | {ok, Pid, State}
stop(State)
    </code>
    <list type="bulleted">
      <item><c>start</c> 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, <c>State</c>, which defaults to <c>[]</c>. This term is passed
      as is to <c>stop</c>.</item>
      <item><c>StartType</c> is usually the atom <c>normal</c>. It has other
      values only in the case of a takeover or failover, see
      <seealso marker="distributed_applications">Distributed Applications</seealso>.
      </item>
      <item><c>StartArgs</c> is defined by the key <c>mod</c> in the
      <seealso marker="#appl_res_file">application
      resource file</seealso>.</item>
      <item><c>stop/1</c> is called <em>after</em> the application has been
      stopped and is to do any necessary cleaning up. The actual stopping of
      the application, that is, the shutdown of the supervision tree, is
      handled automatically as described in
      <seealso marker="#stopping">Starting and Stopping Applications</seealso>.
      </item>
    </list>
    <marker id="ch_app"></marker>
    <p>Example of an application callback module for packaging
      the supervision tree from
      <seealso marker="sup_princ#ex">Supervisor Behaviour</seealso>:</p>
    <code type="none">
-module(ch_app).
-behaviour(application).

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

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

stop(_State) ->
    ok.</code>
    <p>A library application that cannot be started or stopped, does not
      need any application callback module.</p>
  </section>

  <section>
    <marker id="appl_res_file"></marker>
    <title>Application Resource File</title>
    <p>To define an application, an <em>application specification</em> is
      created, which is put in an <em>application resource file</em>, or in
      short an <c>.app</c> file:</p>
    <code type="none">
{application, Application, [Opt1,...,OptN]}.</code>
<list>
    <item><c>Application</c>, an atom, is the name of the application.
      The file must be named <c>Application.app</c>.</item>
    <item>Each <c>Opt</c> is a tuple <c>{Key,Value}</c>, which define a
      certain property of the application. All keys are optional.
      Default values are used for any omitted keys.</item>
</list>
    <p>The contents of a minimal <c>.app</c> file for a library
      application <c>libapp</c> looks as follows:</p>
    <code type="none">
{application, libapp, []}.</code>
    <p>The contents of a minimal <c>.app</c> file <c>ch_app.app</c> for
      a supervision tree application like <c>ch_app</c> looks as follows:</p>
    <code type="none">
{application, ch_app,
 [{mod, {ch_app,[]}}]}.</code>
    <p>The key <c>mod</c> defines the callback module and start argument of
      the application, in this case <c>ch_app</c> and <c>[]</c>, respectively.
      This means that the following is called when the application is to be
      started:</p>
    <code type="none">
ch_app:start(normal, [])</code>
    <p>The following is called when the application is stopped.</p>
    <code type="none">
ch_app:stop([])</code>
    <p>When using <c>systools</c>, the Erlang/OTP tools for packaging
      code (see Section
      <seealso marker="release_structure">Releases</seealso>), the keys
      <c>description</c>, <c>vsn</c>, <c>modules</c>, <c>registered</c>,
      and <c>applications</c> are also to be specified:</p>
    <code type="none">
{application, ch_app,
 [{description, "Channel allocator"},
  {vsn, "1"},
  {modules, [ch_app, ch_sup, ch3]},
  {registered, [ch3]},
  {applications, [kernel, stdlib, sasl]},
  {mod, {ch_app,[]}}
 ]}.</code>
 <list>
   <item><c>description</c> - A short description, a string. Defaults to
   "".</item>
   <item><c>vsn</c> - Version number, a string. Defaults to "".</item>
   <item><c>modules</c> - All modules <em>introduced</em> by this
   application. <c>systools</c> uses this list when generating boot scripts
   and tar files. A module must be defined in only one application.
   Defaults to <c>[]</c>.</item>
   <item><c>registered</c> - All names of registered processes in the
   application. <c>systools</c> uses this list to detect name clashes
   between applications. Defaults to <c>[]</c>.</item>
   <item><c>applications</c> - All applications that must be
   started before this application is started. <c>systools</c> uses this
   list to generate correct boot scripts. Defaults to <c>[]</c>. Notice
   that all applications have dependencies to at least Kernel
   and STDLIB.</item>
 </list>
    <note><p>For details about the syntax and contents of the application
    resource file, see the <seealso marker="kernel:app">app</seealso>
    manual page in Kernel.</p></note>
  </section>

