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# Creating and Upgrading a Target System

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When creating a system using Erlang/OTP, the simplest way is to install Erlang/OTP somewhere, install the application-specific code somewhere else, and then start the Erlang runtime system, making sure the code path includes the application-specific code.

It is often not desirable to use an Erlang/OTP system as is. A developer can create new Erlang/OTP-compliant applications for a particular purpose, and several original Erlang/OTP applications can be irrelevant for the purpose in question. Thus, there is a need to be able to create a new system based on a given Erlang/OTP system, where dispensable applications are removed and new applications are included. Documentation and source code is irrelevant and is therefore not included in the new system.

This chapter is about creating such a system, which is called a target system.

The following sections deal with target systems with different requirements of functionality:

- A basic target system that can be started by calling the ordinary `erl` script.
- A simple target system that also supports code replacement in runtime.
- An embedded target system that also supports starting automatically at boot time, and logging output from the system to files for later inspection.

Here is only considered the case when Erlang/OTP is running on a UNIX system.

The `sasl` application includes the example Erlang module `target_system.erl`, which contains functions for creating and installing a target system. This module is used in the following examples. The source code of the module is listed in Listing of target_system.erl

## Creating a Target System

It is assumed that you have a working Erlang/OTP system structured according to the OTP design principles.

Step 1. Create a `.rel` file (see the rel(4) manual page in SASL), which specifies the ERTS version and lists all applications that are to be included in the new basic target system. An example is the following `mysystem.rel` file:

    %% mysystem.rel
    {release,
     {"MYSYSTEM", "FIRST"},
     {erts, "5.10.4"},
     [{kernel, "2.16.4"},
      {stdlib, "1.19.4"},
      {sasl, "2.3.4"},
      {pea, "1.0"}]}.

The listed applications are not only original Erlang/OTP applications but possibly also new applications that you have written (here exemplified by the application Pea (`pea`)).

Step 2. Start Erlang/OTP from the directory where the `mysystem.rel` file resides:

    % erl -pa /home/user/target_system/myapps/pea-1.0/ebin

The `-pa` argument prepends the path to the `ebin` directory for the Pea application to the code path.

Step 3. Create the target system:

    1> target_system:create("mysystem").

The function `target_system:create/1` performs the following:

1. Reads the file `mysystem.rel` and creates a new file `plain.rel`. The new file is identical to the original, except that it only lists the Kernel and STDLIB applications.

2. From the files `mysystem.rel` and `plain.rel` creates the files `mysystem.script`, `mysystem.boot`, `plain.script`, and `plain.boot` by calling `systools:make_script/2`.

3. Creates the file `mysystem.tar.gz` by calling `systools:make_tar/2`. That file has the following contents:

    erts-5.10.4/bin/
    releases/FIRST/start.boot
    releases/FIRST/mysystem.rel
    releases/mysystem.rel
    lib/kernel-2.16.4/
    lib/stdlib-1.19.4/
    lib/sasl-2.3.4/
    lib/pea-1.0/

The file `releases/FIRST/start.boot` is a copy of our `mysystem.boot`.

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). Hence, the file is now duplicated within the tar archive, eliminating the need for manual copying.

1. Creates the temporary directory `tmp` and extracts the tar file `mysystem.tar.gz` into that directory.
2. Deletes the files `erl` and `start` from `tmp/erts-5.10.4/bin`. These files are created again from source when installing the release.
3. Creates the directory `tmp/bin`.
4. Copies the previously created file `plain.boot` to `tmp/bin/start.boot`.
5. Copies the files `epmd`, `run_erl`, and `to_erl` from the directory `tmp/erts-5.10.4/bin` to the directory `tmp/bin`.
6. Creates the directory `tmp/log`, which is used if the system is started as embedded with the `bin/start` script.
7. Creates the file `tmp/releases/start_erl.data` with the contents "5.10.4 FIRST". This file is to be passed as data file to the `start_erl` script.
8. Recreates the file `mysystem.tar.gz` from the directories in the directory `tmp` and removes `tmp`.

## Installing a Target System

Step 4. Install the created target system in a suitable directory.

    2> target_system:install("mysystem", "/usr/local/erl-target").

The function `target_system:install/2` performs the following:

1. Extracts the tar file `mysystem.tar.gz` into the target directory `/usr/local/erl-target`.
2. In the target directory reads the file `releases/start_erl.data` to find the Erlang runtime system version ("5.10.4").
3. Substitutes `%FINAL_ROOTDIR%` and `%EMU%` for `/usr/local/erl-target` and `beam`, respectively, in the files `erl.src`, `start.src`, and `start_erl.src` of the target `erts-5.10.4/bin` directory, and puts the resulting files `erl`, `start`, and `run_erl` in the target `bin` directory.
4. Finally the target `releases/RELEASES` file is created from data in the file `releases/mysystem.rel`.

