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OTP Design Principles
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11 Release Handling

11.1  Release Handling Principles

An important feature of the Erlang programming language is the ability to change module code in runtime, code replacement, as described in the Erlang Reference Manual.

Based on this feature, the OTP application SASL provides a framework for upgrading and downgrading between different versions of an entire release in runtime. This is called release handling.

The framework consists of:

  • Offline support - systools for generating scripts and building release packages
  • Online support - release_handler for unpacking and installing release packages

The minimal system based on Erlang/OTP, enabling release handling, thus consists of the Kernel, STDLIB, and SASL applications.

Release Handling Workflow

Step 1) A release is created as described in Releases.

Step 2) The release is transferred to and installed at target environment. For information of how to install the first target system, see System Principles.

Step 3) Modifications, for example, error corrections, are made to the code in the development environment.

Step 4) At some point, it is time to make a new version of release. The relevant .app files are updated and a new .rel file is written.

Step 5) For each modified application, an application upgrade file, .appup, is created. In this file, it is described how to upgrade and/or downgrade between the old and new version of the application.

Step 6) Based on the .appup files, a release upgrade file called relup, is created. This file describes how to upgrade and/or downgrade between the old and new version of the entire release.

Step 7) A new release package is made and transferred to the target system.

Step 8) The new release package is unpacked using the release handler.

Step 9) The new version of the release is installed, also using the release handler. This is done by evaluating the instructions in relup. Modules can be added, deleted, or reloaded, applications can be started, stopped, or restarted, and so on. In some cases, it is even necessary to restart the entire emulator.

  • If the installation fails, the system can be rebooted. The old release version is then automatically used.
  • If the installation succeeds, the new version is made the default version, which is to now be used if there is a system reboot.

Release Handling Aspects

Appup Cookbook, contains examples of .appup files for typical cases of upgrades/downgrades that are normally easy to handle in runtime. However, many aspects can make release handling complicated, for example:

  • Complicated or circular dependencies can make it difficult or even impossible to decide in which order things must be done without risking runtime errors during an upgrade or downgrade. Dependencies can be:

    • Between nodes
    • Between processes
    • Between modules
  • During release handling, non-affected processes continue normal execution. This can lead to time-outs or other problems. For example, new processes created in the time window between suspending processes using a certain module, and loading a new version of this module, can execute old code.

It is thus recommended that code is changed in as small steps as possible, and always kept backwards compatible.

11.2  Requirements

For release handling to work properly, the runtime system must have knowledge about which release it is running. It must also be able to change (in runtime) which boot script and system configuration file to use if the system is rebooted, for example, by heart after a failure. Thus, Erlang must be started as an embedded system; for information on how to do this, see Embedded System.

For system reboots to work properly, it is also required that the system is started with heartbeat monitoring, see the erl(1) manual page in ERTS and the heart(3) manual page in Kernel

Other requirements:

  • The boot script included in a release package must be generated from the same .rel file as the release package itself.

    Information about applications is fetched from the script when an upgrade or downgrade is performed.

  • The system must be configured using only one system configuration file, called sys.config.

    If found, this file is automatically included when a release package is created.

  • All versions of a release, except the first one, must contain a relup file.

    If found, this file is automatically included when a release package is created.

11.3  Distributed Systems

If the system consists of several Erlang nodes, each node can use its own version of the release. The release handler is a locally registered process and must be called at each node where an upgrade or downgrade is required. A release handling instruction, sync_nodes, can be used to synchronize the release handler processes at a number of nodes, see the appup(4) manual page in SASL.

11.4  Release Handling Instructions

OTP supports a set of release handling instructions that are used when creating .appup files. The release handler understands a subset of these, the low-level instructions. To make it easier for the user, there are also a number of high-level instructions, which are translated to low-level instructions by systools:make_relup.

Some of the most frequently used instructions are described in this section. The complete list of instructions is included in the appup(4) manual page in SASL.

First, some definitions:

  • Residence module - The module where a process has its tail-recursive loop function(s). If these functions are implemented in several modules, all those modules are residence modules for the process.
  • Functional module - A module that is not a residence module for any process.

For a process implemented using an OTP behaviour, the behaviour module is the residence module for that process. The callback module is a functional module.


If a simple extension has been made to a functional module, it is sufficient to load the new version of the module into the system, and remove the old version. This is called simple code replacement and for this the following instruction is used:

{load_module, Module}


If a more complex change has been made, for example, a change to the format of the internal state of a gen_server, simple code replacement is not sufficient. Instead, it is necessary to:

  • Suspend the processes using the module (to avoid that they try to handle any requests before the code replacement is completed).
  • Ask them to transform the internal state format and switch to the new version of the module.
  • Remove the old version.
  • Resume the processes.

