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

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<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
  <header>
    <copyright>
      <year>2003</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>Appup Cookbook</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>appup_cookbook.xml</file>
  </header>
  <marker id="appup cookbook"></marker>
  <p>This section includes examples of <c>.appup</c> files for
    typical cases of upgrades/downgrades done in runtime.</p>

  <section>
    <title>Changing a Functional Module</title>
    <p>When a functional module has been changed, for example,
      if a new function has been added or a bug has been corrected,
      simple code replacement is sufficient, for example:</p>
    <code type="none">
{"2",
 [{"1", [{load_module, m}]}],
 [{"1", [{load_module, m}]}]
}.</code>
  </section>

  <section>
    <title>Changing a Residence Module</title>
    <p>In a system implemented according to the OTP design principles,
      all processes, except system processes and special processes,
      reside in one of the behaviours <c>supervisor</c>,
      <c>gen_server</c>, <c>gen_fsm</c>,
      <c>gen_statem</c> or <c>gen_event</c>. These
      belong to the STDLIB application and upgrading/downgrading
      normally requires an emulator restart.</p>
    <p>OTP thus provides no support for changing residence modules except
      in the case of <seealso marker="#spec">special processes</seealso>.</p>
  </section>

  <section>
    <title>Changing a Callback Module</title>
     <p>A callback module is a functional module, and for code
      extensions simple code replacement is sufficient.</p>
      <p><em>Example:</em> When adding a function to <c>ch3</c>,
      as described in the example in
      <seealso marker="release_handling#appup">Release Handling</seealso>,
    <c>ch_app.appup</c> looks as follows:</p>
    <code type="none">
{"2",
 [{"1", [{load_module, ch3}]}],
 [{"1", [{load_module, ch3}]}]
}.</code>
    <p>OTP also supports changing the internal state of behaviour
      processes, see <seealso marker="#int_state">Changing Internal State</seealso>.</p>
  </section>

  <section>
    <marker id="int_state"></marker>
    <title>Changing Internal State</title>
    <p>In this case, simple code replacement is not sufficient.
      The process must explicitly transform its state using the callback
      function <c>code_change</c> before switching to the new version
      of the callback module. Thus, synchronized code replacement is
      used.</p>
    <p><em>Example:</em> Consider <c>gen_server</c> <c>ch3</c> from
      <seealso marker="gen_server_concepts#ex">gen_server Behaviour</seealso>.
      The internal state is a term <c>Chs</c>
      representing the available channels. Assume you want to add a counter
      <c>N</c>, which keeps track of the number of <c>alloc</c> requests
      so far. This means that the format must be changed to
      <c>{Chs,N}</c>.</p>
    <p>The <c>.appup</c> file can look as follows:</p>
    <code type="none">
{"2",
 [{"1", [{update, ch3, {advanced, []}}]}],
 [{"1", [{update, ch3, {advanced, []}}]}]
}.</code>
    <p>The third element of the <c>update</c> instruction is a tuple
      <c>{advanced,Extra}</c>, which says that the affected processes
      are to do a state transformation before loading the new version
      of the module. This is done by the processes calling the callback
      function <c>code_change</c> (see the <c>gen_server(3)</c> manual
      page in STDLIB). The term <c>Extra</c>, in this case
      <c>[]</c>, is passed as is to the function:</p>
    <marker id="code_change"></marker>
    <code type="none">
-module(ch3).
...
-export([code_change/3]).
...
code_change({down, _Vsn}, {Chs, N}, _Extra) ->
    {ok, Chs};
code_change(_Vsn, Chs, _Extra) ->
    {ok, {Chs, 0}}.</code>
    <p>The first argument is <c>{down,Vsn}</c> if there is a downgrade,
      or <c>Vsn</c> if there is a upgrade. The term <c>Vsn</c> is
      fetched from the 'original' version of the module, that is,
      the version you are upgrading from, or downgrading to.</p>
    <p>The version is defined by the module attribute <c>vsn</c>, if
      any. There is no such attribute in <c>ch3</c>, so in this case
      the version is the checksum (a huge integer) of the beam file, an
      uninteresting value, which is ignored.</p>
    <p>The other callback functions of <c>ch3</c> must also be modified
      and perhaps a new interface function must be added, but this is not
      shown here.</p>
  </section>

