When you have written one or more applications, you might want to create a complete system with these applications and a subset of the Erlang/OTP applications. This is called a release.

To do this, create a release resource file that defines which applications are included in the release.

The release resource file is used to generate boot scripts and release packages. A system that is transferred to and installed at another site is called a target system. How to use a release package to create a target system is described in System Principles.

To define a release, create a release resource file, or in short a .rel file. In the file, specify the name and version of the release, which ERTS version it is based on, and which applications it consists of:

Name, Vsn, EVsn, and AppVsn are strings.

The file must be named Rel.rel, where Rel is a unique name.

Each Application (atom) and AppVsn is the name and version of an application included in the release. The minimal release based on Erlang/OTP consists of the Kernel and STDLIB applications, so these applications must be included in the list.

If the release is to be upgraded, it must also include the SASL application.

Example: A release of ch_app from Applications has the following .app file:

The .rel file must also contain kernel, stdlib, and sasl, as these applications are required by ch_app. The file is called ch_rel-1.rel:

systools in the SASL application includes tools to build and check releases. The functions read the rel and .app files and performs syntax and dependency checks. The systools:make_script/1,2 function is used to generate a boot script (see System Principles):

1> systools:make_script("ch_rel-1", [local]).
ok

This creates a boot script, both the readable version, ch_rel-1.script, and the binary version, ch_rel-1.boot, used by the runtime system.

This is a useful way to test a generated boot script locally.

When starting Erlang/OTP using the boot script, all applications from the .rel file are automatically loaded and started:

% erl -boot ch_rel-1
Erlang (BEAM) emulator version 5.3

Eshell V5.3  (abort with ^G)
1> 
=PROGRESS REPORT==== 13-Jun-2003::12:01:15 ===
          supervisor: {local,sasl_safe_sup}
             started: [{pid,<0.33.0>},
                       {name,alarm_handler},
                       {mfa,{alarm_handler,start_link,[]}},
                       {restart_type,permanent},
                       {shutdown,2000},
                       {child_type,worker}]

...

=PROGRESS REPORT==== 13-Jun-2003::12:01:15 ===
         application: sasl
          started_at: nonode@nohost

...
=PROGRESS REPORT==== 13-Jun-2003::12:01:15 ===
         application: ch_app
          started_at: nonode@nohost

The systools:make_tar/1,2 function takes a .rel file as input and creates a zipped tar file with the code for the specified applications, a release package:

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

The release package by default contains:

% tar tf ch_rel-1.tar
lib/kernel-2.9/ebin/kernel.app
lib/kernel-2.9/ebin/application.beam
...
lib/stdlib-1.12/ebin/stdlib.app
lib/stdlib-1.12/ebin/beam_lib.beam
...
lib/sasl-1.10/ebin/sasl.app
lib/sasl-1.10/ebin/sasl.beam
...
lib/ch_app-1/ebin/ch_app.app
lib/ch_app-1/ebin/ch_app.beam
lib/ch_app-1/ebin/ch_sup.beam
lib/ch_app-1/ebin/ch3.beam
releases/A/start.boot
releases/A/ch_rel-1.rel
releases/ch_rel-1.rel

A new boot script was generated, without the local option set, before the release package was made. In the release package, all application directories are placed under lib. You do not know where the release package will be installed, so no hard-coded absolute paths are allowed.

The release resource file mysystem.rel is duplicated in the tar file. Originally, this file was only stored in the releases directory to make it possible for the release_handler to extract this file separately. After unpacking the tar file, release_handler would automatically copy the file to releases/FIRST. However, sometimes the tar file is unpacked without involving the release_handler (for example, when unpacking the first target system) and the file is therefore now instead duplicated in the tar file so no manual copying is necessary.

If a relup file and/or a system configuration file called sys.config is found, these files are also included in the release package. See Release Handling.

Options can be set to make the release package include source code and the ERTS binary as well.

For information on how to install the first target system, using a release package, see System Principles. For information on how to install a new release package in an existing system, see Release Handling.

The directory structure for the code installed by the release handler from a release package is as follows:

Applications are not required to be located under directory $ROOT/lib. Several installation directories, which contain different parts of a system, can thus exist. For example, the previous example can be extended as follows:

$SECOND_ROOT/.../SApp1-SAVsn1/ebin
                             /priv
                /SApp2-SAVsn2/ebin
                             /priv
                ...
                /SAppN-SAVsnN/ebin
                             /priv

$THIRD_ROOT/TApp1-TAVsn1/ebin
                        /priv
           /TApp2-TAVsn2/ebin
                        /priv
           ...
           /TAppN-TAVsnN/ebin
                        /priv

$SECOND_ROOT and $THIRD_ROOT are introduced as variables in the call to the systools:make_script/2 function.