## Starting a Target System

Now we have a target system that can be started in various ways. We start it as a basic target system by invoking:

    % /usr/local/erl-target/bin/erl

Here only the Kernel and STDLIB applications are started, that is, the system is started as an ordinary development system. Only two files are needed for all this to work:

1. `bin/erl` (obtained from `erts-5.10.4/bin/erl.src`)
2. `bin/start.boot` (a copy of `plain.boot`)

We can also start a distributed system (requires `bin/epmd`).

To start all applications specified in the original `mysystem.rel` file, use flag `-boot` as follows:

    % /usr/local/erl-target/bin/erl -boot /usr/local/erl-target/releases/FIRST/start

We start a simple target system as above. The only difference is that also the file `releases/RELEASES` is present for code replacement in runtime to work.

To start an embedded target system, the shell script `bin/start` is used. The script calls `bin/run_erl`, which in turn calls `bin/start_erl` (roughly, `start_erl` is an embedded variant of `erl`).

The shell script `start`, which is generated from `erts-5.10.4/bin/start.src` during installation, is merely an example. Edit it to suit your needs. Typically it is executed when the UNIX system boots.

`run_erl` is a wrapper that provides logging of output from the runtime system to file. It also provides a simple mechanism for attaching to the Erlang shell (`to_erl`).

`start_erl` requires:

1. The root directory (`"/usr/local/erl-target"`)
2. The releases directory (`"/usr/local/erl-target/releases"`)
3. The location of the file `start_erl.data`

It performs the following:

1. Reads the runtime system version (`"5.10.4"`) and release version (`"FIRST"`) from the file `start_erl.data`.
2. Starts the runtime system of the version found.
3. Provides the flag `-boot` specifying the boot file of the release version found (`"releases/FIRST/start.boot"`).

`start_erl` also assumes that there is a `sys.config` in the release version directory (`"releases/FIRST/sys.config"`). That is the topic of the next section.

The `start_erl` shell script is normally not to be altered by the user.

## System Configuration Parameters

As was mentioned in the previous section, `start_erl` requires a `sys.config` in the release version directory (`"releases/FIRST/sys.config"`). If there is no such file, the system start fails. Such a file must therefore also be added.

If you have system configuration data that is neither file-location-dependent nor site-dependent, it can be convenient to create `sys.config` early, so it becomes part of the target system tar file created by `target_system:create/1`. In fact, if you in the current directory create not only the file `mysystem.rel`, but also file `sys.config`, the latter file is tacitly put in the appropriate directory.

However, it can also be convenient to replace variables within a `sys.config` on the target after unpacking but before running the release. If you have a `sys.config.src` it will be included and is not required to be a valid Erlang term file like `sys.config`. Before running the release you must have a valid `sys.config` in the same directory, so using `sys.config.src` requires having some tool to populate what is needed and write `sys.config` to disk before booting the release.

## Differences From the Install Script

The previous `install/2` procedure differs somewhat from that of the ordinary `Install` shell script. In fact, `create/1` makes the release package as complete as possible, and leaves it to the `install/2` procedure to finish by only considering location-dependent files.

## Creating the Next Version

In this example the Pea application has been changed, and so have the applications ERTS, Kernel, STDLIB and SASL.

Step 1. Create the file `.rel`:

    %% mysystem2.rel
    {release,
     {"MYSYSTEM", "SECOND"},
     {erts, "6.0"},
     [{kernel, "3.0"},
      {stdlib, "2.0"},
      {sasl, "2.4"},
      {pea, "2.0"}]}.

Step 2. Create the application upgrade file (see appup in SASL) for Pea, for example:

    %% pea.appup
    {"2.0",
     [{"1.0",[{load_module,pea_lib}]}],
     [{"1.0",[{load_module,pea_lib}]}]}.

Step 3. From the directory where the file `mysystem2.rel` resides, start the Erlang/OTP system, giving the path to the new version of Pea:

    % erl -pa /home/user/target_system/myapps/pea-2.0/ebin

Step 4. Create the release upgrade file (see relup in SASL):

    1> systools:make_relup("mysystem2",["mysystem"],["mysystem"],
        [{path,["/home/user/target_system/myapps/pea-1.0/ebin",
        "/my/old/erlang/lib/*/ebin"]}]).

Here `"mysystem"` is the base release and `"mysystem2"` is the release to upgrade to.

The `path` option is used for pointing out the old version of all applications. (The new versions are already in the code path - assuming of course that the Erlang node on which this is executed is running the correct version of Erlang/OTP.)

Step 5. Create the new release:

    2> target_system:create("mysystem2").

Given that the file `relup` generated in Step 4 is now located in the current directory, it is automatically included in the release package.

