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authorErlang/OTP <[email protected]>2009-11-20 14:54:40 +0000
committerErlang/OTP <[email protected]>2009-11-20 14:54:40 +0000
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+<?xml version="1.0" encoding="latin1" ?>
+<!DOCTYPE erlref SYSTEM "erlref.dtd">
+
+<erlref>
+ <header>
+ <copyright>
+ <year>1996</year><year>2009</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_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>
+ <fsummary>Check installation of a release in the system.</fsummary>
+ <type>
+ <v>Vsn = OtherVsn = string()</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>
+ </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>&nbsp;App = atom()</v>
+ <v>&nbsp;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>&nbsp;&nbsp;&nbsp;| {suspend_timeout, Timeout} | {update_paths, Bool}</v>
+ <v>&nbsp;Action = restart | reboot</v>
+ <v>&nbsp;Timeout = default | infinity | int()>0</v>
+ <v>&nbsp;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>
+ </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>&nbsp;App = atom()</v>
+ <v>&nbsp;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>&nbsp;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>&nbsp;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>&nbsp;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>
+