release_handler

The release handler is a process belonging 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 a usage example can be found in Design Principles.

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 should be placed 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 in order to install the new release. The persistent state of the release handler is stored there in a file called RELEASES.

A release package should always contain the release resource file Name.rel and a boot script Name.boot. It may contain a release upgrade file relup and a system configuration file sys.config. The .rel file contains information about the release: its name, version, and which ERTS and application versions it uses. The relup file contains scripts 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 relup. 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 if the system is restarted. 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.

Each version of the 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                  -
    

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.

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 Embedded System. In this case, the system configuration file sys.config is mandatory.

A new release may 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 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 (see sasl(6) for details):

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.

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.

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.

Types:

Checks if the specified version Vsn of the release can be installed. The release must not have status current. Issues warnings if relup or sys.config are not present. If relup 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, or {error,Reason} is returned.

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. Descr defaults to "" if no relup file is found.

Types:

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

Root is the root of the installation ($ROOT) as described above. RelDir is the the directory where the RELEASES file should 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 should 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 should be located under Dir/App-Vsn/ebin. The directories for applications not specified in AppDirs are assumed to be located in $ROOT/lib.

Types:

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: start.boot, relup and sys.config.

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

Types:

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 applications 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 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.

The option error_action defines if the node should be restarted (init:restart()) or rebooted (init:reboot()) in case of an error during the installation. Default is restart.

The option code_change_timeout defines the timeout 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 timeout 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.

The option {update_paths,Bool} indicates if all application code paths should be updated (Bool==true), or if only code paths for modified applications should be updated (Bool==false, default). This option only has effect for other application directories than the default $ROOT/lib/App-Vsn, that is, application directories provided in the AppDirs argument 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 to:

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

If NewVsn is installed with the option {update_paths,true}, afterwards code:lib_dir(myapp) will return /home/user/myapp-1.0.

Types:

Makes the specified version Vsn of the release permanent.

Types:

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.

Types:

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

Types:

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.

Types:

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 should 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 should be located under Dir/App-Vsn/ebin. The directories for applications not specified in AppDirs are assumed to be located in $ROOT/lib.

Types:

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.

Types:

Returns all releases known to the release handler.

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

Types:

Upgrades an application App from the current version to a new version located in Dir according to the .appup 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 should 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 {ok, Unpurged} if evaluating the script is successful, where Unpurged is a list of unpurged modules, or restart_new_emulator if this instruction is encountered in the script, or {error, Reason} if an error occurred when finding or evaluating the script.

Types:

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

App is the name of the application, which must be started. OldVsn is the previous version of the application and can be omitted if Dir is of the format "App-OldVsn". Dir is the library directory of this previous version of App, the corresponding modules as well as the old .app file should be located under Dir/ebin. The .appup file should 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 an 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 {ok, Unpurged} if evaluating the script is successful, where Unpurged is a list of unpurged modules, or restart_new_emulator if this instruction is encountered in the script, or {error, Reason} if an error occurred when finding or evaluating the script.

Types:

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 is useful in order 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 should 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. High-level instructions are translated to low-level instructions and the instructions are sorted in the same manner as when generating a relup script.

Returns {ok, NewVsn, Script} if successful, where NewVsn is the new application version.

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

Types:

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 is useful in order 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 as well as the old .app file should be located under Dir/ebin. The .appup file should 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 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 relup script.

Returns {ok, Script} if successful.

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

Types:

Evaluates an application upgrade or downgrade script Script, the result from calling upgrade_app/2 or downgrade_app/2,3, 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 should be located under Dir/ebin.

Returns {ok, Unpurged} if evaluating the script is successful, where Unpurged is a list of unpurged modules, or restart_new_emulator if this instruction is encountered in the script, or {error, Reason} if an error occurred when evaluating the script.

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

• {bad_rel_file, File} - Specified .rel file File can not 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 up/downgrading between CurrentVsn and Vsn.

• {no_such_directory, Path} - The directory Path does 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 version Vsn of the release 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).

• Posix - Some file operation failed, as above.

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