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  <header>
    <copyright>
      <year>2003</year><year>2020</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
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      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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    <title>Release Handling</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>release_handling.xml</file>
  </header>
  <marker id="release handling"></marker>

  <section>
    <title>Release Handling Principles</title>
    <p>An important feature of the Erlang programming language is
      the ability to change module code in runtime,
      <em>code replacement</em>, as described in the Erlang
      Reference Manual.</p>
    <p>Based on this feature, the OTP application SASL provides a
      framework for upgrading and downgrading between different
      versions of an entire release in runtime. This is called
      <em>release handling</em>.</p>
    <p>The framework consists of:</p>
    <list type="bulleted">
      <item>Offline support - <c>systools</c> for generating scripts
      and building release packages</item>
      <item>Online support - <c>release_handler</c> for unpacking and
      installing release packages</item>
    </list>
    <p>The minimal system based on Erlang/OTP, enabling release handling,
    thus consists of the Kernel, STDLIB, and SASL
    applications.</p>

    <section>
      <title>Release Handling Workflow</title>
      <p><em>Step 1</em>) A release is created as described in
        <seeguide marker="release_structure">Releases</seeguide>.</p>
      <p><em>Step 2</em>) The release is transferred to and installed at
        target environment. For information of how to install the first
	target system, see
        <seeguide marker="system/system_principles:create_target">System Principles</seeguide>.</p>
      <p><em>Step 3</em>) Modifications, for example, error corrections,
        are made to the code in the development environment.</p>
      <p><em>Step 4</em>) At some point, it is time to make a new version
        of release. The relevant <c>.app</c> files are updated and a new
        <c>.rel</c> file is written.</p>
      <p><em>Step 5</em>) For each modified application, an
        <seeguide marker="#appup">application upgrade file</seeguide>,
        <c>.appup</c>, is created. In this file, it is described how to
	upgrade and/or downgrade between the old and new version of the
        application.</p>
      <p><em>Step 6</em>) Based on the <c>.appup</c> files, a
        <seeguide marker="#relup">release upgrade file</seeguide> called
        <c>relup</c>, is created. This file describes how to upgrade and/or
        downgrade between the old and new version of the entire release.</p>
      <p><em>Step 7</em>) A new release package is made and transferred to
        the target system.</p>
      <p><em>Step 8</em>) The new release package is unpacked using the
        release handler.</p>
      <p><em>Step 9</em>) The new version of the release is installed,
        also using the release handler. This is done by evaluating the
        instructions in <c>relup</c>. Modules can be added, deleted, or
        reloaded, applications can be started, stopped, or restarted, and so
        on. In some cases, it is even necessary to restart the entire
        emulator.</p>
      <list type="bulleted">
	<item>If the installation fails, the system can be rebooted.
	The old release version is then automatically used.</item>
	<item>If the installation succeeds, the new version is made
	the default version, which is to now be used if there is a
	system reboot.</item>
      </list>
    </section>

    <section>
      <title>Release Handling Aspects</title>
      <p><seeguide marker="appup_cookbook">Appup Cookbook</seeguide>,
        contains examples of <c>.appup</c> files
	for typical cases of upgrades/downgrades that are normally easy to
	handle in runtime. However, many aspects can make release handling
	complicated, for example:</p>
      <list type="bulleted">
	<item>
          <p>Complicated or circular dependencies can make it difficult
           or even impossible to decide in which order things must be
           done without risking runtime errors during an upgrade or
           downgrade. Dependencies can be:</p>
          <list type="bulleted">
            <item>Between nodes</item>
            <item>Between processes</item>
            <item>Between modules</item>
          </list>
	</item>
	<item>
          <p>During release handling, non-affected processes continue
           normal execution. This can lead to time-outs or other problems.
           For example, new processes created in the time window between
           suspending processes using a certain module, and loading a new
           version of this module, can execute old code.</p>
	</item>
      </list>
      <p>It is thus recommended that code is changed in as small
      steps as possible, and always kept backwards compatible.</p>
    </section>
  </section>

