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  <header>
    <copyright>
      <year>1997</year><year>2021</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.
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          http://www.apache.org/licenses/LICENSE-2.0

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    <title>appup</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <file>appup</file>
  <filesummary>Application upgrade file</filesummary>
  <description>
    <p>The <em>application upgrade file</em> defines how an application
      is upgraded or downgraded in a running system.</p>
    <p>This file is used by the functions in 
    <seeerl marker="systools"><c>systools</c></seeerl> 
    when generating a release upgrade file <c>relup</c>.</p>
  </description>

  <section>
    <title>File Syntax</title>
    <p>The application upgrade file is to be called
      <c>Application.appup</c>, where <c>Application</c> is the
      application name. The file is to be located in the <c>ebin</c>
      directory for the application.</p>
    <p>The <c>.appup</c> file contains one single Erlang term, which
      defines the instructions used to upgrade or downgrade
      the application. The file has the following syntax:</p>
    <code type="none">
{Vsn,
  [{UpFromVsn, Instructions}, ...],
  [{DownToVsn, Instructions}, ...]}.</code>
    <taglist>
      <tag><c>Vsn = string()</c></tag>
        <item><p>Current application version.</p></item>
      <tag><c>UpFromVsn = string() | binary()</c></tag>
        <item><p>An earlier
          application version to upgrade from. If it is a
          string, it is interpreted as a specific version
          number. If it is a binary, it is interpreted as a
          regular expression that can match multiple version
          numbers.</p></item>
      <tag><c>DownToVsn = string() | binary()</c></tag>
        <item><p>An earlier
          application version to downgrade to. If it is a
          string, it is  interpreted as a specific version
          number. If it is a binary, it is interpreted as a
          regular expression that can match multiple version
          numbers.</p></item>
      <tag><c>Instructions</c></tag>
        <item><p>A list of <em>release upgrade instructions</em>, see
          <seefile marker="#Release Upgrade Instructions">Release
          Upgrade Instructions</seefile>. It is recommended to use
          high-level instructions only. These are automatically
          translated to low-level instructions by <c>systools</c> when
          creating the <c>relup</c> file.</p></item>
    </taglist>
    <p>To avoid duplication of upgrade instructions, it is
      allowed to use regular expressions to specify <c>UpFromVsn</c>
      and <c>DownToVsn</c>. To be considered a regular expression, the
      version identifier must be specified as a binary. For example,
      the following match all versions <c>2.1.x</c>, where <c>x</c> is
      any number:</p>
    <code type="none">
&lt;&lt;"2\\.1\\.[0-9]+">></code>
    <p>Notice that the regular expression must match the complete
      version string, so this example works for, for example,
      <c>2.1.1</c>, but not for <c>2.1.1.1</c>.</p>
  </section>

  <section>
    <marker id="Release Upgrade Instructions"></marker>
    <title>Release Upgrade Instructions</title>
    <p>Release upgrade instructions are interpreted by the release
      handler when an upgrade or downgrade is made. For more
      information about release handling, see
      <seeguide marker="system/design_principles:release_handling">OTP
      Design Principles</seeguide> in <em>System Documentation</em>.</p>
    <p>A process is said to <em>use</em> a module <c>Mod</c> if
      <c>Mod</c> is listed in the <c>Modules</c> part of the child
      specification used to start the process, see
      <seeerl marker="stdlib:supervisor"><c>supervisor(3)</c></seeerl>.
