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

<?xml version="1.0" encoding="latin1" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
  <header>
    <copyright>
      <year>2003</year><year>2011</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
    </legalnotice>

    <title>Appup Cookbook</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>appup_cookbook.xml</file>
  </header>
  <p>This chapter contains examples of <c>.appup</c> files for
    typical cases of upgrades/downgrades done in run-time.</p>

  <section>
    <title>Changing a Functional Module</title>
    <p>When a change has been made to a functional module, for example
      if a new function has been added or a bug has been corrected,
      simple code replacement is sufficient.</p>
    <p>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> 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>Example: 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> below.</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>Example: Consider the gen_server <c>ch3</c> from the chapter
      about the <seealso marker="gen_server_concepts#ex">gen_server behaviour</seealso>. The internal state is a term <c>Chs</c>
      representing the available channels. Assume we want add a counter
      <c>N</c> which keeps track of the number of <c>alloc</c> requests
      so far. This means we need to change the format to
      <c>{Chs,N}</c>.</p>
    <p>The <c>.appup</c> file could 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
      should 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 <c>gen_server(3)</c>). The term
      <c>Extra</c>, in this case [], 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> in case of a downgrade,
      or <c>Vsn</c> in case of an upgrade. The term <c>Vsn</c> is
      fetched from the 'original' version of the module, i.e.
      the version we 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> need to be modified
      as well and perhaps a new interface function added, this is not
      shown here).</p>
  </section>

  <section>
    <title>Module Dependencies</title>
    <p>Assume we extend a module by adding a new 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 we also add a call to this function, say in the module
      <c>m1</c>, a run-time error could 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> is, in
      the upgrade case, and vice versa in the downgrade case. We say
      that <c>m1</c><em>is dependent on</em><c>ch3</c>. In a release
      handling instruction, this is expressed by the element
      <c>DepMods</c>:</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>Example: The module <c>m1</c> in the 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 <c>m1</c> and <c>ch3</c> had belonged to the same application,
      the <c>.appup</c> file could have looked like this:</p>
    <code type="none">
{"2",
 [{"1",
   [{load_module, ch3},
    {load_module, m1, [ch3]}]}],
 [{"1",
   [{load_module, ch3},
    {load_module, m1, [ch3]}]}]
}.</code>
    <p>Note that it is <c>m1</c> that is dependent on <c>ch3</c> also
      when downgrading. <c>systools</c> knows the difference between
      up- and downgrading and will generate 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, in order for the release handler to
        find the process.</p>
    </note>
    <p>Example. Consider the example <c>ch4</c> from the chapter about
      <seealso marker="spec_proc#ex">sys and proc_lib</seealso>.
      When started by a supervisor, the child specification could look
      like this:</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 should be loaded when upgrading from
      version "1" to "2" of this application, <c>sp_app.appup</c> could
      look like this:</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 will make 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 [], 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>
    <p>The first argument is the internal state <c>State</c> passed from
      the function <c>sys:handle_system_msg(Request, From, Parent, Module, Deb, State)</c>, 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>.</p>
    <p>The second argument is the name of the module (<c>ch4</c>).</p>
    <p>The third argument is <c>Vsn</c> or <c>{down,Vsn}</c> as
      described for
      <seealso marker="#code_change">gen_server:code_change/3</seealso>.</p>
    <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 we had wanted to
      change the internal state (similar to the example in
      <seealso marker="#int_state">Changing Internal State</seealso>),
      it would have been 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,
      i.e. changing restart strategy and maximum restart frequency
      properties, as well as changing existing child specifications.</p>
    <p>Adding and deleting child processes are also possible, but not
      handled automatically. Instructions must be given by in
      the <c>.appup</c> file.</p>

    <section>
      <title>Changing Properties</title>
      <p>Since the supervisor should change its internal state,
        synchronized code replacement is required. However,
        a special <c>update</c> instruction must be used.</p>
      <p>The new version of the callback module must be loaded first
        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>Example: Assume we want to change the restart strategy of
        <c>ch_sup</c> from the <seealso marker="sup_princ#ex">Supervisor Behaviour</seealso> chapter from one_for_one to one_for_all.
        We change the callback function <c>init/1</c> in
        <c>ch_sup.erl</c>:</p>
      <code type="none">
-module(ch_sup).
...

