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  <header>
    <copyright>
      <year>1997</year><year>2021</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
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      Licensed under the Apache License, Version 2.0 (the "License");
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    <title>app</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <file>app</file>
  <filesummary>Application resource file.</filesummary>
  <description>
    <p>The <em>application resource file</em> specifies the resources an
      application uses, and how the application is started. There must
      always be one application resource file called
      <c>Application.app</c> for each application <c>Application</c> in
      the system.</p>
    <p>The file is read by the application controller when an
      application is loaded/started. It is also used by the functions in
      <c>systools</c>, for example when generating start scripts.</p>
  </description>

  <section>
    <title>File Syntax</title>
    <p>The application resource file is to be called
      <c>Application.app</c>, where <c>Application</c> is the
      application name. The file is to be located in directory <c>ebin</c>
      for the application.</p>
    <p>The file must contain a single Erlang term, which is called an
      <em>application specification</em>:</p>
    <code type="none">
{application, Application,
  [{description,  Description},
   {id,           Id},
   {vsn,          Vsn},
   {modules,      Modules},
   {maxP,         MaxP},
   {maxT,         MaxT},
   {registered,   Names},
   {included_applications, Apps},
   {optional_applications, Apps},
   {applications, Apps},
   {env,          Env},
   {mod,          Start},
   {start_phases, Phases},
   {runtime_dependencies, RTDeps}]}.

             Value                Default
             -----                -------
Application  atom()               -
Description  string()             ""
Id           string()             ""
Vsn          string()             ""
Modules      [Module]             []
MaxP         int()                infinity
MaxT         int()                infinity
Names        [Name]               []
Apps         [App]                []
Env          [{Par,Val}]          []
Start        {Module,StartArgs}   []
Phases       [{Phase,PhaseArgs}]  undefined
RTDeps       [ApplicationVersion] []

