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<!DOCTYPE erlref SYSTEM "erlref.dtd">
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<header>
<copyright>
<year>1996</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
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Software distributed under the License is distributed on an "AS IS"
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<title>release_handler</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
</header>
<module>release_handler</module>
<modulesummary>Unpacking and Installation of Release Packages</modulesummary>
<description>
<p>The <em>release handler</em> is a process belonging to the SASL
application which is responsible for <em>release handling</em>,
that is, unpacking, installation, and removal of release packages.</p>
<p>An introduction to release handling and a usage example can be
found in
<seealso marker="doc/design_principles:release_handling">Design Principles</seealso>.
</p>
<p>A <em>release package</em> is a compressed tar file containing
code for a certain version of a release, created by calling
<seealso marker="systools#make_tar/1">systools:make_tar/1,2</seealso>.
The release package should be placed in the <c>$ROOT/releases</c>
directory of the previous version of the release where
<c>$ROOT</c> is the installation root directory,
<c>code:root_dir()</c>.
Another <c>releases</c> directory can be specified using the SASL
configuration parameter <c>releases_dir</c>, or the OS environment
variable <c>RELDIR</c>. The release handler must have write access
to this directory in order to install the new release.
The persistent state of the release handler is stored there in a
file called <c>RELEASES</c>.</p>
<p>A release package should always contain the release resource file
<c>Name.rel</c> and a boot script <c>Name.boot</c>. It may contain
a release upgrade file <c>relup</c> and a system configuration
file <c>sys.config</c>. The <c>.rel</c> file contains information
about the release: its name, version, and which ERTS and
application versions it uses. The <c>relup</c> file contains
scripts for how to upgrade to, or downgrade from, this version of
the release.</p>
<p>The release package can be <em>unpacked</em>, which extracts
the files. An unpacked release can be <em>installed</em>.
The currently used version of the release is then upgraded or
downgraded to the specified version by evaluating the instructions
in <c>relup</c>. An installed release can be made
<em>permanent</em>. There can only be one permanent release in
the system, and this is the release that is used if the system is
restarted. An installed release, except the permanent one, can be
<em>removed</em>. When a release is removed, all files that
belong to that release only are deleted.</p>
<p>Each version of the release has a status. The status can be
<c>unpacked</c>, <c>current</c>, <c>permanent</c>, or <c>old</c>.
There is always one latest release which either has status
<c>permanent</c> (normal case), or <c>current</c> (installed, but
not yet made permanent). The following table illustrates
the meaning of the status values:</p>
<pre>
Status Action NextStatus
-------------------------------------------
- unpack unpacked
unpacked install current
remove -
current make_permanent permanent
install other old
remove -
permanent make other permanent old
install permanent
old reboot_old permanent
install current
remove -
</pre>
<p>The release handler process is a locally registered process on
each node. When a release is installed in a distributed system,
the release handler on each node must be called. The release
installation may be synchronized between nodes. From an operator
view, it may be unsatisfactory to specify each node. The aim is
to install one release package in the system, no matter how many
nodes there are. If this is the case, it is recommended that
software management functions are written which take care of
this problem. Such a function may have knowledge of the system
architecture, so it can contact each individual release handler
to install the package.</p>
<p>For release handling to work properly, the runtime system needs
to have knowledge about which release it is currently running. It
must also be able to change (in run-time) which boot script and
system configuration file should be used if the system is
restarted. This is taken care of automatically if Erlang is
started as an embedded system. Read about this in <em>Embedded System</em>. In this case, the system configuration file
<c>sys.config</c> is mandatory.</p>
<p>A new release may restart the system. Which program to use is
specified by the SASL configuration parameter <c>start_prg</c>
which defaults to <c>$ROOT/bin/start</c>.</p>
<p>The emulator restart on Windows NT expects that the system is
started using the <c>erlsrv</c> program (as a service).