  <section>
    <marker id="req"></marker>
    <title>Requirements</title>
    <p>For release handling to work properly, the runtime system must
      have knowledge about which release it is running. It
      must also be able to change (in runtime) which boot script and
      system configuration file to use if the system is
      rebooted, for example, by <c>heart</c> after a failure.
      Thus, Erlang must be started as an embedded system; for
      information on how to do this, see Embedded System.</p>
    <p>For system reboots to work properly, it is also required that
      the system is started with heartbeat monitoring, see the
      <c>erl(1)</c> manual page in ERTS and the <c>heart(3)</c>
      manual page in Kernel</p>
    <p>Other requirements:</p>
    <list type="bulleted">
      <item>
        <p>The boot script included in a release package must be
          generated from the same <c>.rel</c> file as the release
          package itself.</p>
        <p>Information about applications is fetched from the script
          when an upgrade or downgrade is performed.</p>
      </item>
      <item>
        <p>The system must be configured using only one system
          configuration file, called <c>sys.config</c>.</p>
        <p>If found, this file is automatically included when a release
          package is created.</p>
      </item>
      <item>
        <p>All versions of a release, except the first one, must
          contain a <c>relup</c> file.</p>
        <p>If found, this file is automatically included when a release
          package is created.</p>
      </item>
    </list>
  </section>

  <section>
    <title>Distributed Systems</title>
    <p>If the system consists of several Erlang nodes, each node can use
      its own version of the release. The release handler is a locally
      registered process and must be called at each node where an
      upgrade or downgrade is required. A release handling
      instruction, <c>sync_nodes</c>, can be used to synchronize the
      release handler processes at a number of nodes, see the
      <c>appup(4)</c> manual page in SASL.</p>
  </section>

  <section>
    <marker id="instr"></marker>
    <title>Release Handling Instructions</title>
    <p>OTP supports a set of <em>release handling instructions</em>
      that are used when creating <c>.appup</c> files. The release
      handler understands a subset of these, the <em>low-level</em>
      instructions. To make it easier for the user, there are also a
      number of <em>high-level</em> instructions, which are translated
      to low-level instructions by <c>systools:make_relup</c>.</p>
    <p>Some of the most frequently used instructions are described in
    this section. The complete list of instructions is included in the
    <c>appup(4)</c> manual page in SASL.</p>
    <p>First, some definitions:</p>
    <list type="bulleted">
      <item><em>Residence module</em> - The module where a process has
      its tail-recursive loop function(s). If these functions are
      implemented in several modules, all those modules are residence
      modules for the process.</item>
      <item><em>Functional module</em> - A module that is not a
      residence module for any process.</item>
    </list>
    <p>For a process implemented using an OTP behaviour, the behaviour
    module is the residence module for that process.
    The callback module is a functional module.</p>

    <section>
      <title>load_module</title>
      <p>If a simple extension has been made to a functional module, it
        is sufficient to load the new version of the module into
        the system, and remove the old version. This is called
        <em>simple code replacement</em> and for this the following
        instruction is used:</p>
      <code type="none">
{load_module, Module}</code>
    </section>

    <section>
      <title>update</title>
      <p>If a more complex change has been made, for example, a change
        to the format of the internal state of a <c>gen_server</c>, simple
	code replacement is not sufficient. Instead, it is necessary to:</p>
	<list type="bulleted">
	  <item>Suspend the processes using the module (to avoid that
	  they try to handle any requests before the code replacement is
	  completed).</item>
	  <item>Ask them to transform the internal state format and
	  switch to the new version of the module.</item>
	  <item>Remove the old version.</item>
	  <item>Resume the processes.</item>
	</list>
	<p>This is called <em>synchronized code replacement</em> and for
	this the following instructions are used:</p>
      <code type="none">
{update, Module, {advanced, Extra}}
{update, Module, supervisor}</code>
      <p><c>update</c> with argument <c>{advanced,Extra}</c> is used
        when changing the internal state of a behaviour as described
        above. It causes behaviour processes to call the callback
        function <c>code_change</c>, passing the term <c>Extra</c> and
        some other information as arguments. See the manual pages for
        the respective behaviours and
	<seeguide marker="appup_cookbook#int_state">Appup Cookbook</seeguide>.</p>
      <p><c>update</c> with argument <c>supervisor</c> is used when
        changing the start specification of a supervisor. See
	<seeguide marker="appup_cookbook#sup">Appup Cookbook</seeguide>.</p>
      <p>When a module is to be updated, the release handler finds
        which processes that are <em>using</em> the module by
        traversing the supervision tree of each running application
        and checking all the child specifications:</p>
      <code type="none">
{Id, StartFunc, Restart, Shutdown, Type, Modules}</code>
      <p>A process uses a module if the name is listed in
        <c>Modules</c> in the child specification for the process.</p>
      <p>If <c>Modules=dynamic</c>, which is the case for event
        managers, the event manager process informs the release handler
        about the list of currently installed event handlers
	(<c>gen_event</c>), and it is checked if the module name is in
        this list instead.</p>
      <p>The release handler suspends, asks for code change, and
        resumes processes by calling the functions
        <c>sys:suspend/1,2</c>, <c>sys:change_code/4,5</c>, and
        <c>sys:resume/1,2</c>, respectively.</p>
    </section>