      In the case of 
      <seeerl marker="stdlib:gen_event"><c>gen_event</c></seeerl>, 
      an event manager process is said to use <c>Mod</c> if <c>Mod</c> 
      is an installed event handler.</p>

    <section>
    <title>High-Level Instructions</title>
    <pre>
{update, Mod}
{update, Mod, supervisor}
{update, Mod, Change}
{update, Mod, DepMods}
{update, Mod, Change, DepMods}
{update, Mod, Change, PrePurge, PostPurge, DepMods}
{update, Mod, Timeout, Change, PrePurge, PostPurge, DepMods}
{update, Mod, ModType, Timeout, Change, PrePurge, PostPurge, DepMods}
  Mod = atom()
  ModType = static | dynamic
  Timeout = int()>0 | default | infinity
  Change = soft | {advanced,Extra}
    Extra = term()
  PrePurge = PostPurge = soft_purge | brutal_purge
  DepMods = [Mod]</pre>
    <p>Synchronized code replacement of processes using module
      <c>Mod</c>.</p>
    <p>All those processes are suspended using 
      <seemfa marker="stdlib:sys#suspend/1"><c>sys:suspend</c></seemfa>,
      the new module version is loaded, and
      then the processes are resumed using 
      <seemfa marker="stdlib:sys#resume/1"><c>sys:resume</c></seemfa>.</p>
    <taglist>
    <tag><c>Change</c></tag>
      <item><p>Defaults to <c>soft</c> and defines the type of
      code change. If it is set to <c>{advanced,Extra}</c>, implemented
      processes using 
      <seeerl marker="stdlib:gen_server"><c>gen_server</c></seeerl>, 
      <seeerl marker="stdlib:gen_fsm"><c>gen_fsm</c></seeerl>,
      <seeerl marker="stdlib:gen_statem"><c>gen_statem</c></seeerl>, or
      <seeerl marker="stdlib:gen_event"><c>gen_event</c></seeerl> 
      transform their internal state by calling the callback function
      <c>code_change</c>. Special processes call the callback
      function <c>system_code_change/4</c>. In both cases, the term
      <c>Extra</c> is passed as an argument to the callback
      function.</p></item>
    <tag><c>PrePurge</c></tag>
      <item><p>Defaults to <c>brutal_purge</c>. It controls
      what action to take with processes executing old code
      before loading the new module version. If the value
      is <c>brutal_purge</c>, the processes are killed. If the value is
      <c>soft_purge</c>, 
      <seemfa marker="release_handler#install_release/1"><c>release_handler:install_release/1</c></seemfa>
      returns <c>{error,{old_processes,Mod}}</c>.</p></item>
    <tag><c>PostPurge</c></tag>
      <item><p>Defaults to <c>brutal_purge</c>. It controls
      what action to take with processes that are executing old code
      when the new module version has been loaded. If the value
      is <c>brutal_purge</c>, the code is purged when the release is
      made permanent and the processes are killed. If the value is
      <c>soft_purge</c>, the release handler purges the old code
      when no remaining processes execute the code.</p></item>
    <tag><c>DepMods</c></tag>
      <item><p>Defaults to <c>[]</c> and defines other modules that
      <c>Mod</c> is dependent on. In the <c>relup</c> file, instructions 
      for suspending processes using <c>Mod</c> come before
      instructions for suspending processes using modules in
      <c>DepMods</c> when upgrading, and conversely when downgrading.
      In case of circular dependencies, the order of the instructions in
      the <c>appup</c> file is kept.</p></item>
    <tag><c>Timeout</c></tag>
      <item><p>Defines the time-out when suspending processes.
      If no value or <c>default</c> is specified, the default value for
      <seemfa marker="stdlib:sys#suspend/1"><c>sys:suspend</c></seemfa> 
      is used.</p></item>
    <tag><c>ModType</c></tag>
      <item><p>Defaults to <c>dynamic</c>. It specifies if
      the code is "dynamic", that is, if a process using the module
      spontaneously switches to new code, or if it is "static".
      When doing an advanced update and upgrade, the new version of a
      dynamic module is loaded before the process is asked to change
      code. When downgrading, the process is asked to change code before
      loading the new version. For static modules, the new version is
      loaded before the process is asked to change code, both in
      the case of upgrading and downgrading. Callback modules are
      dynamic.</p></item>
    </taglist>
    <p><c>update</c> with argument <c>supervisor</c> is used when
      changing the start specification of a supervisor.</p>
    <pre>
{load_module, Mod}
{load_module, Mod, DepMods}
{load_module, Mod, PrePurge, PostPurge, DepMods}
  Mod = atom()
  PrePurge = PostPurge = soft_purge | brutal_purge
  DepMods = [Mod]</pre>
    <p>Simple code replacement of the module <c>Mod</c>.</p>
    <p>For a description of <c>PrePurge</c> and <c>PostPurge</c>,
      see <c>update</c> above.</p>
    <p><c>DepMods</c> defaults to <c>[]</c> and defines which other modules
      <c>Mod</c> is dependent on. In the <c>relup</c> file, instructions for
      loading these modules come before the instruction for loading
      <c>Mod</c> when upgrading, and conversely when downgrading.</p>
    <pre>
{add_module, Mod}
{add_module, Mod, DepMods}
  Mod = atom()
  DepMods = [Mod]</pre>
    <p>Loads a new module <c>Mod</c>.</p>
    <p><c>DepMods</c> defaults to <c>[]</c> and defines which other modules
      <c>Mod</c> is dependent on. In the <c>relup</c> file, instructions
      related to these modules come before the instruction for
      loading <c>Mod</c> when upgrading, and conversely when
      downgrading.</p>
    <pre>
{delete_module, Mod}
{delete_module, Mod, DepMods}
  Mod = atom()</pre>
    <p>Deletes a module <c>Mod</c> using the low-level instructions
      <c>remove</c> and <c>purge</c>.</p>
    <p><c>DepMods</c> defaults to <c>[]</c> and defines which other modules
      <c>Mod</c> is dependent on. In the <c>relup</c> file, instructions
      related to these modules come before the instruction for
      removing <c>Mod</c> when upgrading, and conversely when
      downgrading.</p>
    <pre>
{add_application, Application}
{add_application, Application, Type}
  Application = atom()
  Type = permanent | transient | temporary | load | none</pre>
    <p>Adding an application means that the modules defined by
      the <c>modules</c> key in the <c>.app</c> file are loaded using
      <c>add_module</c>.</p>
    <p><c>Type</c> defaults to <c>permanent</c> and specifies the start type
      of the application. If <c>Type = permanent | transient | temporary</c>,
      the application is loaded and started in the corresponding way, see
      <seeerl marker="kernel:application"><c>application(3)</c></seeerl>.