  <section>
    <title>Module Dependencies</title>
    <p>Assume that a module is extended by adding an interface function,
      as in the example in
      <seealso marker="release_handling#appup">Release Handling</seealso>,
      where a function <c>available/0</c> is added to <c>ch3</c>.</p>
    <p>If a call is added to this function, say in module
      <c>m1</c>, a runtime error could can occur during release upgrade if
      the new version of <c>m1</c> is loaded first and calls
      <c>ch3:available/0</c> before the new version of <c>ch3</c> is
      loaded.</p>
    <p>Thus, <c>ch3</c> must be loaded before <c>m1</c>, in
      the upgrade case, and conversely in the downgrade case.
      <c>m1</c> is said to be <em>dependent on</em> <c>ch3</c>. In a release
      handling instruction, this is expressed by the
      <c>DepMods</c> element:</p>
    <code type="none">
{load_module, Module, DepMods}
{update, Module, {advanced, Extra}, DepMods}</code>
    <p><c>DepMods</c> is a list of modules, on which <c>Module</c> is
      dependent.</p>
    <p><em>Example:</em> The module <c>m1</c> in application <c>myapp</c> is
      dependent on <c>ch3</c> when upgrading from "1" to "2", or
      downgrading from "2" to "1":</p>
    <code type="none">
myapp.appup:

{"2",
 [{"1", [{load_module, m1, [ch3]}]}],
 [{"1", [{load_module, m1, [ch3]}]}]
}.

ch_app.appup:

{"2",
 [{"1", [{load_module, ch3}]}],
 [{"1", [{load_module, ch3}]}]
}.</code>
    <p>If instead <c>m1</c> and <c>ch3</c> belong to the same application,
      the <c>.appup</c> file can look as follows:</p>
    <code type="none">
{"2",
 [{"1",
   [{load_module, ch3},
    {load_module, m1, [ch3]}]}],
 [{"1",
   [{load_module, ch3},
    {load_module, m1, [ch3]}]}]
}.</code>
    <p><c>m1</c> is dependent on <c>ch3</c> also
      when downgrading. <c>systools</c> knows the difference between
      up- and downgrading and generates a correct <c>relup</c>,
      where <c>ch3</c> is loaded before <c>m1</c> when upgrading, but
      <c>m1</c> is loaded before <c>ch3</c> when downgrading.</p>
  </section>

  <section>
    <marker id="spec"></marker>
    <title>Changing Code for a Special Process</title>
    <p>In this case, simple code replacement is not sufficient.
      When a new version of a residence module for a special process
      is loaded, the process must make a fully qualified call to
      its loop function to switch to the new code. Thus, synchronized
      code replacement must be used.</p>
    <note>
      <p>The name(s) of the user-defined residence module(s) must be
        listed in the <c>Modules</c> part of the child specification
        for the special process. Otherwise the release handler cannot
        find the process.</p>
    </note>
    <p><em>Example:</em> Consider the example <c>ch4</c> in
      <seealso marker="spec_proc#ex">sys and proc_lib</seealso>.
      When started by a supervisor, the child specification can look
      as follows:</p>
    <code type="none">
{ch4, {ch4, start_link, []},
 permanent, brutal_kill, worker, [ch4]}</code>
    <p>If <c>ch4</c> is part of the application <c>sp_app</c> and a new
      version of the module is to be loaded when upgrading from
      version "1" to "2" of this application, <c>sp_app.appup</c> can
      look as follows:</p>
    <code type="none">
{"2",
 [{"1", [{update, ch4, {advanced, []}}]}],
 [{"1", [{update, ch4, {advanced, []}}]}]
}.</code>
    <p>The <c>update</c> instruction must contain the tuple
      <c>{advanced,Extra}</c>. The instruction makes the special
      process call the callback function <c>system_code_change/4</c>, a
      function the user must implement. The term <c>Extra</c>, in this
      case <c>[]</c>, is passed as is to <c>system_code_change/4</c>:</p>
    <code type="none">
-module(ch4).
...
-export([system_code_change/4]).
...

system_code_change(Chs, _Module, _OldVsn, _Extra) ->
    {ok, Chs}.</code>
    <list type="bulleted">
      <item>The first argument is the internal state <c>State</c>,
      passed from function
      <c>sys:handle_system_msg(Request, From, Parent, Module, Deb, State)</c>,
      and called by the special process when a system message is received.
      In <c>ch4</c>, the internal state is the set of available channels
      <c>Chs</c>.</item>
      <item>The second argument is the name of the module
      (<c>ch4</c>).</item>
      <item>The third argument is <c>Vsn</c> or <c>{down,Vsn}</c>, as
      described for <c>gen_server:code_change/3</c> in
      <seealso marker="#code_change">Changing Internal State</seealso>.</item>
    </list>
    <p>In this case, all arguments but the first are ignored and
      the function simply returns the internal state again. This is
      enough if the code only has been extended. If instead the
      internal state is changed (similar to the example in
      <seealso marker="#int_state">Changing Internal State</seealso>),
      this is done in this function and <c>{ok,Chs2}</c> returned.</p>
  </section>