## Upgrading the Target System

This part is done on the target node, and for this example we want the node to be running as an embedded system with the `-heart` option, allowing automatic restart of the node. For more information, see Starting a Target System.

We add `-heart` to `bin/start`:

    #!/bin/sh
    ROOTDIR=/usr/local/erl-target/

    if [ -z "$RELDIR" ]
    then
       RELDIR=$ROOTDIR/releases
    fi

    START_ERL_DATA=${1:-$RELDIR/start_erl.data}

    $ROOTDIR/bin/run_erl -daemon /tmp/ $ROOTDIR/log "exec $ROOTDIR/bin/start_erl $ROOTDIR\
    $RELDIR $START_ERL_DATA -heart"

We use the simplest possible `sys.config`, which we store in `releases/FIRST`:

    %% sys.config
    [].

Finally, to prepare the upgrade, we must put the new release package in the `releases` directory of the first target system:

    % cp mysystem2.tar.gz /usr/local/erl-target/releases

Assuming that the node has been started as follows:

    % /usr/local/erl-target/bin/start

It can be accessed as follows:

    % /usr/local/erl-target/bin/to_erl /tmp/erlang.pipe.1

Logs can be found in `/usr/local/erl-target/log`. This directory is specified as an argument to `run_erl` in the start script listed above.

Step 1. Unpack the release:

    1> {ok,Vsn} = release_handler:unpack_release("mysystem2").

Step 2. Install the release:

    2> release_handler:install_release(Vsn).
    {continue_after_restart,"FIRST",[]}
    heart: Tue Apr  1 12:15:10 2014: Erlang has closed.
    heart: Tue Apr  1 12:15:11 2014: Executed "/usr/local/erl-target/bin/start /usr/local/erl-target/releases/new_start_erl.data" -> 0. Terminating.
    [End]

The above return value and output after the call to `release_handler:install_release/1` means that the `release_handler` has restarted the node by using `heart`. This is always done when the upgrade involves a change of the applications ERTS, Kernel, STDLIB, or SASL. For more information, see Upgrade when Erlang/OTP has Changed.

The node is accessible through a new pipe:

    % /usr/local/erl-target/bin/to_erl /tmp/erlang.pipe.2

List the available releases in the system:

    1> release_handler:which_releases().
    [{"MYSYSTEM","SECOND",
      ["kernel-3.0","stdlib-2.0","sasl-2.4","pea-2.0"],
      current},
     {"MYSYSTEM","FIRST",
      ["kernel-2.16.4","stdlib-1.19.4","sasl-2.3.4","pea-1.0"],
      permanent}]

Our new release, "SECOND", is now the current release, but we can also see that our "FIRST" release is still permanent. This means that if the node would be restarted now, it would come up running the "FIRST" release again.

Step 3. Make the new release permanent:

    2> release_handler:make_permanent("SECOND").

Check the releases again:

    3> release_handler:which_releases().
    [{"MYSYSTEM","SECOND",
      ["kernel-3.0","stdlib-2.0","sasl-2.4","pea-2.0"],
      permanent},
     {"MYSYSTEM","FIRST",
      ["kernel-2.16.4","stdlib-1.19.4","sasl-2.3.4","pea-1.0"],
      old}]

We see that the new release version is `permanent`, so it would be safe to restart the node.

## Listing of target_system.erl

This module can also be found in the `examples` directory of the SASL application.

    -module(target_system).
    -export([create/1, create/2, install/2]).

    %% Note: RelFileName below is the *stem* without trailing .rel,
    %% .script etc.
    %%

    %% create(RelFileName)
    %%
    create(RelFileName) ->
        create(RelFileName,[]).

    create(RelFileName,SystoolsOpts) ->
        RelFile = RelFileName ++ ".rel",
        Dir = filename:dirname(RelFileName),
        PlainRelFileName = filename:join(Dir,"plain"),
        PlainRelFile = PlainRelFileName ++ ".rel",
        io:fwrite("Reading file: ~ts ...~n", [RelFile]),
        {ok, [RelSpec]} = file:consult(RelFile),
        io:fwrite("Creating file: ~ts from ~ts ...~n",
                  [PlainRelFile, RelFile]),
        {release,
         {RelName, RelVsn},
         {erts, ErtsVsn},
         AppVsns} = RelSpec,
        PlainRelSpec = {release,
                        {RelName, RelVsn},
                        {erts, ErtsVsn},
                        lists:filter(fun({kernel, _}) ->
                                             true;
                                        ({stdlib, _}) ->
                                             true;
                                        (_) ->
                                             false
                                     end, AppVsns)
                       },
        {ok, Fd} = file:open(PlainRelFile, [write]),
        io:fwrite(Fd, "~p.~n", [PlainRelSpec]),
        file:close(Fd),

        io:fwrite("Making \"~ts.script\" and \"~ts.boot\" files ...~n",
    	      [PlainRelFileName,PlainRelFileName]),
        make_script(PlainRelFileName,SystoolsOpts),

        io:fwrite("Making \"~ts.script\" and \"~ts.boot\" files ...~n",
                  [RelFileName, RelFileName]),
        make_script(RelFileName,SystoolsOpts),