    <section>
      <title>add_module and delete_module</title>
      <p>If a new module is introduced, the following instruction is
        used:</p>
      <code type="none">
{add_module, Module}</code>
      <p>The instruction loads the module and is necessary
        when running Erlang in embedded mode. It is not strictly
        required when running Erlang in interactive (default) mode,
        since the code server then automatically searches for and
        loads unloaded modules.</p>
      <p>The opposite of <c>add_module</c> is <c>delete_module</c>, which
        unloads a module:</p>
      <code type="none">
{delete_module, Module}</code>
      <p>Any process, in any application, with <c>Module</c>
        as residence module, is killed when the instruction is
        evaluated. The user must therefore ensure that all such
        processes are terminated before deleting the module, to avoid
        a situation with failing supervisor restarts.</p>
    </section>

    <section>
      <title>Application Instructions</title>
      <p>The following is the instruction for adding an application:</p>
      <code type="none">
{add_application, Application}</code>
      <p>Adding an application means that the modules defined by
        the <c>modules</c> key in the <c>.app</c> file are loaded using
        a number of <c>add_module</c> instructions, and then the application
        is started.</p>
      <p>The following is the instruction for removing an application:</p>
      <code type="none">
{remove_application, Application}</code>
      <p>Removing an application means that the application is stopped,
        the modules are unloaded using a number of <c>delete_module</c>
        instructions, and then the application specification is unloaded
        from the application controller.</p>
      <p>The following is the instruction for restarting an application:</p>
      <code type="none">
{restart_application, Application}</code>
      <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.</p>
    </section>

    <section>
      <title>apply (Low-Level)</title>
      <p>To call an arbitrary function from the release handler,
        the following instruction is used:</p>
      <code type="none">
{apply, {M, F, A}}</code>
      <p>The release handler evalutes <c>apply(M, F, A)</c>.</p>
    </section>

    <section>
      <marker id="restart_new_emulator_instr"></marker>
      <title>restart_new_emulator (Low-Level)</title>
      <p>This instruction is used when changing to a new emulator
        version, or when any of the core applications Kernel,
	STDLIB, or SASL is upgraded. If a system reboot
	is needed for another reason, the <c>restart_emulator</c>
	instruction is to be used instead.</p>
      <p>This instruction requires that the system is started with
        heartbeat monitoring, see the <c>erl(1)</c> manual page in
	ERTS and the <c>heart(3)</c> manual page in Kernel.</p>
      <p>The <c>restart_new_emulator</c> instruction must always be
        the first instruction in a relup. If the relup is
        generated by <c>systools:make_relup/3,4</c>, this is
        automatically ensured.</p>
      <p>When the release handler encounters the instruction, it first
        generates a temporary boot file, which starts the new versions
        of the emulator and the core applications, and the old version
        of all other applications. Then it shuts down
        the current emulator by calling <c>init:reboot()</c>, see the
        <c>init(3)</c> manual page in Kernel.
	All processes are terminated gracefully and
        the system is rebooted by the <c>heart</c> program, using the
        temporary boot file. After the reboot, the rest of the relup
        instructions are executed. This is done as a part of the
        temporary boot script.</p>
      <warning>
	<p>This mechanism causes the new versions of the emulator and
	core applications to run with the old version of other
	applications during startup. Thus, take extra care to
	avoid incompatibility. Incompatible changes in the core
	applications can in some situations be necessary. If possible,
	such changes are preceded by deprecation over two major
	releases before the actual change. To ensure the
	application is not crashed by an incompatible change, always
	remove any call to deprecated functions as soon as
	possible.</p>
      </warning>
      <p>An info report is written when the upgrade is completed. To
        programmatically find out if the upgrade is complete,
        call <c>release_handler:which_releases(current)</c> and check
        if it returns the expected (that is, the new) release.</p>
      <p>The new release version must be made permanent when the new
        emulator is operational. Otherwise, the old version will be
        used if there is a new system reboot.</p>
      <p>On UNIX, the release handler tells the <c>heart</c> program
        which command to use to reboot the system. The environment
        variable <c>HEART_COMMAND</c>, normally used by the <c>heart</c>
        program, is ignored in this case. The command instead defaults
        to <c>$ROOT/bin/start</c>. Another command can be set by using
	the SASL configuration parameter <c>start_prg</c>, see
	the <c>sasl(6)</c> manual page.</p>
    </section>