init(_Args) ->
    {ok, {{one_for_all, 1, 60}, ...}}.</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 above:</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 should be restarted, if needed later on.</p>
      <p>Note that the id of the child specification cannot be changed.</p>
      <p>Note also that changing the <c>Modules</c> field of the child
        specification may 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 above, changing child specifications does not affect
        existing child processes. New child specifications are
        automatically added, but not deleted. Also, child processes are
        not automatically started or terminated. Instead, this must be
        done explicitly using <c>apply</c> instructions.</p>
      <p>Example: Assume we want to add a new child process <c>m1</c> to
        <c>ch_sup</c> when upgrading <c>ch_app</c> from "1" to "2".
        This means <c>m1</c> should 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>Note that the order of the instructions is important.</p>
      <p>Note also that 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> etc. must be written,
        and it is this function that should 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>Note again that the order of the instructions is important.
        When upgrading, <c>m1</c> must be loaded and the supervisor's
        child specification changed, before the new child process can
        be started. When downgrading, the child process must be
        terminated before child specification is changed and the module
        is deleted.</p>
    </section>
  </section>

  <section>
    <title>Adding or Deleting a Module</title>
    <p>Example: 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> above.</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 <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>Example: When adding a new child <c>m1</c> to <c>ch_sup</c>, as
      in the <seealso marker="#sup_add">example above</seealso>, 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.
      Hence, 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, <seealso marker="#app_spec">see above</seealso>.</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>Example: Assume we have 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, our example application
      <c>ch_app</c> should be included in <c>prim_app</c>. That is,
      its topmost supervisor <c>ch_sup</c> should be started as a child
      process to <c>prim_sup</c>.</p>
    <p>1) 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>2) 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>3) Create a new <c>.rel</c> file, including <c>ch_app</c>:</p>
    <code type="none">
{release,
 ...,
 [...,
  {prim_app, "2"},
  {ch_app, "1"}]}.</code>

    <section>
      <title>Application Restart</title>
      <p>4a) One way to start the included application is to restart
        the entire <c>prim_app</c> application. Normally, we would then
        use the <c>restart_application</c> instruction in
        the <c>.appup</c> file for <c>prim_app</c>.</p>
      <p>However, if we did this and then generated a <c>relup</c> file,
        not only would it contain instructions for restarting (i.e.
        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 due to the fact that
        <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>4b) 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> should be 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 very
      application dependent and OTP provides no special support for it.</p>
    <p>Example, changing code for a port program: Assume that
      the Erlang process controlling the port is a gen_server
      <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 should be updated, we can extend the code for
      the gen_server with a <c>code_change</c> function which closes
      the old port and opens a new port. (If necessary, the gen_server
      may first request data that needs to 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>Make sure 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>There are two upgrade instructions that will restart the emulator:</p>
    <taglist>
      <tag><c>restart_new_emulator</c></tag>
      <item>Intended for when erts, kernel, stdlib or sasl is
        upgraded. It is automatically added when the relup file is
        generated by <c>systools:make_relup/3,4</c>. It is executed
        before all other upgrade instructions. See
        <seealso marker="release_handling#restart_new_emulator_instr">Release
        Handling</seealso> for more information about this
        instruction.</item>
      <tag><c>restart_emulator</c></tag>
      <item>Used when a restart of the emulator is required after all
        other upgrade instructions are executed. See
        <seealso marker="release_handling#restart_emulator_instr">Release
        Handling</seealso> for more information about this
        instruction.</item>
    </taglist>

    <p>If an emulator restart is necessary and no upgrade instructions
      are needed, i.e. if the restart itself is enough for the
      upgraded applications to start running the new versions, a very
      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 etc. can be used without having to specify <c>.appup</c>
      files.</p>
  </section>
</chapter>