release_handler(3)

NAME
release_handler - Unpacking and Installation of Release Packages

DESCRIPTION

The release handler process belongs to the SASL application, which is responsible for release handling, that is, unpacking, installation, and removal of release packages.

An introduction to release handling and an example is provided in OTP Design Principles in System Documentation.

A release package is a compressed tar file containing code for a certain version of a release, created by calling systools:make_tar/1,2. The release package is to be located in the $ROOT/releases directory of the previous version of the release, where $ROOT is the installation root directory, code:root_dir(). Another releases directory can be specified using the SASL configuration parameter releases_dir or the OS environment variable RELDIR. The release handler must have write access to this directory to install the new release. The persistent state of the release handler is stored there in a file called RELEASES.

A release package is always to contain:

    *   A release resource file, Name.rel

    *   A boot script, Name.boot

The .rel file contains information about the release: its name, version, and which ERTS and application versions it uses.

A release package can also contain:

    *   A release upgrade file, relup

    *   A system configuration file, sys.config

The relup file contains instructions for how to upgrade to, or downgrade from, this version of the release.

The release package can be unpacked, which extracts the files. An unpacked release can be installed. The currently used version of the release is then upgraded or downgraded to the specified version by evaluating the instructions in the relup file. An installed release can be made permanent. Only one permanent release can exist in the system, and this release is used if the system is restarted. An installed release, except the permanent one, can be removed. When a release is removed, all files belonging to that release only are deleted.

Each release version has a status, which can be unpacked, current, permanent, or old. There is always one latest release, which either has status permanent (normal case) or current (installed, but not yet made permanent). The meaning of the status values are illustrated in the following table:

        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                -

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 can be synchronized between nodes. From an operator view, it can be unsatisfactory to specify each node. The aim is to install one release package in the system, no matter how many nodes there are. It is recommended that software management functions are written that take care of this problem. Such a function can have knowledge of the system architecture, so it can contact each individual release handler to install the package.

For release handling to work properly, the runtime system must know which release it is running. It must also be able to change (in runtime) which boot script and system configuration file are to 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 Embedded System in System Documentation. In this case, the system configuration file sys.config is mandatory.

The installation of a new release can restart the system. Which program to use is specified by the SASL configuration parameter start_prg, which defaults to $ROOT/bin/start.

The emulator restart on Windows NT expects that the system is started using the erlsrv program (as a service). Furthermore, the release handler expects that the service is named NodeName_Release, where NodeName is the first part of the Erlang node name (up to, but not including the "@") and Release is the current release version. The release handler furthermore expects that a program like start_erl.exe is specified as "machine" to erlsrv. During upgrading with restart, a new service is registered and started. The new service is set to automatic and the old service is removed when the new release is made permanent.

The release handler at a node running on a diskless machine, or with a read-only file system, must be configured accordingly using the following SASL configuration parameters (for details, see sasl(6)):

    masters:
        This node uses some master nodes to store and fetch release information. All master nodes must be operational whenever release information is written by this node.

    client_directory:
        The client_directory in the directory structure of the master nodes must be specified.

    static_emulator:
        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, as the executable files are statically written into memory.

The release handler can also be used to unpack and install release packages when not running Erlang as an embedded system. However, in this case the user must somehow ensure that correct boot scripts and configuration files are used if the system must be restarted.

Functions are provided for using another file structure than the structure defined in OTP. These functions can be used to test a release upgrade locally.

EXPORTS

check_install_release(Vsn) -> {ok, OtherVsn, Descr} | {error, Reason}
check_install_release(Vsn,Opts) -> {ok, OtherVsn, Descr} | {error, Reason}

    Types:
        Vsn = OtherVsn = string()
        Opts = [Opt]
        Opt = purge
        Descr = term()
        Reason = term()

    Checks if the specified version Vsn of the release can be installed. The release must not have status current. Issues warnings if relup file or sys.config is not present. If relup file is present, its contents are checked and {error,Reason} is returned if an error is found. Also checks that all required applications are present and that all new code can be loaded; {error,Reason} is returned if an error is found.