    <section>
      <marker id="restart_emulator_instr"></marker>
      <title>restart_emulator (Low-Level)</title>
      <p>This instruction is not related to upgrades of ERTS or any
        of the core applications. It can be used by any application to
        force a restart of the emulator after all upgrade instructions
        are executed.</p>
      <p>A relup script can only have one <c>restart_emulator</c>
        instruction and it must always be placed at the end. If
	the relup is generated by <c>systools:make_relup/3,4</c>, this
	is automatically ensured.</p>
      <p>When the release handler encounters the instruction, it shuts
        down the emulator by calling <c>init:reboot()</c>, see the
        <c>init(3)</c> manual page in Kernel.
	All processes are terminated gracefully and the system
        can then be rebooted by the <c>heart</c> program using the
        new release version. No more upgrade instruction is
        executed after the restart.</p>
    </section>
  </section>

  <section>
    <marker id="appup"></marker>
    <title>Application Upgrade File</title>
    <p>To define how to upgrade/downgrade between the current version
      and previous versions of an application, an
      <em>application upgrade file</em>, or in short an <c>.appup</c>
      file is created.
      The file is to be called <c>Application.appup</c>, where
      <c>Application</c> is the application name:</p>
    <code type="none">
{Vsn,
 [{UpFromVsn1, InstructionsU1},
  ...,
  {UpFromVsnK, InstructionsUK}],
 [{DownToVsn1, InstructionsD1},
  ...,
  {DownToVsnK, InstructionsDK}]}.</code>
  <list type="bulleted">
    <item><c>Vsn</c>, a string, is the current version of the application,
    as defined in the <c>.app</c> file.</item>
    <item>Each <c>UpFromVsn</c> is a previous version of the application
    to upgrade from.</item>
    <item>Each <c>DownToVsn</c> is a previous version of the application
    to downgrade to.</item>
    <item>Each <c>Instructions</c> is a list of release handling
    instructions.</item>
  </list>
    <p>For information about the syntax and contents of the <c>.appup</c>
      file, see the <c>appup(4)</c> manual page in SASL.</p>
    <p><seeguide marker="appup_cookbook">Appup Cookbook</seeguide>
      includes examples of <c>.appup</c> files for typical upgrade/downgrade
      cases.</p>
    <p><em>Example:</em> Consider the release <c>ch_rel-1</c> from
      <seeguide marker="release_structure#ch_rel">Releases</seeguide>.
      Assume you want to add a function <c>available/0</c> to server
      <c>ch3</c>, which returns the number of available channels (when
      trying out the example, change in a copy of the original
      directory, so that the first versions are still available):</p>
    <code type="none">
-module(ch3).
-behaviour(gen_server).