This is called synchronized code replacement and for this the following instructions are used:

{update, Module, {advanced, Extra}}
{update, Module, supervisor}

update with argument {advanced,Extra} is used when changing the internal state of a behaviour as described above. It causes behaviour processes to call the callback function code_change, passing the term Extra and some other information as arguments. See the manual pages for the respective behaviours and Appup Cookbook.

update with argument supervisor is used when changing the start specification of a supervisor. See Appup Cookbook.

When a module is to be updated, the release handler finds which processes that are using the module by traversing the supervision tree of each running application and checking all the child specifications:

{Id, StartFunc, Restart, Shutdown, Type, Modules}

A process uses a module if the name is listed in Modules in the child specification for the process.

If Modules=dynamic, which is the case for event managers, the event manager process informs the release handler about the list of currently installed event handlers (gen_fsm), and it is checked if the module name is in this list instead.

The release handler suspends, asks for code change, and resumes processes by calling the functions sys:suspend/1,2, sys:change_code/4,5, and sys:resume/1,2, respectively.

add_module and delete_module

If a new module is introduced, the following instruction is used:

{add_module, Module}

The instruction loads the module and is necessary when running Erlang in embedded mode. It is not strictly required when running Erlang in interactive (default) mode, since the code server then automatically searches for and loads unloaded modules.

The opposite of add_module is delete_module, which unloads a module:

{delete_module, Module}

Any process, in any application, with Module as residence module, is killed when the instruction is evaluated. The user must therefore ensure that all such processes are terminated before deleting the module, to avoid a situation with failing supervisor restarts.

Application Instructions

The following is the instruction for adding an application:

{add_application, Application}

Adding an application means that the modules defined by the modules key in the .app file are loaded using a number of add_module instructions, and then the application is started.

The following is the instruction for removing an application:

{remove_application, Application}

Removing an application means that the application is stopped, the modules are unloaded using a number of delete_module instructions, and then the application specification is unloaded from the application controller.

The following is the instruction for restarting an application:

{restart_application, Application}

Restarting an application means that the application is stopped and then started again similar to using the instructions remove_application and add_application in sequence.

apply (Low-Level)

To call an arbitrary function from the release handler, the following instruction is used:

{apply, {M, F, A}}

The release handler evalutes apply(M, F, A).

restart_new_emulator (Low-Level)

This instruction is used when changing to a new emulator version, or when any of the core applications Kernel, STDLIB, or SASL is upgraded. If a system reboot is needed for another reason, the restart_emulator instruction is to be used instead.

This instruction requires that the system is started with heartbeat monitoring, see the erl(1) manual page in ERTS and the heart(3) manual page in Kernel.

The restart_new_emulator instruction must always be the first instruction in a relup. If the relup is generated by systools:make_relup/3,4, this is automatically ensured.

When the release handler encounters the instruction, it first generates a temporary boot file, which starts the new versions of the emulator and the core applications, and the old version of all other applications. Then it shuts down the current emulator by calling init:reboot(), see the init(3) manual page in Kernel. All processes are terminated gracefully and the system is rebooted by the heart program, using the temporary boot file. After the reboot, the rest of the relup instructions are executed. This is done as a part of the temporary boot script.


This mechanism causes the new versions of the emulator and core applications to run with the old version of other applications during startup. Thus, take extra care to avoid incompatibility. Incompatible changes in the core applications can in some situations be necessary. If possible, such changes are preceded by deprecation over two major releases before the actual change. To ensure the application is not crashed by an incompatible change, always remove any call to deprecated functions as soon as possible.

An info report is written when the upgrade is completed. To programmatically find out if the upgrade is complete, call release_handler:which_releases(current) and check if it returns the expected (that is, the new) release.

The new release version must be made permanent when the new emulator is operational. Otherwise, the old version will be used if there is a new system reboot.

On UNIX, the release handler tells the heart program which command to use to reboot the system. The environment variable HEART_COMMAND, normally used by the heart program, is ignored in this case. The command instead defaults to $ROOT/bin/start. Another command can be set by using the SASL configuration parameter start_prg, see the sasl(6) manual page.

restart_emulator (Low-Level)

This instruction is not related to upgrades of ERTS or any of the core applications. It can be used by any application to force a restart of the emulator after all upgrade instructions are executed.

A relup script can only have one restart_emulator instruction and it must always be placed at the end. If the relup is generated by systools:make_relup/3,4, this is automatically ensured.

