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# Releases

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It is recommended to read this section alongside `rel`, `systools`, and `script` in SASL.

## Release Concept

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 Creating and Upgrading a Target System in System Principles.

## Release Resource File

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:

    {release, {Name,Vsn}, {erts, EVsn},
     [{Application1, AppVsn1},
       ...
      {ApplicationN, AppVsnN}]}.

`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.

Here is an example showing the `.app` file for a release of `ch_app` from the Applications section:

    {application, ch_app,
     [{description, "Channel allocator"},
      {vsn, "1"},
      {modules, [ch_app, ch_sup, ch3]},
      {registered, [ch3]},
      {applications, [kernel, stdlib, sasl]},
      {mod, {ch_app,[]}}
     ]}.

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`:

    {release,
     {"ch_rel", "A"},
     {erts, "14.2.5"},
     [{kernel, "9.2.4"},
      {stdlib, "5.2.3"},
      {sasl, "4.2.1"},
      {ch_app, "1"}]
    }.

## Generating Boot Scripts

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

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

This call creates both the human-readable boot script, `ch_rel-1.script`, and the binary boot script, `ch_rel-1.boot`, used by the runtime system.

- `"ch_rel-1"` is the name of the `.rel` file, minus the extension.
- `local` is an option that means that the directories where the applications are found are used in the boot script, instead of `$ROOT/lib` (`$ROOT` is the root directory of the installed release).

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/OTP 26 [erts-14.2.5] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit]

    Eshell V14.2.5 (press Ctrl+G to abort, type help(). for help)
    1> application:which_applications().
    [{ch_app,"Channel allocator","1"},
     {sasl,"SASL  CXC 138 11","4.2.1"},
     {stdlib,"ERTS  CXC 138 10","5.2.3"},
     {kernel,"ERTS  CXC 138 10","9.2.4"}]

## Creating a Release Package

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:

- The `.app` files
- The `.rel` file
- The object code for all applications, structured according to the application directory structure
- The binary boot script renamed to `start.boot`

    % tar tf ch_rel-1.tar
    lib/kernel-9.2.4/ebin/kernel.app
    lib/kernel-9.2.4/ebin/application.beam
    ...
    lib/stdlib-5.2.3/ebin/stdlib.app
    lib/stdlib-5.2.3/ebin/argparse.beam
    ...
    lib/sasl-4.2.1/ebin/sasl.app
    lib/sasl-4.2.1/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/ch_rel-1.rel
    releases/A/ch_rel-1.rel
    releases/A/start.boot

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`, or a `sys.config.src`, 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.

## Directory Structure

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

    $ROOT/lib/App1-AVsn1/ebin
                        /priv
             /App2-AVsn2/ebin
                        /priv
             ...
             /AppN-AVsnN/ebin
                        /priv
         /erts-EVsn/bin
         /releases/Vsn
         /bin

- `lib` - Application directories
- `erts-EVsn/bin` - Erlang runtime system executables
- `releases/Vsn` - `.rel` file and boot script `start.boot`; if present in the release package, `relup` and/or `sys.config` or `sys.config.src`
- `bin` - Top-level Erlang runtime system executables

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.

### Disk-Less and/or Read-Only Clients

If a complete system consists of disk-less and/or read-only client nodes, a `clients` directory is to be added to the `$ROOT` directory. A read-only node is a node with a read-only file system.

The `clients` directory is to have one subdirectory per supported client node. The name of each client directory is to be the name of the corresponding client node. As a minimum, each client directory is to contain the `bin` and `releases` subdirectories. These directories are used to store information about installed releases and to appoint the current release to the client. The `$ROOT` directory thus contains the following:

    $ROOT/...
        /clients/ClientName1/bin
                            /releases/Vsn
                /ClientName2/bin
                            /releases/Vsn
                ...
                /ClientNameN/bin
                            /releases/Vsn

This structure is to be used if all clients are running the same type of Erlang machine. If there are clients running different types of Erlang machines, or on different operating systems, the `clients` directory can be divided into one subdirectory per type of Erlang machine. Alternatively, one `$ROOT` can be set up per type of machine. For each type, some of the directories specified for the `$ROOT` directory are to be included:

    $ROOT/...
        /clients/Type1/lib
                      /erts-EVsn
                      /bin
                      /ClientName1/bin
                                  /releases/Vsn
                      /ClientName2/bin
                                  /releases/Vsn
                      ...
                      /ClientNameN/bin
                                  /releases/Vsn
                ...
                /TypeN/lib
                      /erts-EVsn
                      /bin
                      ...

With this structure, the root directory for clients of `Type1` is `$ROOT/clients/Type1`.

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