Module = Name = App = Par = Phase = atom()
Val = StartArgs = PhaseArgs = term()
ApplicationVersion = string()</code>
    <taglist>
      <tag><c>Application</c></tag>
      <item>Application name.</item>
    </taglist>
    <p>For the application controller, all keys are optional.
      The respective default values are used for any omitted keys.</p>
    <p>The functions in <c>systools</c> require more information. If
      they are used, the following keys are mandatory:</p>
    <list type="bulleted">
      <item><c>description</c></item>
      <item><c>vsn</c></item>
      <item><c>modules</c></item>
      <item><c>registered</c></item>
      <item><c>applications</c></item>
    </list>
    <p>The other keys are ignored by <c>systools</c>.</p>
    <taglist>
      <tag><c>description</c></tag>
      <item>
        <p>A one-line description of the application.</p>
      </item>
      <tag><c>id</c></tag>
      <item>
        <p>Product identification, or similar.</p>
      </item>
      <tag><c>vsn</c></tag>
      <item>
        <p>Version of the application.</p>
      </item>
      <tag><c>modules</c></tag>
      <item>
        <p>All modules introduced by this application. <c>systools</c>
          uses this list when generating start scripts and tar files. A
          module can only be defined in one application.</p>
      </item>
      <tag><c>maxP</c></tag>
      <item>
        <p><em>Deprecated - is ignored</em></p>
        <p>Maximum number of processes allowed in the application.</p>
      </item>
      <tag><c>maxT</c></tag>
      <item>
        <p>Maximum time, in milliseconds, that the application is
          allowed to run. After the specified time, the application
          terminates automatically.</p>
      </item>
      <tag><c>registered</c></tag>
      <item>
        <p>All names of registered processes started in this
          application. <c>systools</c> uses this list to detect name
          clashes between different applications.</p>
      </item>
      <tag><c>included_applications</c></tag>
      <item>
        <p>All applications included by this application.
          When this application is started, all included applications
          are loaded automatically, but not started, by
          the application controller. It is assumed that the top-most
          supervisor of the included application is started by a
          supervisor of this application.</p>
      </item>
      <tag><c>applications</c></tag>
      <item>
        <p>All applications that must be started before this
          application. If an application is also listed in
          <c>optional_applications</c>, then the application
          is not required to exist (but if it exists, it is
          also guaranteed to be started before this one).</p>
        <p><c>systools</c> uses this list to generate correct start
          scripts. Defaults to the empty list, but notice that all
          applications have dependencies to (at least) Kernel and STDLIB.</p>
      </item>
      <tag><c>optional_applications</c></tag>
      <item>
        <p>A list of <c>applications</c> that are optional.
          Note if you want an optional dependency to be
          automatically started before the current application
          whenever it is available, it must be listed on both
          <c>applications</c> and <c>optional_applications</c>.</p>
      </item>
      <tag><c>env</c></tag>
      <item>
        <p>Configuration parameters used by the application. The value
          of a configuration parameter is retrieved by calling
          <c>application:get_env/1,2</c>. The values in the application
          resource file can be overridden by values in a configuration
          file (see <seefile marker="config"><c>config(4)</c></seefile>)
          or by command-line flags (see
          <seecom marker="erts:erl"><c>erts:erl(1)</c></seecom>).</p>
      </item>
      <tag><c>mod</c></tag>
      <item>
        <p>Specifies the application callback module and a start argument, see
          <seeerl marker="application"><c>application(3)</c></seeerl>.</p>
        <p>Key <c>mod</c> is necessary for an application
          implemented as a supervision tree, otherwise the application
          controller does not know how to start it. <c>mod</c>
          can be omitted for applications without processes, typically
          code libraries, for example, STDLIB.</p>
      </item>
      <tag><c>start_phases</c></tag>
      <item>
        <p>A list of start phases and corresponding start arguments for
          the application. If this key is present, the application
          master, in addition to the usual call to
          <c>Module:start/2</c>, also calls
          <c>Module:start_phase(Phase,Type,PhaseArgs)</c> for each
          start phase defined by key <c>start_phases</c>. Only
          after this extended start procedure,
          <c>application:start(Application)</c> returns.</p>
        <p>Start phases can be used to synchronize startup of an
          application and its included applications. In this case,
          key <c>mod</c> must be specified as follows:</p>
        <code type="none">
{mod, {application_starter,[Module,StartArgs]}}</code>
        <p>The application master then calls <c>Module:start/2</c>
          for the primary application, followed by calls to
          <c>Module:start_phase/3</c> for each start phase (as defined
          for the primary application), both for the primary application
          and for each of its included applications, for which the start
          phase is defined.</p>
        <p>This implies that for an included application, the set of
          start phases must be a subset of the set of phases defined
          for the primary application. For more information, see
          <seeguide marker="system/design_principles:applications">OTP Design Principles</seeguide>.
        </p>
      </item>
      <tag>
        <marker id="runtime_dependencies"></marker>
        <c>runtime_dependencies</c></tag>
      <item>
        <p>A list of application versions that the application
          depends on. An example of such an application version is
          <c>"kernel-3.0"</c>. Application versions specified as runtime
          dependencies are minimum requirements. That is, a larger
          application version than the one specified in the
          dependency satisfies the requirement. For information about
          how to compare application versions, see section
          <seeguide marker="system/system_principles:versions">Versions</seeguide>
          in the System Principles User's Guide.</p>
        <p>Notice that the application version
          specifies a source code version. One more, indirect,
          requirement is that the installed binary application of
          the specified version is built so that it is
          compatible with the rest of the system.</p>
        <p>Some dependencies can only be required in specific runtime
          scenarios. When such optional dependencies exist, these are
          specified and documented in the corresponding "App" documentation
          of the specific application.</p>
	  <warning><p>The <c>runtime_dependencies</c> key was introduced in
	  OTP 17.0. The type of its value might be subject to changes during
	  the OTP 17 release.</p></warning>
	  <warning><p>All runtime dependencies specified in OTP applications
	  during the OTP 17 release may not be completely correct. This
	  is actively being worked on. Declared runtime dependencies in OTP
	  applications are expected to be correct in OTP 18.</p></warning>
      </item>
    </taglist>
  </section>

  <section>
    <title>See Also</title>
    <p><seeerl marker="application"><c>application(3)</c></seeerl>,
      <seeerl marker="sasl:systools"><c>systools(3)</c></seeerl></p>
  </section>
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