Furthermore the release handler expects that the service is named
<em>NodeName</em>_<em>Release</em>, where <em>NodeName</em> is
the first part of the Erlang nodename (up to, but not including
the "@") and <em>Release</em> is the current release of
the application. The release handler furthermore expects that a
program like <c>start_erl.exe</c> is specified as "machine" to
<c>erlsrv</c>. During upgrading with restart, a new service will
be registered and started. The new service will be set to
automatic and the old service removed as soon as the new release
is made permanent.</p>
<p>The release handler at a node which runs on a diskless machine,
or with a read-only file system, must be configured accordingly
using the following <c>sasl</c> configuration parameters (see
<seealso marker="sasl_app">sasl(6)</seealso> for details):</p>
<taglist>
<tag><c>masters</c></tag>
<item>
<p>This node uses a number of master nodes in order to store
and fetch release information. All master nodes must be up
and running whenever release information is written by this
node.</p>
</item>
<tag><c>client_directory</c></tag>
<item>
<p>The <c>client_directory</c> in the directory structure of
the master nodes must be specified.</p>
</item>
<tag><c>static_emulator</c></tag>
<item>
<p>This parameter specifies if the Erlang emulator is
statically installed at the client node. A node with a static
emulator cannot dynamically switch to a new emulator because
the executable files are statically written into memory.</p>
</item>
</taglist>
<p>It is also possible to use the release handler to unpack and
install release packages when not running Erlang as an embedded
system, but in this case the user must somehow make sure that
correct boot scripts and configuration files are used if
the system needs to be restarted.</p>
<p>There are additional functions for using another file structure
than the structure defined in OTP. These functions can be used
to test a release upgrade locally.</p>
</description>
<funcs>
<func>
<name>check_install_release(Vsn) -> {ok, OtherVsn, Descr} | {error, Reason}</name>
<name>check_install_release(Vsn,Opts) -> {ok, OtherVsn, Descr} | {error, Reason}</name>
<fsummary>Check installation of a release in the system.</fsummary>
<type>
<v>Vsn = OtherVsn = string()</v>
<v>Opts = [Opt]</v>
<v>Opt = purge</v>
<v>Descr = term()</v>
<v>Reason = term()</v>
</type>
<desc>
<p>Checks if the specified version <c>Vsn</c> of the release
can be installed. The release must not have status
<c>current</c>. Issues warnings if <c>relup</c> or
<c>sys.config</c> are not present. If <c>relup</c> is present,
its contents are checked and <c>{error,Reason}</c> is
returned if an error is found. Also checks that all required
applications are present and that all new code can be loaded,
or <c>{error,Reason}</c> is returned.</p>
<p>This function evaluates all instructions that occur before
the <c>point_of_no_return</c> instruction in the release
upgrade script.</p>
<p>Returns the same as <c>install_release/1</c>. <c>Descr</c>
defaults to "" if no <c>relup</c> file is found.</p>
<p>If the option <c>purge</c> is given, all old code that can
be soft purged will be purged after all other checks are
successfully completed. This can be useful in order to
reduce the time needed by <seealso
marker="#install_release/1">install_release</seealso>.</p>
</desc>
</func>
<func>
<name>create_RELEASES(Root, RelDir, RelFile, AppDirs) -> ok | {error, Reason}</name>
<fsummary>Create an initial RELEASES file.</fsummary>
<type>
<v>Root = RelDir = RelFile = string()</v>
<v>AppDirs = [{App, Vsn, Dir}]</v>
<v> App = atom()</v>
<v> Vsn = Dir = string()</v>
<v>Reason = term()</v>
</type>
<desc>
<p>Creates an initial RELEASES file to be used by the release
handler. This file must exist in order to install new
releases.</p>
<p><c>Root</c> is the root of the installation (<c>$ROOT</c>) as
described above. <c>RelDir</c> is the the directory where
the <c>RELEASES</c> file should be created (normally
<c>$ROOT/releases</c>). <c>RelFile</c> is the name
of the <c>.rel</c> file that describes the initial release,
including the extension <c>.rel</c>.</p>
<p><c>AppDirs</c> can be used to specify from where the modules
for the specified applications should be loaded. <c>App</c> is
the name of an application, <c>Vsn</c> is the version, and
<c>Dir</c> is the name of the directory where <c>App-Vsn</c>
is located. The corresponding modules should be located under
<c>Dir/App-Vsn/ebin</c>. The directories for applications not
specified in <c>AppDirs</c> are assumed to be located in
<c>$ROOT/lib</c>.</p>
</desc>
</func>
<func>
<name>install_file(Vsn, File) -> ok | {error, Reason}</name>
<fsummary>Install a release file in the release structure.</fsummary>
<type>
<v>Vsn = File = string()</v>
<v>Reason = term()</v>
</type>
<desc>
<p>Installs a release dependent file in the release structure.