When the release handler encounters the instruction, it shuts down the emulator by calling init:reboot(), see the init(3) manual page in Kernel. All processes are terminated gracefully and the system can then be rebooted by the heart program using the new release version. No more upgrade instruction is executed after the restart.

11.5  Application Upgrade File

To define how to upgrade/downgrade between the current version and previous versions of an application, an application upgrade file, or in short an .appup file is created. The file is to be called Application.appup, where Application is the application name:

 [{UpFromVsn1, InstructionsU1},
  {UpFromVsnK, InstructionsUK}],
 [{DownToVsn1, InstructionsD1},
  {DownToVsnK, InstructionsDK}]}.
  • Vsn, a string, is the current version of the application, as defined in the .app file.
  • Each UpFromVsn is a previous version of the application to upgrade from.
  • Each DownToVsn is a previous version of the application to downgrade to.
  • Each Instructions is a list of release handling instructions.

For information about the syntax and contents of the .appup file, see the appup(4) manual page in SASL.

Appup Cookbook includes examples of .appup files for typical upgrade/downgrade cases.

Example: Consider the release ch_rel-1 from Releases. Assume you want to add a function available/0 to server ch3, which returns the number of available channels (when trying out the example, change in a copy of the original directory, so that the first versions are still available):


-export([alloc/0, free/1]).
-export([init/1, handle_call/3, handle_cast/2]).

start_link() ->
    gen_server:start_link({local, ch3}, ch3, [], []).

alloc() ->
    gen_server:call(ch3, alloc).

free(Ch) ->
    gen_server:cast(ch3, {free, Ch}).

available() ->
    gen_server:call(ch3, available).

init(_Args) ->
    {ok, channels()}.

handle_call(alloc, _From, Chs) ->
    {Ch, Chs2} = alloc(Chs),
    {reply, Ch, Chs2};
handle_call(available, _From, Chs) ->
    N = available(Chs),
    {reply, N, Chs}.

handle_cast({free, Ch}, Chs) ->
    Chs2 = free(Ch, Chs),
    {noreply, Chs2}.

A new version of the file must now be created, where the version is updated:

{application, ch_app,
 [{description, "Channel allocator"},
  {vsn, "2"},
  {modules, [ch_app, ch_sup, ch3]},
  {registered, [ch3]},
  {applications, [kernel, stdlib, sasl]},
  {mod, {ch_app,[]}}

To upgrade ch_app from "1" to "2" (and to downgrade from "2" to "1"), you only need to load the new (old) version of the ch3 callback module. Create the application upgrade file ch_app.appup in the ebin directory:

 [{"1", [{load_module, ch3}]}],
 [{"1", [{load_module, ch3}]}]

11.6  Release Upgrade File

To define how to upgrade/downgrade between the new version and previous versions of a release, a release upgrade file, or in short relup file, is to be created.

This file does not need to be created manually, it can be generated by systools:make_relup/3,4. The relevant versions of the .rel file, .app files, and .appup files are used as input. It is deducted which applications are to be added and deleted, and which applications that must be upgraded and/or downgraded. The instructions for this are fetched from the .appup files and transformed into a single list of low-level instructions in the right order.

If the relup file is relatively simple, it can be created manually. It it only to contain low-level instructions.

For details about the syntax and contents of the release upgrade file, see the relup(4) manual page in SASL.

Example, continued from the previous section: You have a new version "2" of ch_app and an .appup file. A new version of the .rel file is also needed. This time the file is called ch_rel-2.rel and the release version string is changed from "A" to "B":

 {"ch_rel", "B"},
 {erts, "5.3"},
 [{kernel, "2.9"},
  {stdlib, "1.12"},
  {sasl, "1.10"},
  {ch_app, "2"}]

Now the relup file can be generated:

1> systools:make_relup("ch_rel-2", ["ch_rel-1"], ["ch_rel-1"]).

This generates a relup file with instructions for how to upgrade from version "A" ("ch_rel-1") to version "B" ("ch_rel-2") and how to downgrade from version "B" to version "A".

Both the old and new versions of the .app and .rel files must be in the code path, as well as the .appup and (new) .beam files. The code path can be extended by using the option path:

1> systools:make_relup("ch_rel-2", ["ch_rel-1"], ["ch_rel-1"],

11.7  Installing a Release

When you have made a new version of a release, a release package can be created with this new version and transferred to the target environment.

To install the new version of the release in runtime, the release handler is used. This is a process belonging to the SASL application, which handles unpacking, installation, and removal of release packages. It is communicated through the release_handler module. For details, see the release_handler(3) manual page in SASL.