  <section>
    <marker id="app_dir"></marker>
    <title>Directory Structure</title>
    <p>When packaging code using <c>systools</c>, the code for each
      application is placed in a separate directory,
      <c>lib/Application-Vsn</c>, where <c>Vsn</c> is the version number.</p>
    <p>This can be useful to know, even if <c>systools</c> is not used,
      since Erlang/OTP is packaged according to the OTP principles
      and thus comes with this directory structure. The code server
      (see the <c>code(3)</c> manual page in Kernel) automatically
      uses code from
      the directory with the highest version number, if more than one
      version of an application is present.</p>
    <p>The application directory structure can also be used in the
      development environment. The version number can then
      be omitted from the name.</p>
    <p>The application directory has the following sub-directories:</p>
    <list type="bulleted">
      <item><c>src</c> - Contains the Erlang source code.</item>
      <item><c>ebin</c> - Contains the Erlang object code, the
      <c>beam</c> files. The <c>.app</c> file is also placed here.</item>
      <item><c>priv</c> - Used for application specific files. For
      example, C executables are placed here. The function
      <c>code:priv_dir/1</c> is to be used to access this directory.</item>
      <item><c>include</c> - Used for include files.</item>
    </list>
  </section>

  <section>
    <marker id="application_controller"></marker>
    <title>Application Controller</title>
    <p>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 <em>application controller</em> process,
      registered as <c>application_controller</c>.</p>
    <p>All operations on applications are coordinated by the application
      controller. It is interacted through the functions in
      the module <c>application</c>, see the <c>application(3)</c>
      manual page in Kernel. In particular, applications can be
      loaded, unloaded, started, and stopped.</p>
  </section>

  <section>
    <title>Loading and Unloading Applications</title>
    <p>Before an application can be started, it must be <em>loaded</em>.
      The application controller reads and stores the information from
      the <c>.app</c> file:</p>
    <pre>
1> <input>application:load(ch_app).</input>
ok
2> <input>application:loaded_applications().</input>
[{kernel,"ERTS  CXC 138 10","2.8.1.3"},
 {stdlib,"ERTS  CXC 138 10","1.11.4.3"},
 {ch_app,"Channel allocator","1"}]</pre>
    <p>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.</p>
    <pre>
3> <input>application:unload(ch_app).</input>
ok
4> <input>application:loaded_applications().</input>
[{kernel,"ERTS  CXC 138 10","2.8.1.3"},
 {stdlib,"ERTS  CXC 138 10","1.11.4.3"}]</pre>
    <note>
      <p>Loading/unloading an application does not load/unload the code
      used by the application. Code loading is done the usual way.</p>
    </note>
  </section>

  <section>
    <marker id="stopping"></marker>
    <title>Starting and Stopping Applications</title>
    <p>An application is started by calling:</p>
    <pre>
5> <input>application:start(ch_app).</input>
ok
6> <input>application:which_applications().</input>
[{kernel,"ERTS  CXC 138 10","2.8.1.3"},
 {stdlib,"ERTS  CXC 138 10","1.11.4.3"},
 {ch_app,"Channel allocator","1"}]</pre>
    <p>If the application is not already loaded, the application
      controller first loads it using <c>application:load/1</c>. It
      checks the value of the <c>applications</c> key, to ensure
      that all applications that are to be started before this
      application are running.</p>
    <marker id="application_master"></marker>
    <p>The application controller then creates an
    <em>application master</em> for the application. The application master
      is the group leader of all the processes in the application.
      The application master starts the application by calling
      the application callback function <c>start/2</c> in the module,
      and with the start argument, defined by the <c>mod</c> key in
      the <c>.app</c> file.</p>
    <p>An application is stopped, but not unloaded, by calling:</p>
    <pre>
7> <input>application:stop(ch_app).</input>
ok</pre>
    <p>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
      reversed start order. The application master then calls
      the application callback function <c>stop/1</c> in the module
      defined by the <c>mod</c> key.</p>
  </section>