A release dependent file is a file that must be in
the release structure when a new release is installed:
<c>start.boot</c>, <c>relup</c> and <c>sys.config</c>.</p>
<p>The function can be called, for example, when these files
are generated at the target. It should be called after
<c>set_unpacked/2</c> has been called.</p>
</desc>
</func>
<func>
<name>install_release(Vsn) -> {ok, OtherVsn, Descr} | {error, Reason}</name>
<name>install_release(Vsn, [Opt]) -> {ok, OtherVsn, Descr} | {error, Reason}</name>
<fsummary>Install a release in the system.</fsummary>
<type>
<v>Vsn = OtherVsn = string()</v>
<v>Opt = {error_action, Action} | {code_change_timeout, Timeout}</v>
<v> | {suspend_timeout, Timeout} | {update_paths, Bool}</v>
<v> Action = restart | reboot</v>
<v> Timeout = default | infinity | int()>0</v>
<v> Bool = boolean()</v>
<v>Descr = term()</v>
<v>Reason = {illegal_option, Opt} | {already_installed, Vsn} | {change_appl_data, term()} | term()</v>
</type>
<desc>
<p>Installs the specified version <c>Vsn</c> of the release.
Looks first for a <c>relup</c> file for <c>Vsn</c> and a
script <c>{UpFromVsn,Descr1,Instructions1}</c> in this file
for upgrading from the current version. If not found,
the function looks for a <c>relup</c> file for the current
version and a script <c>{Vsn,Descr2,Instructions2}</c> in this
file for downgrading to <c>Vsn</c>.</p>
<p>If a script is found, the first thing that happens is that
the applications specifications are updated according to
the <c>.app</c> files and <c>sys.config</c> belonging to
the release version <c>Vsn</c>.</p>
<p>After the application specifications have been updated,
the instructions in the script are evaluated and the function
returns <c>{ok,OtherVsn,Descr}</c> if successful.
<c>OtherVsn</c> and <c>Descr</c> are the version
(<c>UpFromVsn</c> or <c>Vsn</c>) and description
(<c>Descr1</c> or <c>Descr2</c>) as specified in the script.</p>
<p>If a recoverable error occurs, the function returns
<c>{error,Reason}</c> and the original application
specifications are restored. If a non-recoverable error
occurs, the system is restarted.</p>
<p>The option <c>error_action</c> defines if the node should be
restarted (<c>init:restart()</c>) or rebooted
(<c>init:reboot()</c>) in case of an error during
the installation. Default is <c>restart</c>.</p>
<p>The option <c>code_change_timeout</c> defines the timeout
for all calls to <c>sys:change_code</c>. If no value is
specified or <c>default</c> is given, the default value
defined in <c>sys</c> is used.</p>
<p>The option <c>suspend_timeout</c> defines the timeout for
all calls to <c>sys:suspend</c>. If no value is specified,
the values defined by the <c>Timeout</c> parameter of
the <c>upgrade</c> or <c>suspend</c> instructions are used.
If <c>default</c> is specified, the default value defined in
<c>sys</c> is used.</p>
<p>The option <c>{update_paths,Bool}</c> indicates if all
application code paths should be updated (<c>Bool==true</c>),
or if only code paths for modified applications should be
updated (<c>Bool==false</c>, default). This option only has
effect for other application directories than the default
<c>$ROOT/lib/App-Vsn</c>, that is, application directories
provided in the <c>AppDirs</c> argument in a call to
<c>create_RELEASES/4</c> or <c>set_unpacked/2</c>.</p>
<p>Example: In the current version <c>CurVsn</c> of a release,
the application directory of <c>myapp</c> is
<c>$ROOT/lib/myapp-1.0</c>. A new version <c>NewVsn</c> is
unpacked outside the release handler, and the release handler
is informed about this with a call to:</p>
<code type="none">
release_handler:set_unpacked(RelFile, [{myapp,"1.0","/home/user"},...]).