-export([start_link/0]).
-export([alloc/0, free/1]).
-export([available/0]).
-export([init/1, handle_call/3, handle_cast/2]).

start_link() ->
    gen_server:start_link({local, ch3}, ch3, [], []).

alloc() ->
    gen_server:call(ch3, alloc).

free(Ch) ->
    gen_server:cast(ch3, {free, Ch}).

available() ->
    gen_server:call(ch3, available).

init(_Args) ->
    {ok, channels()}.

handle_call(alloc, _From, Chs) ->
    {Ch, Chs2} = alloc(Chs),
    {reply, Ch, Chs2};
handle_call(available, _From, Chs) ->
    N = available(Chs),
    {reply, N, Chs}.

handle_cast({free, Ch}, Chs) ->
    Chs2 = free(Ch, Chs),
    {noreply, Chs2}.</code>
    <p>A new version of the <c>ch_app.app</c> file must now be created,
      where the version is updated:</p>
    <code type="none">
{application, ch_app,
 [{description, "Channel allocator"},
  {vsn, "2"},
  {modules, [ch_app, ch_sup, ch3]},
  {registered, [ch3]},
  {applications, [kernel, stdlib, sasl]},
  {mod, {ch_app,[]}}
 ]}.</code>
    <p>To upgrade <c>ch_app</c> from <c>"1"</c> to <c>"2"</c> (and
      to downgrade from <c>"2"</c> to <c>"1"</c>), you only need to
      load the new (old) version of the <c>ch3</c> callback module.
      Create the application upgrade file <c>ch_app.appup</c> in
      the <c>ebin</c> directory:</p>
    <code type="none">
{"2",
 [{"1", [{load_module, ch3}]}],
 [{"1", [{load_module, ch3}]}]
}.</code>
  </section>

  <section>
    <marker id="relup"></marker>
    <title>Release Upgrade File</title>
    <p>To define how to upgrade/downgrade between the new version and
      previous versions of a release, a <em>release upgrade file</em>,
      or in short <c>relup</c> file, is to be created.</p>
    <p>This file does not need to be created manually, it can be
      generated by <c>systools:make_relup/3,4</c>. The relevant versions
      of the <c>.rel</c> file, <c>.app</c> files, and <c>.appup</c> files
      are used as input. It is deduced which applications are to be
      added and deleted, and which applications that must be upgraded
      and/or downgraded. The instructions for this are fetched from
      the <c>.appup</c> files and transformed into a single list of
      low-level instructions in the right order.</p>
    <p>If the <c>relup</c> file is relatively simple, it can be created
      manually. It is only to contain low-level instructions.</p>
    <p>For details about the syntax and contents of the release upgrade
      file, see the <c>relup(4)</c> manual page in SASL.</p>
    <p><em>Example, continued from the previous section:</em> You have a
      new version "2" of <c>ch_app</c> and an <c>.appup</c> file. A new
      version of the <c>.rel</c> file is also needed. This time the file
      is called <c>ch_rel-2.rel</c> and the release version string is
      changed from "A" to "B":</p>
    <code type="none">
{release,
 {"ch_rel", "B"},
 {erts, "5.3"},
 [{kernel, "2.9"},
  {stdlib, "1.12"},
  {sasl, "1.10"},
  {ch_app, "2"}]
}.</code>
    <p>Now the <c>relup</c> file can be generated:</p>
    <pre>
1> <input>systools:make_relup("ch_rel-2", ["ch_rel-1"], ["ch_rel-1"]).</input>
ok</pre>
    <p>This generates a <c>relup</c> file with instructions for
      how to upgrade from version "A" ("ch_rel-1") to version "B"
      ("ch_rel-2") and how to downgrade from version "B" to version "A".</p>
    <p>Both the old and new versions of the <c>.app</c> and
      <c>.rel</c> files must be in the code path, as well as the
      <c>.appup</c> and (new) <c>.beam</c> files. The code path can be
      extended by using the option <c>path</c>:</p>
    <pre>
1> <input>systools:make_relup("ch_rel-2", ["ch_rel-1"], ["ch_rel-1"],</input>
<input>[{path,["../ch_rel-1",</input>
<input>"../ch_rel-1/lib/ch_app-1/ebin"]}]).</input>
ok</pre>
  </section>