    Evaluates all instructions that occur before the point_of_no_return instruction in the release upgrade script.

    Returns the same as install_release/1. Descr defaults to "" if no relup file is found.

    If option purge is specified, all old code that can be soft-purged is purged after all other checks are successfully completed. This can be useful to reduce the time needed by install_release/1.

create_RELEASES(Root, RelDir, RelFile, AppDirs) -> ok | {error, Reason}

    Types:
        Root = RelDir = RelFile = string()
        AppDirs = [{App, Vsn, Dir}]
         App = atom()
         Vsn = Dir = string()
        Reason = term()

    Creates an initial RELEASES file to be used by the release handler. This file must exist to install new releases.

    Root is the root of the installation ($ROOT) as described earlier. RelDir is the directory where the RELEASES file is to be created (normally $ROOT/releases). RelFile is the name of the .rel file that describes the initial release, including the extension .rel.

    AppDirs can be used to specify from where the modules for the specified applications are to be loaded. App is the name of an application, Vsn is the version, and Dir is the name of the directory where App-Vsn is located. The corresponding modules are to be located under Dir/App-Vsn/ebin. The directories for applications not specified in AppDirs are assumed to be located in $ROOT/lib.

install_file(Vsn, File) -> ok | {error, Reason}

    Types:
        Vsn = File = string()
        Reason = term()

    Installs a release-dependent file in the release structure. The release-dependent file must be in the release structure when a new release is installed: start.boot, relup, and sys.config.

    The function can be called, for example, when these files are generated at the target. The function is to be called after set_unpacked/2 has been called.

install_release(Vsn) -> {ok, OtherVsn, Descr} | {error, Reason}
install_release(Vsn, [Opt]) -> {ok, OtherVsn, Descr} | {continue_after_restart, OtherVsn, Descr} | {error, Reason}

    Types:
        Vsn = OtherVsn = string()
        Opt = {error_action, Action} | {code_change_timeout, Timeout}
         | {suspend_timeout, Timeout} | {update_paths, Bool}
         Action = restart | reboot
         Timeout = default | infinity | pos_integer()
         Bool = boolean()
        Descr = term()
        Reason = {illegal_option, Opt} | {already_installed, Vsn} | {change_appl_data, term()} | {missing_base_app, OtherVsn, App} | {could_not_create_hybrid_boot, term()} | term()
        App = atom()

    Installs the specified version Vsn of the release. Looks first for a relup file for Vsn and a script {UpFromVsn,Descr1,Instructions1} in this file for upgrading from the current version. If not found, the function looks for a relup file for the current version and a script {Vsn,Descr2,Instructions2} in this file for downgrading to Vsn.

    If a script is found, the first thing that happens is that the application specifications are updated according to the .app files and sys.config belonging to the release version Vsn.

    After the application specifications have been updated, the instructions in the script are evaluated and the function returns {ok,OtherVsn,Descr} if successful. OtherVsn and Descr are the version (UpFromVsn or Vsn) and description (Descr1 or Descr2) as specified in the script.

    If {continue_after_restart,OtherVsn,Descr} is returned, the emulator is restarted before the upgrade instructions are executed. This occurs if the emulator or any of the applications Kernel, STDLIB, or SASL are updated. The new emulator version and these core applications execute after the restart. For all other applications the old versions are started and the upgrade is performed as normal by executing the upgrade instructions.

    If a recoverable error occurs, the function returns {error,Reason} and the original application specifications are restored. If a non-recoverable error occurs, the system is restarted.

    Options:

        error_action:
            Defines if the node is to be restarted (init:restart()) or rebooted (init:reboot()) if there is an error during the installation. Default is restart.

        code_change_timeout:
            Defines the time-out for all calls to sys:change_code. If no value is specified or default is specified, the default value defined in sys is used.

        suspend_timeout:
            Defines the time-out for all calls to sys:suspend. If no value is specified, the values defined by the Timeout parameter of the upgrade or suspend instructions are used. If default is specified, the default value defined in sys is used.