=> {ok,NewVsn}
</code>
<p>If <c>NewVsn</c> is installed with the option
<c>{update_paths,true}</c>, afterwards
<c>code:lib_dir(myapp)</c> will return
<c>/home/user/myapp-1.0</c>.</p>
<note>
<p>Installing a new release might be quite time consuming if
there are many processes in the system. The reason is that
each process must be checked for references to old code
before a module can be purged. This check might lead to
garbage collections and copying of data.</p>
<p>If you wish to speed up the execution of
<c>install_release</c>, then you may call <seealso
marker="#check_install_release/1">check_install_release</seealso>
first, using the option <c>purge</c>. This will do the same
check for old code, and then purge all modules that can be
soft purged. The purged modules will then no longer have any
old code, and <c>install_release</c> will not need to do the
checks.</p>
<p>Obviously, this will not reduce the overall time for the
upgrade, but it will allow checks and purge to be executed
in the background before the real upgrade is started.</p>
</note>
</desc>
</func>
<func>
<name>make_permanent(Vsn) -> ok | {error, Reason}</name>
<fsummary>Make the specified release version permanent.</fsummary>
<type>
<v>Vsn = string()</v>
<v>Reason = {bad_status, Status} | term()</v>
</type>
<desc>
<p>Makes the specified version <c>Vsn</c> of the release
permanent.</p>
</desc>
</func>
<func>
<name>remove_release(Vsn) -> ok | {error, Reason}</name>
<fsummary>Remove a release from the system.</fsummary>
<type>
<v>Vsn = string()</v>
<v>Reason = {permanent, Vsn} | client_node | term()</v>
</type>
<desc>
<p>Removes a release and its files from the system.
The release must not be the permanent release. Removes only
the files and directories not in use by another release.</p>
</desc>
</func>
<func>
<name>reboot_old_release(Vsn) -> ok | {error, Reason}</name>
<fsummary>Reboot the system from an old release.</fsummary>
<type>
<v>Vsn = string()</v>
<v>Reason = {bad_status, Status} | term()</v>
</type>
<desc>
<p>Reboots the system by making the old release permanent, and
calls <c>init:reboot()</c> directly. The release must have
status <c>old</c>.</p>
</desc>
</func>
<func>
<name>set_removed(Vsn) -> ok | {error, Reason}</name>
<fsummary>Mark a release as removed.</fsummary>
<type>
<v>Vsn = string()</v>
<v>Reason = {permanent, Vsn} | term()</v>
</type>
<desc>
<p>Makes it possible to handle removal of releases outside
the release handler. Tells the release handler that
the release is removed from the system. This function does
not delete any files.</p>
</desc>
</func>
<func>
<name>set_unpacked(RelFile, AppDirs) -> {ok, Vsn} | {error, Reason}</name>
<fsummary>Mark a release as unpacked.</fsummary>
<type>
<v>RelFile = string()</v>
<v>AppDirs = [{App, Vsn, Dir}]</v>
<v> App = atom()</v>
<v> Vsn = Dir = string()</v>
<v>Reason = term()</v>
</type>
<desc>
<p>Makes it possible to handle unpacking of releases outside
the release handler. Tells the release handler that
the release is unpacked. <c>Vsn</c> is extracted from
the release resource file <c>RelFile</c>.</p>
<p><c>AppDirs</c> can be used to specify from where the modules
for the specified applications should be loaded. <c>App</c> is
the name of an application, <c>Vsn</c> is the version, and
<c>Dir</c> is the name of the directory where <c>App-Vsn</c>
is located. The corresponding modules should be located under
<c>Dir/App-Vsn/ebin</c>. The directories for applications not
specified in <c>AppDirs</c> are assumed to be located in
<c>$ROOT/lib</c>.</p>
</desc>
</func>
<func>
<name>unpack_release(Name) -> {ok, Vsn} | {error, Reason}</name>
<fsummary>Unpack a release package.</fsummary>
<type>
<v>Name = Vsn = string()</v>
<v>Reason = client_node | term()</v>
</type>
<desc>
<p>Unpacks a release package <c>Name.tar.gz</c> located in
the <c>releases</c> directory.</p>
<p>Performs some checks on the package - for example checks
that all mandatory files are present - and extracts its
contents.</p>
</desc>
</func>
<func>