  <section>
    <marker id="sup"></marker>
    <title>Changing a Supervisor</title>
    <p>The supervisor behaviour supports changing the internal state,
      that is, changing the restart strategy and maximum restart frequency
      properties, as well as changing the existing child specifications.</p>
    <p>Child processes can be added or deleted, but this is not
      handled automatically. Instructions must be given by in
      the <c>.appup</c> file.</p>

    <section>
      <title>Changing Properties</title>
      <p>Since the supervisor is to change its internal state,
        synchronized code replacement is required. However,
        a special <c>update</c> instruction must be used.</p>
      <p>First, the new version of the callback module must be loaded,
        both in the case of upgrade and downgrade. Then the new return
        value of <c>init/1</c> can be checked and the internal state be
        changed accordingly.</p>
      <p>The following <c>upgrade</c> instruction is used for
        supervisors:</p>
      <code type="none">
{update, Module, supervisor}</code>
      <p><em>Example:</em> To change the restart strategy of
        <c>ch_sup</c> (from
	<seealso marker="sup_princ#ex">Supervisor Behaviour</seealso>)
	from <c>one_for_one</c> to <c>one_for_all</c>, change the callback
	function <c>init/1</c> in <c>ch_sup.erl</c>:</p>
      <code type="none">
-module(ch_sup).
...

init(_Args) ->
    {ok, {#{strategy => one_for_all, ...}, ...}}.</code>
      <p>The file <c>ch_app.appup</c>:</p>
      <code type="none">
{"2",
 [{"1", [{update, ch_sup, supervisor}]}],
 [{"1", [{update, ch_sup, supervisor}]}]
}.</code>
    </section>

    <section>
      <title>Changing Child Specifications</title>
      <p>The instruction, and thus the <c>.appup</c> file, when
        changing an existing child specification, is the same as when
        changing properties as described earlier:</p>
      <code type="none">
{"2",
 [{"1", [{update, ch_sup, supervisor}]}],
 [{"1", [{update, ch_sup, supervisor}]}]
}.</code>
      <p>The changes do not affect existing child processes. For
        example, changing the start function only specifies how
        the child process is to be restarted, if needed later on.</p>
      <p>The id of the child specification cannot be changed.</p>
      <p>Changing the <c>Modules</c> field of the child
        specification can affect the release handling process itself,
        as this field is used to identify which processes are affected
        when doing a synchronized code replacement.</p>
    </section>
    <marker id="sup_add"></marker>

    <section>
      <title>Adding and Deleting Child Processes</title>
      <p>As stated earlier, changing child specifications does not affect
        existing child processes. New child specifications are
        automatically added, but not deleted. Child processes are
        not automatically started or terminated, this must be
        done using <c>apply</c> instructions.</p>
      <p><em>Example:</em> Assume a new child process <c>m1</c> is to be
        added to <c>ch_sup</c> when upgrading <c>ch_app</c> from "1" to "2".
        This means <c>m1</c> is to be deleted when downgrading from
        "2" to "1":</p>
      <code type="none">
{"2",
 [{"1",
   [{update, ch_sup, supervisor},
    {apply, {supervisor, restart_child, [ch_sup, m1]}}
   ]}],
 [{"1",
   [{apply, {supervisor, terminate_child, [ch_sup, m1]}},
    {apply, {supervisor, delete_child, [ch_sup, m1]}},
    {update, ch_sup, supervisor}
   ]}]
}.</code>
      <p>The order of the instructions is important.</p>
      <p>The supervisor must be registered as
        <c>ch_sup</c> for the script to work. If the supervisor is not
        registered, it cannot be accessed directly from the script.
        Instead a help function that finds the pid of the supervisor
        and calls <c>supervisor:restart_child</c>, and so on, must be
	written. This function is then to be called from the script
        using the <c>apply</c> instruction.</p>
      <p>If the module <c>m1</c> is introduced in version "2" of
        <c>ch_app</c>, it must also be loaded when upgrading and
        deleted when downgrading:</p>
      <code type="none">
{"2",
 [{"1",
   [{add_module, m1},
    {update, ch_sup, supervisor},
    {apply, {supervisor, restart_child, [ch_sup, m1]}}
   ]}],
 [{"1",
   [{apply, {supervisor, terminate_child, [ch_sup, m1]}},
    {apply, {supervisor, delete_child, [ch_sup, m1]}},
    {update, ch_sup, supervisor},
    {delete_module, m1}
   ]}]
}.</code>
      <p>As stated earlier, the order of the instructions is important.
        When upgrading, <c>m1</c> must be loaded, and the supervisor
        child specification changed, before the new child process can
        be started. When downgrading, the child process must be
        terminated before the child specification is changed and the module
        is deleted.</p>
    </section>
  </section>