        {update_paths,Bool}:
            Indicates if all application code paths are to be updated (Bool==true) or if only code paths for modified applications are to be updated (Bool==false, default). This option has only effect for other application directories than the default $ROOT/lib/App-Vsn, that is, application directories specified in argument AppDirs in a call to create_RELEASES/4 or set_unpacked/2.

            Example:

            In the current version CurVsn of a release, the application directory of myapp is $ROOT/lib/myapp-1.0. A new version NewVsn is unpacked outside the release handler and the release handler is informed about this with a call as follows:

        release_handler:set_unpacked(RelFile, [{myapp,"1.0","/home/user"},...]).
        => {ok,NewVsn}

            If NewVsn is installed with option {update_paths,true}, then code:lib_dir(myapp) returns /home/user/myapp-1.0.

        Note:
    Installing a new release can be 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 can lead to garbage collections and copying of data.

    To speed up the execution of install_release, first call check_install_release, using option purge. This does the same check for old code. Then purges all modules that can be soft-purged. The purged modules do then no longer have any old code, and install_release does not need to do the checks.

    This does not reduce the overall time for the upgrade, but it allows checks and purge to be executed in the background before the real upgrade is started.

        Note:
    When upgrading the emulator from a version older than OTP R15, an attempt is made to load new application beam code into the old emulator. Sometimes the new beam format cannot be read by the old emulator, so the code loading fails and the complete upgrade is terminated. To overcome this problem, the new application code is to be compiled with the old emulator. For more information about emulator upgrade from pre OTP R15 versions, see Design Principles in System Documentation.

make_permanent(Vsn) -> ok | {error, Reason}

    Types:
        Vsn = string()
        Reason = {bad_status, Status} | term()

    Makes the specified release version Vsn permanent.

remove_release(Vsn) -> ok | {error, Reason}

    Types:
        Vsn = string()
        Reason = {permanent, Vsn} | client_node | term()

    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.

reboot_old_release(Vsn) -> ok | {error, Reason}

    Types:
        Vsn = string()
        Reason = {bad_status, Status} | term()

    Reboots the system by making the old release permanent, and calls init:reboot() directly. The release must have status old.

set_removed(Vsn) -> ok | {error, Reason}

    Types:
        Vsn = string()
        Reason = {permanent, Vsn} | term()

    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.

set_unpacked(RelFile, AppDirs) -> {ok, Vsn} | {error, Reason}

    Types:
        RelFile = string()
        AppDirs = [{App, Vsn, Dir}]
         App = atom()
         Vsn = Dir = string()
        Reason = term()

    Makes it possible to handle unpacking of releases outside the release handler. Tells the release handler that the release is unpacked. Vsn is extracted from the release resource file RelFile.

    AppDirs can be used to specify from where the modules for the specified applications are to be loaded. App is the name of an application, Vsn is the version, and Dir is the name of the directory where App-Vsn is located. The corresponding modules are to be located under Dir/App-Vsn/ebin. The directories for applications not specified in AppDirs are assumed to be located in $ROOT/lib.

unpack_release(Name) -> {ok, Vsn} | {error, Reason}

    Types:
        Name = Vsn = string()
        Reason = client_node | term()

    Unpacks a release package Name.tar.gz located in the releases directory.

    Performs some checks on the package, for example, checks that all mandatory files are present, and extracts its contents.

which_releases() -> [{Name, Vsn, Apps, Status}]

    Types:
        Name = Vsn = string()
        Apps = ["App-Vsn"]
        Status = unpacked | current | permanent | old

    Returns all releases known to the release handler.

which_releases(Status) -> [{Name, Vsn, Apps, Status}]

    Types:
        Name = Vsn = string()
        Apps = ["App-Vsn"]
        Status = unpacked | current | permanent | old

    Returns all releases, known to the release handler, of a specific status.