      If <c>Type = load</c>, the application is only loaded.
      If <c>Type = none</c>, the application is not loaded and not
      started, although the code for its modules is loaded.</p>
    <pre>
{remove_application, Application}
  Application = atom()</pre>
    <p>Removing an application means that the application is stopped,
      the modules are unloaded using <c>delete_module</c>, and then
      the application specification is unloaded from the application
      controller.</p>
    <pre>
{restart_application, Application}
  Application = atom()</pre>
    <p>Restarting an application means that the application is stopped
    and then started again, similar to using the instructions
    <c>remove_application</c> and <c>add_application</c> in sequence.
    Note that, even if the application has been started before the
    release upgrade is performed, <c>restart_application</c> may only
    <c>load</c> it rather than <c>start</c> it, depending on the
    application's <c>start type</c>:
    If <c>Type = load</c>, the application is only loaded.
    If <c>Type = none</c>, the application is not loaded and not
    started, although the code for its modules is loaded.
    </p>
    </section>

    <section>
    <title>Low-Level Instructions</title>
    <pre>
{load_object_code, {App, Vsn, [Mod]}}
  App = Mod = atom()
  Vsn = string()</pre>
    <p>Reads each <c>Mod</c> from directory <c>App-Vsn/ebin</c> as
      a binary. It does not load the modules. The instruction is to be
      placed first in the script to read all new code from the file
      to make the suspend-load-resume cycle less time-consuming.</p>
    <pre>
point_of_no_return</pre>
    <p>If a crash occurs after this instruction, the system cannot
      recover and is restarted from the old release version.
      The instruction must only occur once in a script. It is to be
      placed after all <c>load_object_code</c> instructions.</p>
    <pre>
{load, {Mod, PrePurge, PostPurge}}
  Mod = atom()
  PrePurge = PostPurge = soft_purge | brutal_purge</pre>
    <p>Before this instruction occurs, <c>Mod</c> must have been loaded
      using <c>load_object_code</c>. This instruction loads the module.
      <c>PrePurge</c> is ignored. For a description of <c>PostPurge</c>,
      see the high-level instruction <c>update</c> earlier.</p>
    <pre>
{remove, {Mod, PrePurge, PostPurge}}
  Mod = atom()
  PrePurge = PostPurge = soft_purge | brutal_purge</pre>
    <p>Makes the current version of <c>Mod</c> old.
      <c>PrePurge</c> is ignored. For a description of <c>PostPurge</c>,
      see the high-level instruction <c>update</c> earlier.</p>
    <pre>
{purge, [Mod]}
  Mod = atom()</pre>
    <p>Purges each module <c>Mod</c>, that is, removes the old code.
      Notice that any process executing purged code is killed.</p>
    <pre>
{suspend, [Mod | {Mod, Timeout}]}
  Mod = atom()
  Timeout = int()>0 | default | infinity</pre>
    <p>Tries to suspend all processes using a module <c>Mod</c>. If a
      process does not respond, it is ignored. This can cause
      the process to die, either because it crashes when it
      spontaneously switches to new code, or as a result of a purge
      operation. If no <c>Timeout</c> is specified or <c>default</c> is
      specified, the default value for 
      <seemfa marker="stdlib:sys#suspend/1"><c>sys:suspend</c></seemfa> 
      is used.</p>
    <pre>
{resume, [Mod]}
  Mod = atom()</pre>
    <p>Resumes all suspended processes using a module <c>Mod</c>.</p>
    <pre>
{code_change, [{Mod, Extra}]}
{code_change, Mode, [{Mod, Extra}]}
  Mod = atom()
  Mode = up | down
  Extra = term()</pre>
    <p><c>Mode</c> defaults to <c>up</c> and specifies if it is an
      upgrade or downgrade. This instruction sends a <c>code_change</c>
      system message to all processes using a module <c>Mod</c> by
      calling function 
      <seemfa marker="stdlib:sys#change_code/4"><c>sys:change_code</c></seemfa>, 
      passing term <c>Extra</c> as argument.</p>
    <pre>
{stop, [Mod]}
  Mod = atom()</pre>
    <p>Stops all processes using a module <c>Mod</c> by calling
      <seemfa marker="stdlib:supervisor#terminate_child/2"><c>supervisor:terminate_child/2</c></seemfa>. 