<name>which_releases() -> [{Name, Vsn, Apps, Status}]</name>
<fsummary>Return all known releases</fsummary>
<type>
<v>Name = Vsn = string()</v>
<v>Apps = ["App-Vsn"]</v>
<v>Status = unpacked | current | permanent | old</v>
</type>
<desc>
<p>Returns all releases known to the release handler.</p>
</desc>
</func>
</funcs>
<section>
<title>Application Upgrade/Downgrade</title>
<p>The following functions can be used to test upgrade and downgrade
of single applications (instead of upgrading/downgrading an entire
release). A script corresponding to <c>relup</c> is created
on-the-fly, based on the <c>.appup</c> file for the application,
and evaluated exactly in the same way as <c>release_handler</c>
does.</p>
<warning>
<p>These function is primarily intended for simplified testing of
of <c>.appup</c> files. They are not run within the context of
the <c>release_handler</c> process. They must therefore
<em>not</em> be used together with calls to
<c>install_release/1,2</c>, as this will cause
<c>release_handler</c> to end up in an inconsistent state.</p>
<p>No persistent information is updated, why these functions can
be used on any Erlang node, embedded or not. Also, using these
functions does not effect which code will be loaded in case of
a reboot.</p>
<p>If the upgrade or downgrade fails, the application may end up
in an inconsistent state.</p>
</warning>
</section>
<funcs>
<func>
<name>upgrade_app(App, Dir) -> {ok, Unpurged} | restart_new_emulator | {error, Reason}</name>
<fsummary>Upgrade to a new application version</fsummary>
<type>
<v>App = atom()</v>
<v>Dir = string()</v>
<v>Unpurged = [Module]</v>
<v> Module = atom()</v>
<v>Reason = term()</v>
</type>
<desc>
<p>Upgrades an application <c>App</c> from the current
version to a new version located in <c>Dir</c> according to
the <c>.appup</c> script.</p>
<p><c>App</c> is the name of the application, which must be
started. <c>Dir</c> is the new library directory of
<c>App</c>, the corresponding modules as well as
the <c>.app</c> and <c>.appup</c> files should be located
under <c>Dir/ebin</c>.</p>
<p>The function looks in the <c>.appup</c> file and tries to
find an upgrade script from the current version of
the application using
<seealso marker="#upgrade_script/2">upgrade_script/2</seealso>.
This script is evaluated using
<seealso marker="#eval_appup_script/4">eval_appup_script/4</seealso>,
exactly in the same way as
<seealso marker="#install_release/1">install_release/1,2</seealso>
does.</p>
<p>Returns <c>{ok, Unpurged}</c> if evaluating the script is
successful, where <c>Unpurged</c> is a list of unpurged
modules, or <c>restart_new_emulator</c> if this instruction is
encountered in the script, or <c>{error, Reason}</c> if
an error occurred when finding or evaluating the script.</p>
</desc>
</func>
<func>
<name>downgrade_app(App, Dir) -></name>
<name>downgrade_app(App, OldVsn, Dir) -> {ok, Unpurged} | restart_new_emulator | {error, Reason}</name>
<fsummary>Downgrade to a previous application version</fsummary>
<type>
<v>App = atom()</v>
<v>Dir = OldVsn = string()</v>
<v>Unpurged = [Module]</v>
<v> Module = atom()</v>
<v>Reason = term()</v>
</type>
<desc>
<p>Downgrades an application <c>App</c> from the current
version to a previous version <c>OldVsn</c> located in
<c>Dir</c> according to the <c>.appup</c> script.</p>
<p><c>App</c> is the name of the application, which must be
started. <c>OldVsn</c> is the previous version of
the application and can be omitted if <c>Dir</c> is of
the format <c>"App-OldVsn"</c>. <c>Dir</c> is the library
directory of this previous version of <c>App</c>,
the corresponding modules as well as the old <c>.app</c> file
should be located under <c>Dir/ebin</c>. The <c>.appup</c>
file should be located in the <c>ebin</c> directory of
the <em>current</em> library directory of the application
(<c>code:lib_dir(App)</c>).</p>
<p>The function looks in the <c>.appup</c> file and tries to
find an downgrade script to the previous version of
the application using
<seealso marker="#downgrade_script/3">downgrade_script/3</seealso>.