  <section>
    <marker id="rel_handler"></marker>
    <title>Installing a Release</title>
    <p>When you have made a new version of a release, a release package
      can be created with this new version and transferred to the target
      environment.</p>
    <p>To install the new version of the release in runtime,
      the <em>release handler</em> is used. This is a process belonging
      to the SASL application, which handles unpacking, installation,
      and removal of release packages. It is communicated through
      the <c>release_handler</c> module. For details, see the
      <c>release_handler(3)</c> manual page in SASL.</p>
    <p>Assuming there is an operational target system with
      installation root directory <c>$ROOT</c>, the release package with
      the new version of the release is to be copied to
      <c>$ROOT/releases</c>.</p>
    <p>First, <em>unpack</em> the release package.
      The files are then extracted from the package:</p>
    <code type="none">
release_handler:unpack_release(ReleaseName) => {ok, Vsn}</code>
   <list type="bulleted">
     <item><c>ReleaseName</c> is the name of the release package except
     the <c>.tar.gz</c> extension.</item>
     <item><c>Vsn</c> is the version of the unpacked release, as
     defined in its <c>.rel</c> file.</item>
   </list>
    <p>A directory <c>$ROOT/lib/releases/Vsn</c> is created, where
      the <c>.rel</c> file, the boot script <c>start.boot</c>,
      the system configuration file <c>sys.config</c>, and <c>relup</c>
      are placed. For applications with new version numbers,
      the application directories are placed under <c>$ROOT/lib</c>.
      Unchanged applications are not affected.</p>
    <p>An unpacked release can be <em>installed</em>. The release
      handler then evaluates the instructions in <c>relup</c>, step by
      step:</p>
    <code type="none">
release_handler:install_release(Vsn) => {ok, FromVsn, []}</code>
    <p>If an error occurs during the installation, the system is
      rebooted using the old version of the release. If installation
      succeeds, the system is afterwards using the new version of
      the release, but if anything happens and the system is
      rebooted, it starts using the previous version again.</p>
      <p>To be made the default version, the newly installed release
      must be made <em>permanent</em>, which means the previous
      version becomes <em>old</em>:</p>
    <code type="none">
release_handler:make_permanent(Vsn) => ok</code>
    <p>The system keeps information about which versions are old and
      permanent in the files <c>$ROOT/releases/RELEASES</c> and
      <c>$ROOT/releases/start_erl.data</c>.</p>
    <p>To downgrade from <c>Vsn</c> to <c>FromVsn</c>,
      <c>install_release</c> must be called again:</p>
    <code type="none">
release_handler:install_release(FromVsn) => {ok, Vsn, []}</code>
    <p>An installed, but not permanent, release can be <em>removed</em>.
      Information about the release is then deleted from
      <c>$ROOT/releases/RELEASES</c> and the release-specific code,
      that is, the new application directories and
      the <c>$ROOT/releases/Vsn</c> directory, are removed.</p>
    <code type="none">
release_handler:remove_release(Vsn) => ok</code>