APPLICATION UPGRADE/DOWNGRADE

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 the instructions in the relup file is created on-the-fly, based on the .appup file for the application, and evaluated exactly in the same way as release_handler does.

    Warning:
These functions are primarily intended for simplified testing of .appup files. They are not run within the context of the release_handler process. They must therefore not be used together with calls to install_release/1,2, as this causes the release_handler to end up in an inconsistent state.

No persistent information is updated, so these functions can be used on any Erlang node, embedded or not. Also, using these functions does not affect which code is loaded if there is a reboot.

If the upgrade or downgrade fails, the application can end up in an inconsistent state.

EXPORTS

upgrade_app(App, Dir) -> {ok, Unpurged} | restart_emulator | {error, Reason}

    Types:
        App = atom()
        Dir = string()
        Unpurged = [Module]
         Module = atom()
        Reason = term()

    Upgrades an application App from the current version to a new version located in Dir according to the .appup file.

    App is the name of the application, which must be started. Dir is the new library directory of App. The corresponding modules as well as the .app and .appup files are to be located under Dir/ebin.

    The function looks in the .appup file and tries to find an upgrade script from the current version of the application using upgrade_script/2. This script is evaluated using eval_appup_script/4, exactly in the same way as install_release/1,2 does.

    Returns one of the following:

        *   {ok, Unpurged} if evaluating the script is successful, where Unpurged is a list of unpurged modules

        *   restart_emulator if this instruction is encountered in the script

        *   {error, Reason} if an error occurred when finding or evaluating the script

    If the restart_new_emulator instruction is found in the script, upgrade_app/2 returns {error,restart_new_emulator}. This because restart_new_emulator requires a new version of the emulator to be started before the rest of the upgrade instructions can be executed, and this can only be done by install_release/1,2.

downgrade_app(App, Dir) ->
downgrade_app(App, OldVsn, Dir) -> {ok, Unpurged} | restart_emulator | {error, Reason}

    Types:
        App = atom()
        Dir = OldVsn = string()
        Unpurged = [Module]
         Module = atom()
        Reason = term()

    Downgrades an application App from the current version to a previous version OldVsn located in Dir according to the .appup file.

    App is the name of the application, which must be started. OldVsn is the previous application version and can be omitted if Dir is of the format "App-OldVsn". Dir is the library directory of the previous version of App. The corresponding modules and the old .app file are to be located under Dir/ebin. The .appup file is to be located in the ebin directory of the current library directory of the application (code:lib_dir(App)).

    The function looks in the .appup file and tries to find a downgrade script to the previous version of the application using downgrade_script/3. This script is evaluated using eval_appup_script/4, exactly in the same way as install_release/1,2 does.

    Returns one of the following:

        *   {ok, Unpurged} if evaluating the script is successful, where Unpurged is a list of unpurged modules

        *   restart_emulator if this instruction is encountered in the script

        *   {error, Reason} if an error occurred when finding or evaluating the script

upgrade_script(App, Dir) -> {ok, NewVsn, Script}

    Types:
        App = atom()
        Dir = string()
        NewVsn = string()
        Script = Instructions

    Tries to find an application upgrade script for App from the current version to a new version located in Dir.

    The upgrade script can then be evaluated using eval_appup_script/4. It is recommended to use upgrade_app/2 instead, but this function (upgrade_script) is useful to inspect the contents of the script.

    App is the name of the application, which must be started. Dir is the new library directory of App. The corresponding modules as well as the .app and .appup files are to be located under Dir/ebin.

    The function looks in the .appup file and tries to find an upgrade script from the current application version. High-level instructions are translated to low-level instructions. The instructions are sorted in the same manner as when generating a relup file.

    Returns {ok, NewVsn, Script} if successful, where NewVsn is the new application version. For details about Script, see appup(4).