  <section>
    <title>Adding or Deleting a Module</title>
    <p><em>Example:</em> A new functional module <c>m</c> is added to
      <c>ch_app</c>:</p>
    <code type="none">
{"2",
 [{"1", [{add_module, m}]}],
 [{"1", [{delete_module, m}]}]</code>
  </section>

  <section>
    <title>Starting or Terminating a Process</title>
    <p>In a system structured according to the OTP design principles,
      any process would be a child process belonging to a supervisor, see
      <seealso marker="#sup_add">Adding and Deleting Child Processes</seealso>
      in Changing a Supervisor.</p>
  </section>

  <section>
    <title>Adding or Removing an Application</title>
    <p>When adding or removing an application, no <c>.appup</c> file
      is needed. When generating <c>relup</c>, the <c>.rel</c> files
      are compared and the <c>add_application</c> and
      <c>remove_application</c> instructions are added automatically.</p>
  </section>

  <section>
    <title>Restarting an Application</title>
    <p>Restarting an application is useful when a change is too
      complicated to be made without restarting the processes, for
      example, if the supervisor hierarchy has been restructured.</p>
    <p><em>Example:</em> When adding a child <c>m1</c> to <c>ch_sup</c>, as in
      <seealso marker="#sup_add">Adding and Deleting Child Processes</seealso>
      in Changing a Supervisor, an alternative to updating
      the supervisor is to restart the entire application:</p>
    <code type="none">
{"2",
 [{"1", [{restart_application, ch_app}]}],
 [{"1", [{restart_application, ch_app}]}]
}.</code>
  </section>

  <section>
    <marker id="app_spec"></marker>
    <title>Changing an Application Specification</title>
    <p>When installing a release, the application specifications are
      automatically updated before evaluating the <c>relup</c> script.
      Thus, no instructions are needed in the <c>.appup</c> file:</p>
    <pre>
{"2",
 [{"1", []}],
 [{"1", []}]
}.</pre>
  </section>

  <section>
    <title>Changing Application Configuration</title>
    <p>Changing an application configuration by updating the <c>env</c>
      key in the <c>.app</c> file is an instance of changing an
      application specification, see the previous section.</p>
    <p>Alternatively, application configuration parameters can be
      added or updated in <c>sys.config</c>.</p>
  </section>

  <section>
    <title>Changing Included Applications</title>
    <p>The release handling instructions for adding, removing, and
      restarting applications apply to primary applications only.
      There are no corresponding instructions for included
      applications. However, since an included application is really a
      supervision tree with a topmost supervisor, started as a child
      process to a supervisor in the including application, a
      <c>relup</c> file can be manually created.</p>
    <p><em>Example:</em> Assume there is a release containing an application
      <c>prim_app</c>, which have a supervisor <c>prim_sup</c> in its
      supervision tree.</p>
    <p>In a new version of the release, the application <c>ch_app</c>
      is to be included in <c>prim_app</c>. That is,
      its topmost supervisor <c>ch_sup</c> is to be started as a child
      process to <c>prim_sup</c>.</p>
    <p>The workflow is as follows:</p>
    <p><em>Step 1)</em> Edit the code for <c>prim_sup</c>:</p>
    <code type="none">
init(...) ->
    {ok, {...supervisor flags...,
          [...,
           {ch_sup, {ch_sup,start_link,[]},
            permanent,infinity,supervisor,[ch_sup]},
           ...]}}.</code>
    <p><em>Step 2)</em> Edit the <c>.app</c> file for <c>prim_app</c>:</p>
    <code type="none">
{application, prim_app,
 [...,
  {vsn, "2"},
  ...,
  {included_applications, [ch_app]},
  ...
 ]}.</code>
    <p><em>Step 3)</em> Create a new <c>.rel</c> file, including
      <c>ch_app</c>:</p>
    <code type="none">
{release,
 ...,
 [...,
  {prim_app, "2"},
  {ch_app, "1"}]}.</code>
  <p>The included application can be started in two ways.
  This is described in the next two sections.</p>