    <section>
      <title>Example (continued from the previous sections)</title>
    <p><em>Step 1)</em> Create a target system as described in
      System Principles of the first version <c>"A"</c>
      of <c>ch_rel</c> from
      <seeguide marker="release_structure#ch_rel">Releases</seeguide>.
      This time <c>sys.config</c> must be included in the release package.
      If no configuration is needed, the file is to contain the empty
      list:</p>
      <code type="none">
[].</code>
    <p><em>Step 2)</em> Start the system as a simple target system. In
      reality, it is to be started as an embedded system. However, using
      <c>erl</c> with the correct boot script and config file is enough for
      illustration purposes:</p>
      <pre>
% <input>cd $ROOT</input>
% <input>bin/erl -boot $ROOT/releases/A/start -config $ROOT/releases/A/sys</input>
...</pre>
    <p><c>$ROOT</c> is the installation directory of the target system.</p>
    <p><em>Step 3)</em> In another Erlang shell, generate start scripts and
      create a release package for the new version <c>"B"</c>. Remember to
      include (a possible updated) <c>sys.config</c> and the <c>relup</c> file,
      see <seeguide marker="#relup">Release Upgrade File</seeguide>.</p>
      <pre>
1> <input>systools:make_script("ch_rel-2").</input>
ok
2> <input>systools:make_tar("ch_rel-2").</input>
ok</pre>
    <p>The new release package now also contains version "2" of <c>ch_app</c>
      and the <c>relup</c> file:</p>
      <code type="none">
% tar tf ch_rel-2.tar
lib/kernel-2.9/ebin/kernel.app
lib/kernel-2.9/ebin/application.beam
...
lib/stdlib-1.12/ebin/stdlib.app
lib/stdlib-1.12/ebin/beam_lib.beam
...
lib/sasl-1.10/ebin/sasl.app
lib/sasl-1.10/ebin/sasl.beam
...
lib/ch_app-2/ebin/ch_app.app
lib/ch_app-2/ebin/ch_app.beam
lib/ch_app-2/ebin/ch_sup.beam
lib/ch_app-2/ebin/ch3.beam
releases/B/start.boot
releases/B/relup
releases/B/sys.config
releases/B/ch_rel-2.rel
releases/ch_rel-2.rel</code>
    <p><em>Step 4)</em> Copy the release package <c>ch_rel-2.tar.gz</c>
      to the <c>$ROOT/releases</c> directory.</p>
    <p><em>Step 5)</em> In the running target system, unpack the release
      package:</p>
      <pre>
1> <input>release_handler:unpack_release("ch_rel-2").</input>
{ok,"B"}</pre>
    <p>The new application version <c>ch_app-2</c> is installed under
      <c>$ROOT/lib</c> next to <c>ch_app-1</c>. The <c>kernel</c>,
      <c>stdlib</c>, and <c>sasl</c> directories are not affected, as
      they have not changed.</p>
    <p>Under <c>$ROOT/releases</c>, a new directory <c>B</c> is created,
      containing <c>ch_rel-2.rel</c>, <c>start.boot</c>,
      <c>sys.config</c>, and <c>relup</c>.</p>
    <p><em>Step 6)</em> Check if the function <c>ch3:available/0</c> is
      available:</p>
      <pre>
2> <input>ch3:available().</input>
** exception error: undefined function ch3:available/0</pre>
    <p><em>Step 7)</em> Install the new release. The instructions in
      <c>$ROOT/releases/B/relup</c> are executed one by one, resulting
      in the new version of <c>ch3</c> being loaded. The function
      <c>ch3:available/0</c> is now available:</p>
      <pre>
3> <input>release_handler:install_release("B").</input>
{ok,"A",[]}
4> <input>ch3:available().</input>
3
5> <input>code:which(ch3).</input>
".../lib/ch_app-2/ebin/ch3.beam"
6> <input>code:which(ch_sup).</input>
".../lib/ch_app-1/ebin/ch_sup.beam"</pre>
    <p>Processes in <c>ch_app</c> for which code have not
      been updated, for example, the supervisor, are still evaluating
      code from <c>ch_app-1</c>.</p>
    <p><em>Step 8)</em> If the target system is now rebooted, it uses
      version "A" again. The "B" version must be made permanent, to be
      used when the system is rebooted.</p>
      <pre>
7> <input>release_handler:make_permanent("B").</input>
ok</pre>
    </section>
  </section>

  <section>
    <marker id="sys"></marker>
    <title>Updating Application Specifications</title>
    <p>When a new version of a release is installed, the application
      specifications are automatically updated for all loaded
      applications.</p>
    <note>
      <p>The information about the new application specifications is
        fetched from the boot script included in the release package.
        Thus, it is important that the boot script is generated from
        the same <c>.rel</c> file as is used to build the release
        package itself.</p>
    </note>
    <p>Specifically, the application configuration parameters are
      automatically updated according to (in increasing priority
      order):</p>
    <list type="bulleted">
      <item>The data in the boot script, fetched from the new
       application resource file <c>App.app</c></item>
      <item>The new <c>sys.config</c></item>
      <item>Command-line arguments <c>-App Par Val</c></item>
    </list>
    <p>This means that parameter values set in the other system
      configuration files and values set using
      <c>application:set_env/3</c> are disregarded.</p>
    <p>When an installed release is made permanent, the system process
      <c>init</c> is set to point out the new <c>sys.config</c>.</p>
    <p>After the installation, the application controller compares
      the old and new configuration parameters for all running
      applications and call the callback function:</p>
    <code type="none">
Module:config_change(Changed, New, Removed)</code>
    <list type="bulleted">
      <item><c>Module</c> is the application callback module as defined
      by the <c>mod</c> key in the <c>.app</c> file.</item>
      <item><c>Changed</c> and <c>New</c> are lists of <c>{Par,Val}</c> for
      all changed and added configuration parameters, respectively.</item>
      <item><c>Removed</c> is a list of all parameters <c>Par</c> that have
      been removed.</item>
    </list>
    <p>The function is optional and can be omitted when implementing an
      application callback module.</p>
  </section>
</chapter>