Assuming there is an operational target system with installation root directory $ROOT, the release package with the new version of the release is to be copied to $ROOT/releases.

First, unpack the release package. The files are then extracted from the package:

release_handler:unpack_release(ReleaseName) => {ok, Vsn}
  • ReleaseName is the name of the release package except the .tar.gz extension.
  • Vsn is the version of the unpacked release, as defined in its .rel file.

A directory $ROOT/lib/releases/Vsn is created, where the .rel file, the boot script start.boot, the system configuration file sys.config, and relup are placed. For applications with new version numbers, the application directories are placed under $ROOT/lib. Unchanged applications are not affected.

An unpacked release can be installed. The release handler then evaluates the instructions in relup, step by step:

release_handler:install_release(Vsn) => {ok, FromVsn, []}

If an error occurs during the installation, the system is rebooted using the old version of the release. If installation succeeds, the system is afterwards using the new version of the release, but if anything happens and the system is rebooted, it starts using the previous version again.

To be made the default version, the newly installed release must be made permanent, which means the previous version becomes old:

release_handler:make_permanent(Vsn) => ok

The system keeps information about which versions are old and permanent in the files $ROOT/releases/RELEASES and $ROOT/releases/

To downgrade from Vsn to FromVsn, install_release must be called again:

release_handler:install_release(FromVsn) => {ok, Vsn, []}

An installed, but not permanent, release can be removed. Information about the release is then deleted from $ROOT/releases/RELEASES and the release-specific code, that is, the new application directories and the $ROOT/releases/Vsn directory, are removed.

release_handler:remove_release(Vsn) => ok

Example (continued from the previous sections)

Step 1) Create a target system as described in System Principles of the first version "A" of ch_rel from Releases. This time sys.config must be included in the release package. If no configuration is needed, the file is to contain the empty list:


Step 2) Start the system as a simple target system. In reality, it is to be started as an embedded system. However, using erl with the correct boot script and config file is enough for illustration purposes:

% cd $ROOT
% bin/erl -boot $ROOT/releases/A/start -config $ROOT/releases/A/sys

$ROOT is the installation directory of the target system.

Step 3) In another Erlang shell, generate start scripts and create a release package for the new version "B". Remember to include (a possible updated) sys.config and the relup file, see Release Upgrade File.

1> systools:make_script("ch_rel-2").
2> systools:make_tar("ch_rel-2").

The new release package now also contains version "2" of ch_app and the relup file:

% tar tf ch_rel-2.tar 

Step 4) Copy the release package ch_rel-2.tar.gz to the $ROOT/releases directory.

Step 5) In the running target system, unpack the release package:

1> release_handler:unpack_release("ch_rel-2").

The new application version ch_app-2 is installed under $ROOT/lib next to ch_app-1. The kernel, stdlib, and sasl directories are not affected, as they have not changed.

Under $ROOT/releases, a new directory B is created, containing ch_rel-2.rel, start.boot, sys.config, and relup.

Step 6) Check if the function ch3:available/0 is available:

2> ch3:available().
** exception error: undefined function ch3:available/0

Step 7) Install the new release. The instructions in $ROOT/releases/B/relup are executed one by one, resulting in the new version of ch3 being loaded. The function ch3:available/0 is now available:

3> release_handler:install_release("B").
4> ch3:available().
5> code:which(ch3).
6> code:which(ch_sup).

Processes in ch_app for which code have not been updated, for example, the supervisor, are still evaluating code from ch_app-1.

Step 8) If the target system is now rebooted, it uses version "A" again. The "B" version must be made permanent, to be used when the system is rebooted.

7> release_handler:make_permanent("B").

11.8  Updating Application Specifications

When a new version of a release is installed, the application specifications are automatically updated for all loaded applications.


The information about the new application specifications is fetched from the boot script included in the release package. Thus, it is important that the boot script is generated from the same .rel file as is used to build the release package itself.

Specifically, the application configuration parameters are automatically updated according to (in increasing priority order):

  • The data in the boot script, fetched from the new application resource file
  • The new sys.config
  • Command-line arguments -App Par Val

This means that parameter values set in the other system configuration files and values set using application:set_env/3 are disregarded.

When an installed release is made permanent, the system process init is set to point out the new sys.config.

After the installation, the application controller compares the old and new configuration parameters for all running applications and call the callback function:

Module:config_change(Changed, New, Removed)
  • Module is the application callback module as defined by the mod key in the .app file.
  • Changed and New are lists of {Par,Val} for all changed and added configuration parameters, respectively.
  • Removed is a list of all parameters Par that have been removed.

The function is optional and can be omitted when implementing an application callback module.