<?xml version="1.0" encoding="utf-8" ?>
<!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>