This script is evaluated using
<seealso marker="#eval_appup_script/4">eval_appup_script/4</seealso>,
exactly in the same way as
<seealso marker="#install_release/1">install_release/1,2</seealso>
does.</p>
<p>Returns <c>{ok, Unpurged}</c> if evaluating the script is
successful, where <c>Unpurged</c> is a list of unpurged
modules, or <c>restart_new_emulator</c> if this instruction is
encountered in the script, or <c>{error, Reason}</c> if
an error occurred when finding or evaluating the script.</p>
</desc>
</func>
<func>
<name>upgrade_script(App, Dir) -> {ok, NewVsn, Script}</name>
<fsummary>Find an application upgrade script</fsummary>
<type>
<v>App = atom()</v>
<v>Dir = string()</v>
<v>NewVsn = string()</v>
<v>Script = Instructions -- see appup(4)</v>
</type>
<desc>
<p>Tries to find an application upgrade script for <c>App</c>
from the current version to a new version located in
<c>Dir</c>.</p>
<p>The upgrade script can then be evaluated using
<seealso marker="#eval_appup_script/4">eval_appup_script/4</seealso>.
It is recommended to use
<seealso marker="#upgrade_app/2">upgrade_app/2</seealso>
instead, but this function is useful in order to inspect
the contents of the script.</p>
<p><c>App</c> is the name of the application, which must be
started. <c>Dir</c> is the new library directory of
<c>App</c>, the corresponding modules as well as
the <c>.app</c> and <c>.appup</c> files should be located
under <c>Dir/ebin</c>.</p>
<p>The function looks in the <c>.appup</c> file and tries to
find an upgrade script from the current version of
the application. High-level instructions are translated to
low-level instructions and the instructions are sorted in
the same manner as when generating a <c>relup</c> script.</p>
<p>Returns <c>{ok, NewVsn, Script}</c> if successful, where
<c>NewVsn</c> is the new application version.</p>
<p>Failure: If a script cannot be found, the function fails
with an appropriate error reason.</p>
</desc>
</func>
<func>
<name>downgrade_script(App, OldVsn, Dir) -> {ok, Script}</name>
<fsummary>Find an application downgrade script</fsummary>
<type>
<v>App = atom()</v>
<v>OldVsn = Dir = string()</v>
<v>Script = Instructions -- see appup(4)</v>
</type>
<desc>
<p>Tries to find an application downgrade script for <c>App</c>
from the current version to a previous version <c>OldVsn</c>
located in <c>Dir</c>.</p>
<p>The downgrade script can then be evaluated using
<seealso marker="#eval_appup_script/4">eval_appup_script/4</seealso>.
It is recommended to use
<seealso marker="#downgrade_app/2">downgrade_app/2,3</seealso>
instead, but this function is useful in order to inspect
the contents of the script.</p>
<p><c>App</c> is the name of the application, which must be
started. <c>Dir</c> is the previous library directory of
<c>App</c>, the corresponding modules as well as
the old <c>.app</c> file should be located under
<c>Dir/ebin</c>. The <c>.appup</c> file should be located in
the <c>ebin</c> directory of the <em>current</em> library
directory of the application (<c>code:lib_dir(App)</c>).</p>
<p>The function looks in the <c>.appup</c> file and tries to
find an downgrade script from the current version of
the application. High-level instructions are translated to
low-level instructions and the instructions are sorted in
the same manner as when generating a <c>relup</c> script.</p>
<p>Returns <c>{ok, Script}</c> if successful.</p>
<p>Failure: If a script cannot be found, the function fails
with an appropriate error reason.</p>
</desc>
</func>
<func>
<name>eval_appup_script(App, ToVsn, ToDir, Script) -> {ok, Unpurged} | restart_new_emulator | {error, Reason}</name>
<fsummary>Evaluate an application upgrade or downgrade script</fsummary>
<type>
<v>App = atom()</v>
<v>ToVsn = ToDir = string()</v>
<v>Script -- see upgrade_script/2, downgrade_script/3</v>
<v>Unpurged = [Module]</v>
<v> Module = atom()</v>
<v>Reason = term()</v>
</type>
<desc>
<p>Evaluates an application upgrade or downgrade script
<c>Script</c>, the result from calling
<seealso marker="#upgrade_app/2">upgrade_app/2</seealso> or
<seealso marker="#downgrade_app/3">downgrade_app/2,3</seealso>,
exactly in the same way as
<seealso marker="#install_release/1">install_release/1,2</seealso>
does.</p>
<p><c>App</c> is the name of the application, which must be
started. <c>ToVsn</c> is the version to be upgraded/downgraded
to, and <c>ToDir</c> is the library directory of this version.
