release_handler(3)

MODULE
release_handler - A process to Unpack and Install Releases

DESCRIPTION

The release handler process is a SASL process that handles unpacking, installation, and removal of release packages. As an example, a release package could contain applications, a new emulator, and new configuration parameters. In this text, the directory ROOT refers to the installation root directory (code:root_dir()). A release package is a compressed tar file that is written to the releases directory, for example via ftp. The location of this directory is specified with the configuration parameter releases_dir, or the OS environment variable RELDIR. Default is ROOT/releases. The release handler must have write access to this directory in order to install new releases. The persistent state of the release handler, for example information about installed releases, is stored in a file called RELEASES in the releases directory.

The package can be unpacked, which extracts the files from the package. When the release is unpacked, it can be installed. This operation evaluates the release upgrade script. An installed release can be made permanent. There can only be one permanent release in the system, and this is the release that is used when the system is started. An installed release, except the permanent one, can be removed. When a release is removed, all files that belong to that release only are deleted. The system can be rebooted from an old release.

Each release has a status. The status 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 following table illustrates the meaning of the status values.
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                  -

A release package always contains two special files, the ReleaseName.rel file and the relup file. The ReleaseName.rel file contains information about the release, such as its name, version, and which system and library versions it uses. The relup file contains release upgrade scripts. There is one release upgrade script for each old version that can be updated to the new version.

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.

A new release may restart the system, using start_prg. This is a configuration parameter to the application sasl. The default is 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 nodename (up to, but not including the "@") and Release is the current release of the application. 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 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.

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 sasl configuration parameters:

    masters:
        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.

    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 because the executable files are statically written into memory.

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.

EXPORTS

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

    Types
        Vsn = FromVsn = string()
        Descr = term()

    The release must not have status current. Checks that there is a relup release upgrade script from the FromVsn (current version) to Vsn. Checks that all required libs (or applications) are present and that all new code can be loaded. Checks that there is a start.boot file and a sys.config for the new release.

    This function evaluates all instructions that occur before the point_of_no_return instruction in the release upgrade script.

    Returns the same as install_release/1.

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

    Types
        Root = RelDir = RelFile = string()
        LibDirs = [{LibName, LibVsn, Dir}]
        LibName = atom()
        LibVsn = Dir = string()

    This function can be called to create an initial RELEASES file to be used by the release_handler. This file must exist in order to install new releases. When the system is installed, a default RELEASES file is created in the default releases directory ROOT/releases.

    Root is the root of the installation as described above. RelDir is the the releases directory where the RELASES file should be created. RelFile is the name of the .rel file that describes the initial release.

    LibDirs can be used to specify from where the modules for an application should be loaded. LibName is the name of the lib (or application), LibVsn is the version, and Dir is the name of the directory where the lib directory LibName-LibVsn is located. The corresponding modules should be located under Dir/LibName-LibVsn/ebin.

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

    Types
        FileName = string()
        Vsn = string()

    Installs a release dependent file in the release structure. A release dependent file is a file that must be in the release structure when the release is installed. Currently there are three such mandatory files, start.boot, sys.config and relup.

    This function should be called to install release dependent files, for example when these files are generated at the target. It should be called when set_unpacked/2 has been called.

install_release(Vsn) -> {ok, FromVsn, Descr} | {error, Reason}
install_release(Vsn, Opt) -> {ok, FromVsn, Descr} | {error, Reason}

    Types
        Vsn = FromVsn = string()
        Opt = [{error_action, Error_action} | {code_change_timeout, Timeout} | {suspend_timeout, Timeout}]
        Error_action = restart | reboot
        Descr = term()
        Timeout = default | infinity | int() > 0

    The release must not have status current. Installs the delivered release in the system by evaluating the release upgrade script found in the relup file. This function returns {ok, FromVsn, Descr} if successful, or {error, Reason} if a recoverable error occurs. Descr is a user defined parameter, found in the relup file, used to describe the release. The system is restarted if a non-recoverable error occurs. There can be many installed releases at the same time in the system.

    It is possible to define if the node should be restarted or rebooted in case of an error during the installation. Default is restart.

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

    The option 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.

    Note that if an old or the permanent release is installed, a downgrade will occur. There must a corresponding downgrade script in the relup file.

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

    Types
        Vsn = string()

    Makes the current release permanent. This causes the specified release to be used at system start-up.

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

    Types
        Vsn = string()

    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 = {no_such_release, Vsn}

    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 = {no_such_release, Vsn} | {permanent, Vsn}

    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, LibDirs) -> {ok, Vsn} | {error, Reason}

    Types
        RelFile = string()
        LibDirs = [{LibName, LibVsn, Dir}]
        LibName = atom()
        LibVsn = Dir = string()
        Vsn = string()

    Makes it possible to handle the unpacking of releases outside the release_handler. Makes the release_handler aware that the release is unpacked. Vsn is extracted from the release file RelFile and is used as parameter to the other functions.

    LibDirs can be used to specify from where the modules for an application should be loaded. LibName is the name of the lib (or application), LibVsn is the version, and Dir is the name of the directory where the lib directory LibName-LibVsn is located. The corresponding modules should be located under Dir/LibName-LibVsn/ebin.

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

    Types
        ReleaseName = string()
        Vsn = string()

    The ReleaseName is the name of the release package. This is the name of the package file, without .tar.gz. ReleaseName may or may not be the same as the release version. Vsn is extracted from the release package and is used as parameter to the other functions.

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

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

    Types
        Name = string()
        Vsn = string()
        Lib = string()
        Status = unpacked | current | permanent | old

    Returns all releases known to the release handler. Name is the name of the system. Lib is the name of a library. This name may be the application name followed by its version, for example "kernel-1.0".

See Also

systools(3)

AUTHOR
Martin Björklund - support@erlang.ericsson.se