    Failure: If a script cannot be found, the function fails with an appropriate error reason.

downgrade_script(App, OldVsn, Dir) -> {ok, Script}

    Types:
        App = atom()
        OldVsn = Dir = string()
        Script = Instructions

    Tries to find an application downgrade script for App from the current version to a previous version OldVsn located in Dir.

    The downgrade script can then be evaluated using eval_appup_script/4. It is recommended to use downgrade_app/2,3 instead, but this function (downgrade_script) is useful to inspect the contents of the script.

    App is the name of the application, which must be started. Dir is the previous library directory of App. The corresponding modules and the old .app file are to be located under Dir/ebin. The .appup file is to be located in the ebin directory of the current library directory of the application (code:lib_dir(App)).

    The function looks in the .appup file and tries to find a downgrade script from the current application version. High-level instructions are translated to low-level instructions. The instructions are sorted in the same manner as when generating a relup file.

    Returns {ok, Script} if successful. For details about Script, see appup(4).

    Failure: If a script cannot be found, the function fails with an appropriate error reason.

eval_appup_script(App, ToVsn, ToDir, Script) -> {ok, Unpurged} | restart_emulator | {error, Reason}

    Types:
        App = atom()
        ToVsn = ToDir = string()
        Script
            See upgrade_script/2, downgrade_script/3
        Unpurged = [Module]
         Module = atom()
        Reason = term()

    Evaluates an application upgrade or downgrade script Script, the result from calling upgrade_script/2 or downgrade_script/3, exactly in the same way as install_release/1,2 does.

    App is the name of the application, which must be started. ToVsn is the version to be upgraded/downgraded to, and ToDir is the library directory of this version. The corresponding modules as well as the .app and .appup files are to be located under Dir/ebin.

    Returns one of the following:

        *   {ok, Unpurged} if evaluating the script is successful, where Unpurged is a list of unpurged modules

        *   restart_emulator if this instruction is encountered in the script

        *   {error, Reason} if an error occurred when finding or evaluating the script

    If the restart_new_emulator instruction is found in the script, eval_appup_script/4 returns {error,restart_new_emulator}. This because restart_new_emulator requires a new version of the emulator to be started before the rest of the upgrade instructions can be executed, and this can only be done by install_release/1,2.

TYPICAL ERROR REASONS

    {bad_masters, Masters}:
        The master nodes Masters are not alive.

    {bad_rel_file, File}:
        Specified .rel file File cannot be read or does not contain a single term.

    {bad_rel_data, Data}:
        Specified .rel file does not contain a recognized release specification, but another term Data.

    {bad_relup_file, File}:
        Specified relup file Relup contains bad data.

    {cannot_extract_file, Name, Reason}:
        Problems when extracting from a tar file, erl_tar:extract/2 returned {error, {Name, Reason}}.

    {existing_release, Vsn}:
        Specified release version Vsn is already in use.

    {Master, Reason, When}:
        Some operation, indicated by the term When, failed on the master node Master with the specified error reason Reason.

    {no_matching_relup, Vsn, CurrentVsn}:
        Cannot find a script for upgrading/downgrading between CurrentVsn and Vsn.

    {no_such_directory, Path}:
        The directory Pathdoes not exist.

    {no_such_file, Path}:
        The path Path (file or directory) does not exist.

    {no_such_file, {Master, Path}}:
        The path Path (file or directory) does not exist at the master node Master.

    {no_such_release, Vsn}:
        The specified release version Vsn does not exist.

    {not_a_directory, Path}:
        Path exists but is not a directory.

    {Posix, File}:
        Some file operation failed for File. Posix is an atom named from the Posix error codes, such as enoent, eacces, or eisdir. See file(3) in Kernel.

    Posix:
        Some file operation failed, as for the previous item in the list.

SEE ALSO

OTP Design Principles, config(4), rel(4), relup(4), script(4), sys(3), systools(3)