The corresponding modules as well as the <c>.app</c> and
<c>.appup</c> files should be located under <c>Dir/ebin</c>.</p>
<p>Returns <c>{ok, Unpurged}</c> if evaluating the script is
successful, where <c>Unpurged</c> is a list of unpurged
modules, or <c>restart_new_emulator</c> if this instruction is
encountered in the script, or <c>{error, Reason}</c> if
an error occurred when evaluating the script.</p>
</desc>
</func>
</funcs>
<section>
<title>Typical Error Reasons</title>
<list type="bulleted">
<item>
<p><c>{bad_masters, Masters}</c> - The master nodes
<c>Masters</c> are not alive.</p>
</item>
<item>
<p><c>{bad_rel_file, File}</c> - Specified <c>.rel</c> file
<c>File</c> can not be read, or does not contain a single
term.</p>
</item>
<item>
<p><c>{bad_rel_data, Data}</c> - Specified <c>.rel</c> file
does not contain a recognized release specification, but
another term <c>Data</c>.</p>
</item>
<item>
<p><c>{bad_relup_file, File}</c> - Specified <c>relup</c> file
<c>Relup</c> contains bad data.</p>
</item>
<item>
<p><c>{cannot_extract_file, Name, Reason}</c> - Problems when
extracting from a tar file, <c>erl_tar:extract/2</c> returned
<c>{error, {Name, Reason}}</c>.</p>
</item>
<item>
<p><c>{existing_release, Vsn}</c> - Specified release version
<c>Vsn</c> is already in use.</p>
</item>
<item>
<p><c>{Master, Reason, When}</c> - Some operation, indicated by
the term <c>When</c>, failed on the master node <c>Master</c>
with the specified error reason <c>Reason</c>.</p>
</item>
<item>
<p><c>{no_matching_relup, Vsn, CurrentVsn}</c> - Cannot find a
script for up/downgrading between <c>CurrentVsn</c> and
<c>Vsn</c>.</p>
</item>
<item>
<p><c>{no_such_directory, Path}</c> - The directory <c>Path</c>
does not exist.</p>
</item>
<item>
<p><c>{no_such_file, Path}</c> - The path <c>Path</c> (file or
directory) does not exist.</p>
</item>
<item>
<p><c>{no_such_file, {Master, Path}}</c> - The path <c>Path</c>
(file or directory) does not exist at the master node
<c>Master</c>.</p>
</item>
<item>
<p><c>{no_such_release, Vsn}</c> - The specified version
<c>Vsn</c> of the release does not exist.</p>
</item>
<item>
<p><c>{not_a_directory, Path}</c> - <c>Path</c> exists, but is
not a directory.</p>
</item>
<item>
<p><c>{Posix, File}</c> - Some file operation failed for
<c>File</c>. <c>Posix</c> is an atom named from the Posix
error codes, such as <c>enoent</c>, <c>eacces</c> or
<c>eisdir</c>. See <c>file(3)</c>.</p>
</item>
<item>
<p><c>Posix</c> - Some file operation failed, as above.</p>
</item>
</list>
</section>
<section>
<title>SEE ALSO</title>
<p><seealso marker="doc/design_principles:release_handling">OTP Design Principles</seealso>,
<seealso marker="kernel:config">config(4)</seealso>,
<seealso marker="relup">relup(4)</seealso>,
<seealso marker="rel">rel(4)</seealso>,
<seealso marker="script">script(4)</seealso>,
<seealso marker="stdlib:sys">sys(3)</seealso>,
<seealso marker="systools">systools(3)</seealso></p>
</section>
</erlref>