      This instruction is useful
      when the simplest way to change code is to stop and restart the
      processes that run the code.</p>
    <pre>
{start, [Mod]}
  Mod = atom()</pre>
    <p>Starts all stopped processes using a module <c>Mod</c> by calling
      <seemfa marker="stdlib:supervisor#restart_child/2"><c>supervisor:restart_child/2</c></seemfa>.</p>
    <pre>
{sync_nodes, Id, [Node]}
{sync_nodes, Id, {M, F, A}}
  Id = term()
  Node = node()
  M = F = atom()
  A = [term()]</pre>
    <p><c>apply(M, F, A)</c> must return a list of nodes.</p>
    <p>This instruction synchronizes the release installation with other
      nodes. Each <c>Node</c> must evaluate this command with the same
      <c>Id</c>. The local node waits for all other nodes to evaluate
      the instruction before execution continues. If a node goes
      down, it is considered to be an unrecoverable error, and
      the local node is restarted from the old release. There is no
      time-out for this instruction, which means that it can hang
      forever.</p>
    <pre>
{apply, {M, F, A}}
  M = F = atom()
  A = [term()]</pre>
    <p>Evaluates <c>apply(M, F, A)</c>.</p>
    <p>If the instruction appears before instruction
      <c>point_of_no_return</c>, a failure is caught.
      <seemfa marker="release_handler#install_release/1"><c>release_handler:install_release/1</c></seemfa> 
      then returns <c>{error,{'EXIT',Reason}}</c>, unless <c>{error,Error}</c> 
      is thrown or returned. Then it returns <c>{error,Error}</c>.</p>
    <p>If the instruction appears after instruction
      <c>point_of_no_return</c> and the function call fails, the
      system is restarted.</p>
    <pre>
restart_new_emulator</pre>
    <p>This instruction is used when the application ERTS,
      Kernel, STDLIB, or SASL is
      upgraded. It shuts down the current emulator and starts a new
      one. All processes are terminated gracefully, and the new
      version of ERTS, Kernel, STDLIB, and
      SASL are used when the emulator restarts.
      Only one <c>restart_new_emulator</c> instruction is allowed
      in the <c>relup</c> file, and it must be placed first.
      <seemfa marker="systools#make_relup/3"><c>systools:make_relup/3,4</c></seemfa>
      ensures this when the <c>relup</c> file is generated. The rest of the
      instructions in the <c>relup</c> file is executed after the
      restart as a part of the boot script.</p>
    <p>An info report is written when the upgrade is completed.
      To programmatically determine if the upgrade is complete,
      call <seemfa marker="release_handler#which_releases/0">
      <c>release_handler:which_releases/0,1</c></seemfa> and check if the
      expected release has status <c>current</c>.</p>
    <p>The new release must still be made permanent after the upgrade
      is completed, otherwise the old emulator is started if there is
      an emulator restart.</p>
    <warning>
      <p>As stated earlier, instruction <c>restart_new_emulator</c>
        causes the emulator to be restarted with new versions of
        ERTS>, Kernel, STDLIB, and SASL.
        However, all other applications do at startup run their old
        versions in this new emulator. This is usually no problem,
        but every now and then incompatible changes occur to the
        core applications, which can cause
        trouble in this setting. Such incompatible changes (when
        functions are removed) are normally preceded by a deprecation
        over two major releases. To ensure that your application is not
        crashed by an incompatible change, always remove any call to
        deprecated functions as soon as possible.</p>
    </warning>
    <pre>
restart_emulator</pre>
    <p>This instruction is similar to <c>restart_new_emulator</c>,
      except it must be placed at the end of the <c>relup</c> file.
      It is not related to an upgrade of the emulator or the core
      applications, but can be used by any application when a complete
      reboot of the system is required.</p>
    <p>When generating the <c>relup</c> file,
      <seemfa marker="systools#make_relup/3"><c>systools:make_relup/3,4</c></seemfa>
      ensures that there is only one <c>restart_emulator</c>
      instruction and that it is the last instruction in the
      <c>relup</c> file.</p>
    </section>
  </section>

  <section>
    <title>See Also</title>
    <p><seeerl marker="release_handler"><c>release_handler(3)</c></seeerl>,
      <seefile marker="relup"><c>relup(4)</c></seefile>,
      <seeerl marker="stdlib:supervisor"><c>supervisor(3)</c></seeerl>,
      <seeerl marker="systools"><c>systools(3)</c></seeerl></p>
  </section>
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