    <section>
      <title>Application Restart</title>
      <p><em>Step 4a)</em> One way to start the included application is to
        restart the entire <c>prim_app</c> application. Normally, the
	<c>restart_application</c> instruction in the <c>.appup</c> file
        for <c>prim_app</c> would be used.</p>
      <p>However, if this is done and a <c>relup</c> file is generated,
        not only would it contain instructions for restarting (that is,
        removing and adding) <c>prim_app</c>, it would also contain
        instructions for starting <c>ch_app</c> (and stopping it, in
        the case of downgrade). This is because <c>ch_app</c> is included
	in the new <c>.rel</c> file, but not in the old one.</p>
      <p>Instead, a correct <c>relup</c> file can be created manually,
        either from scratch or by editing the generated version.
        The instructions for starting/stopping <c>ch_app</c> are
        replaced by instructions for loading/unloading the application:</p>
      <code type="none">
{"B",
 [{"A",
   [],
   [{load_object_code,{ch_app,"1",[ch_sup,ch3]}},
    {load_object_code,{prim_app,"2",[prim_app,prim_sup]}},
    point_of_no_return,
    {apply,{application,stop,[prim_app]}},
    {remove,{prim_app,brutal_purge,brutal_purge}},
    {remove,{prim_sup,brutal_purge,brutal_purge}},
    {purge,[prim_app,prim_sup]},
    {load,{prim_app,brutal_purge,brutal_purge}},
    {load,{prim_sup,brutal_purge,brutal_purge}},
    {load,{ch_sup,brutal_purge,brutal_purge}},
    {load,{ch3,brutal_purge,brutal_purge}},
    {apply,{application,load,[ch_app]}},
    {apply,{application,start,[prim_app,permanent]}}]}],
 [{"A",
   [],
   [{load_object_code,{prim_app,"1",[prim_app,prim_sup]}},
    point_of_no_return,
    {apply,{application,stop,[prim_app]}},
    {apply,{application,unload,[ch_app]}},
    {remove,{ch_sup,brutal_purge,brutal_purge}},
    {remove,{ch3,brutal_purge,brutal_purge}},
    {purge,[ch_sup,ch3]},
    {remove,{prim_app,brutal_purge,brutal_purge}},
    {remove,{prim_sup,brutal_purge,brutal_purge}},
    {purge,[prim_app,prim_sup]},
    {load,{prim_app,brutal_purge,brutal_purge}},
    {load,{prim_sup,brutal_purge,brutal_purge}},
    {apply,{application,start,[prim_app,permanent]}}]}]
}.</code>
    </section>

    <section>
      <title>Supervisor Change</title>
      <p><em>Step 4b)</em> Another way to start the included
        application (or stop it
        in the case of downgrade) is by combining instructions for
        adding and removing child processes to/from <c>prim_sup</c> with
        instructions for loading/unloading all <c>ch_app</c> code and
        its application specification.</p>
      <p>Again, the <c>relup</c> file is created manually. Either from
        scratch or by editing a generated version. Load all code for
        <c>ch_app</c> first, and also load the application
        specification, before <c>prim_sup</c> is updated. When
        downgrading, <c>prim_sup</c> is to updated first, before
        the code for <c>ch_app</c> and its application specification
        are unloaded.</p>
      <code type="none">
{"B",
 [{"A",
   [],
   [{load_object_code,{ch_app,"1",[ch_sup,ch3]}},
    {load_object_code,{prim_app,"2",[prim_sup]}},
    point_of_no_return,
    {load,{ch_sup,brutal_purge,brutal_purge}},
    {load,{ch3,brutal_purge,brutal_purge}},
    {apply,{application,load,[ch_app]}},
    {suspend,[prim_sup]},
    {load,{prim_sup,brutal_purge,brutal_purge}},
    {code_change,up,[{prim_sup,[]}]},
    {resume,[prim_sup]},
    {apply,{supervisor,restart_child,[prim_sup,ch_sup]}}]}],
 [{"A",
   [],
   [{load_object_code,{prim_app,"1",[prim_sup]}},
    point_of_no_return,
    {apply,{supervisor,terminate_child,[prim_sup,ch_sup]}},
    {apply,{supervisor,delete_child,[prim_sup,ch_sup]}},
    {suspend,[prim_sup]},
    {load,{prim_sup,brutal_purge,brutal_purge}},
    {code_change,down,[{prim_sup,[]}]},
    {resume,[prim_sup]},
    {remove,{ch_sup,brutal_purge,brutal_purge}},
    {remove,{ch3,brutal_purge,brutal_purge}},
    {purge,[ch_sup,ch3]},
    {apply,{application,unload,[ch_app]}}]}]
}.</code>
    </section>
  </section>

