<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">
<chapter>
<header>
<copyright>
<year>2003</year><year>2014</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>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
<seealso marker="release_structure">Releases</seealso>.</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 System Principles.</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
<seealso marker="#appup">application upgrade file</seealso>,
<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
<seealso marker="#relup">release upgrade file</seealso> 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><seealso marker="appup_cookbook">Appup Cookbook</seealso>,
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
<seealso marker="appup_cookbook#int_state">Appup Cookbook</seealso>.</p>
<p><c>update</c> with argument <c>supervisor</c> is used when
changing the start specification of a supervisor. See
<seealso marker="appup_cookbook#sup">Appup Cookbook</seealso>.</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_fsm</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><seealso marker="appup_cookbook">Appup Cookbook</seealso>
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
<seealso marker="release_structure#ch_rel">Releases</seealso>.
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 deducted 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 it 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
<seealso marker="release_structure#ch_rel">Releases</seealso>.
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 <seealso marker="#relup">Release Upgrade File</seealso>.</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>