  <section>
    <title>Configuring an Application</title>
    <p>An application can be configured using
    <em>configuration parameters</em>. These are a list of
    <c>{Par,Val}</c> tuples
    specified by a key <c>env</c> in the <c>.app</c> file:</p>
    <code type="none">
{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"}]}
 ]}.</code>
    <p><c>Par</c> is to be an atom. <c>Val</c> is any term.
      The application can retrieve the value of a configuration
      parameter by calling <c>application:get_env(App, Par)</c> or a
      number of similar functions, see the <c>application(3)</c>
      manual page in Kernel.</p>
    <p><em>Example:</em></p>
    <pre>
% <input>erl</input>
Erlang (BEAM) emulator version 5.2.3.6 [hipe] [threads:0]

Eshell V5.2.3.6  (abort with ^G)
1> <input>application:start(ch_app).</input>
ok
2> <input>application:get_env(ch_app, file).</input>
{ok,"/usr/local/log"}</pre>
    <p>The values in the <c>.app</c> file can be overridden by values
      in a <em>system configuration file</em>. This is a file that
      contains configuration parameters for relevant applications:</p>
    <code type="none">
[{Application1, [{Par11,Val11},...]},
 ...,
 {ApplicationN, [{ParN1,ValN1},...]}].</code>
    <p>The system configuration is to be called <c>Name.config</c> and
      Erlang is to be started with the command-line argument
      <c>-config Name</c>. For details, see the <c>config(4)</c>
      manual page in Kernel.</p>
    <p><em>Example:</em></p>
    <p>A file <c>test.config</c> is created with the following contents:</p>
    <code type="none">
[{ch_app, [{file, "testlog"}]}].</code>
    <p>The value of <c>file</c> overrides the value of <c>file</c>
      as defined in the <c>.app</c> file:</p>
    <pre>
% <input>erl -config test</input>
Erlang (BEAM) emulator version 5.2.3.6 [hipe] [threads:0]

Eshell V5.2.3.6  (abort with ^G)
1> <input>application:start(ch_app).</input>
ok
2> <input>application:get_env(ch_app, file).</input>
{ok,"testlog"}</pre>
    <p>If
      <seealso marker="release_handling#sys">release handling</seealso>
      is used, exactly one system configuration file is to be used and
      that file is to be called <c>sys.config</c>.</p>
    <p>The values in the <c>.app</c> file and the values in a
      system configuration file can be overridden directly from
      the command line:</p>
    <pre>
% <input>erl -ApplName Par1 Val1 ... ParN ValN</input></pre>
    <p><em>Example:</em></p>
    <pre>
% <input>erl -ch_app file '"testlog"'</input>
Erlang (BEAM) emulator version 5.2.3.6 [hipe] [threads:0]

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

  <section>
    <title>Application Start Types</title>
    <p>A <em>start type</em> is defined when starting the application:</p>
    <code type="none">
application:start(Application, Type)</code>
    <p><c>application:start(Application)</c> is the same as calling
      <c>application:start(Application, temporary)</c>. The type can
      also be <c>permanent</c> or <c>transient</c>:</p>
    <list type="bulleted">
      <item>If a permanent application terminates, all other
       applications and the runtime system are also terminated.</item>
      <item>If a transient application terminates with reason
      <c>normal</c>, this is reported but no other applications are
       terminated. If a transient application terminates abnormally,
       that is with any other reason than <c>normal</c>, all other
       applications and the runtime system are also terminated.</item>
      <item>If a temporary application terminates, this is reported but
       no other applications are terminated.</item>
    </list>
    <p>An application can always be stopped explicitly by
      calling <c>application:stop/1</c>. Regardless of the mode, no
      other applications are affected.</p>
    <p>The transient mode is of little practical use, since when
      a supervision tree terminates, the reason is set to
      <c>shutdown</c>, not <c>normal</c>.</p>
  </section>
</chapter>