  <section>
    <title>Changing Non-Erlang Code</title>
    <p>Changing code for a program written in another programming
      language than Erlang, for example, a port program, is
      application-dependent and OTP provides no special support for it.</p>
    <p><em>Example:</em> When changing code for a port program, assume that
      the Erlang process controlling the port is a <c>gen_server</c>
      <c>portc</c> and that the port is opened in the callback function
      <c>init/1</c>:</p>
    <code type="none">
init(...) ->
    ...,
    PortPrg = filename:join(code:priv_dir(App), "portc"),
    Port = open_port({spawn,PortPrg}, [...]),
    ...,
    {ok, #state{port=Port, ...}}.</code>
    <p>If the port program is to be updated, the code for the
      <c>gen_server</c> can be extended with a <c>code_change</c> function,
      which closes the old port and opens a new port.
      (If necessary, the <c>gen_server</c> can
      first request data that must be saved from the port
      program and pass this data to the new port):</p>
    <code type="none">
code_change(_OldVsn, State, port) ->
    State#state.port ! close,
    receive
        {Port,close} ->
            true
    end,
    PortPrg = filename:join(code:priv_dir(App), "portc"),
    Port = open_port({spawn,PortPrg}, [...]),
    {ok, #state{port=Port, ...}}.</code>
    <p>Update the application version number in the <c>.app</c> file
      and write an <c>.appup</c> file:</p>
    <code type="none">
["2",
 [{"1", [{update, portc, {advanced,port}}]}],
 [{"1", [{update, portc, {advanced,port}}]}]
].</code>
    <p>Ensure that the <c>priv</c> directory, where the C program is
      located, is included in the new release package:</p>
    <pre>
1> <input>systools:make_tar("my_release", [{dirs,[priv]}]).</input>
...</pre>
  </section>

  <section>
    <title>Emulator Restart and Upgrade</title>
    <p>Two upgrade instructions restart the emulator:</p>
    <list type="bulleted">
      <item><p><c>restart_new_emulator</c></p>
      <p>Intended when ERTS, Kernel, STDLIB, or
      SASL is upgraded. It is automatically added when the
      <c>relup</c> file is generated by <c>systools:make_relup/3,4</c>.
      It is executed before all other upgrade instructions.
      For more information about this instruction, see
      restart_new_emulator (Low-Level) in
      <seealso marker="release_handling#restart_new_emulator_instr">Release Handling Instructions</seealso>.
      </p></item>
      <item><p><c>restart_emulator</c></p>
      <p>Used when a restart of the emulator is required after all
      other upgrade instructions are executed.
      For more information about this instruction, see
      restart_emulator (Low-Level) in
      <seealso marker="release_handling#restart_emulator_instr">Release Handling Instructions</seealso>.
      </p></item>
    </list>
    <p>If an emulator restart is necessary and no upgrade instructions
      are needed, that is, if the restart itself is enough for the
      upgraded applications to start running the new versions, a
      simple <c>relup</c> file can be created manually:</p>
    <code type="none">
{"B",
 [{"A",
   [],
   [restart_emulator]}],
 [{"A",
   [],
   [restart_emulator]}]
}.</code>
    <p>In this case, the release handler framework with automatic
      packing and unpacking of release packages, automatic path
      updates, and so on, can be used without having to specify
      <c>.appup</c> files.</p>
  </section>

  <section>
    <title>Emulator Upgrade From Pre OTP R15</title>
    <p>From OTP R15, an emulator upgrade is performed by restarting
    the emulator with new versions of the core applications
    (Kernel, STDLIB, and SASL) before loading code
    and running upgrade instruction for other applications. For this
    to work, the release to upgrade from must include OTP R15 or
    later.</p>
    <p>For the case where the release to upgrade from includes an
    earlier emulator version, <c>systools:make_relup</c> creates a
    backwards compatible relup file. This means that all upgrade
    instructions are executed before the emulator is
    restarted. The new application code is therefore loaded into
    the old emulator. If the new code is compiled with the new
    emulator, there can be cases where the beam format has changed
    and beam files cannot be loaded. To overcome this problem, compile
    the new code with the old emulator.</p>
  </section>
</chapter>