        TarFileName = RelFileName ++ ".tar.gz",
        io:fwrite("Creating tar file ~ts ...~n", [TarFileName]),
        make_tar(RelFileName,SystoolsOpts),

        TmpDir = filename:join(Dir,"tmp"),
        io:fwrite("Creating directory ~tp ...~n",[TmpDir]),
        file:make_dir(TmpDir),

        io:fwrite("Extracting ~ts into directory ~ts ...~n", [TarFileName,TmpDir]),
        extract_tar(TarFileName, TmpDir),

        TmpBinDir = filename:join([TmpDir, "bin"]),
        ErtsBinDir = filename:join([TmpDir, "erts-" ++ ErtsVsn, "bin"]),
        io:fwrite("Deleting \"erl\" and \"start\" in directory ~ts ...~n",
                  [ErtsBinDir]),
        file:delete(filename:join([ErtsBinDir, "erl"])),
        file:delete(filename:join([ErtsBinDir, "start"])),

        io:fwrite("Creating temporary directory ~ts ...~n", [TmpBinDir]),
        file:make_dir(TmpBinDir),

        io:fwrite("Copying file \"~ts.boot\" to ~ts ...~n",
                  [PlainRelFileName, filename:join([TmpBinDir, "start.boot"])]),
        copy_file(PlainRelFileName++".boot",filename:join([TmpBinDir, "start.boot"])),

        io:fwrite("Copying files \"epmd\", \"run_erl\" and \"to_erl\" from \n"
                  "~ts to ~ts ...~n",
                  [ErtsBinDir, TmpBinDir]),
        copy_file(filename:join([ErtsBinDir, "epmd"]),
                  filename:join([TmpBinDir, "epmd"]), [preserve]),
        copy_file(filename:join([ErtsBinDir, "run_erl"]),
                  filename:join([TmpBinDir, "run_erl"]), [preserve]),
        copy_file(filename:join([ErtsBinDir, "to_erl"]),
                  filename:join([TmpBinDir, "to_erl"]), [preserve]),

        %% This is needed if 'start' script created from 'start.src' shall
        %% be used as it points out this directory as log dir for 'run_erl'
        TmpLogDir = filename:join([TmpDir, "log"]),
        io:fwrite("Creating temporary directory ~ts ...~n", [TmpLogDir]),
        ok = file:make_dir(TmpLogDir),

        StartErlDataFile = filename:join([TmpDir, "releases", "start_erl.data"]),
        io:fwrite("Creating ~ts ...~n", [StartErlDataFile]),
        StartErlData = io_lib:fwrite("~s ~s~n", [ErtsVsn, RelVsn]),
        write_file(StartErlDataFile, StartErlData),

        io:fwrite("Recreating tar file ~ts from contents in directory ~ts ...~n",
    	      [TarFileName,TmpDir]),
        {ok, Tar} = erl_tar:open(TarFileName, [write, compressed]),
        %% {ok, Cwd} = file:get_cwd(),
        %% file:set_cwd("tmp"),
        ErtsDir = "erts-"++ErtsVsn,
        erl_tar:add(Tar, filename:join(TmpDir,"bin"), "bin", []),
        erl_tar:add(Tar, filename:join(TmpDir,ErtsDir), ErtsDir, []),
        erl_tar:add(Tar, filename:join(TmpDir,"releases"), "releases", []),
        erl_tar:add(Tar, filename:join(TmpDir,"lib"), "lib", []),
        erl_tar:add(Tar, filename:join(TmpDir,"log"), "log", []),
        erl_tar:close(Tar),
        %% file:set_cwd(Cwd),
        io:fwrite("Removing directory ~ts ...~n",[TmpDir]),
        remove_dir_tree(TmpDir),
        ok.

    install(RelFileName, RootDir) ->
        TarFile = RelFileName ++ ".tar.gz",
        io:fwrite("Extracting ~ts ...~n", [TarFile]),
        extract_tar(TarFile, RootDir),
        StartErlDataFile = filename:join([RootDir, "releases", "start_erl.data"]),
        {ok, StartErlData} = read_txt_file(StartErlDataFile),
        [ErlVsn, _RelVsn| _] = string:tokens(StartErlData, " \n"),
        ErtsBinDir = filename:join([RootDir, "erts-" ++ ErlVsn, "bin"]),
        BinDir = filename:join([RootDir, "bin"]),
        io:fwrite("Substituting in erl.src, start.src and start_erl.src to "
                  "form erl, start and start_erl ...\n"),
        subst_src_scripts(["erl", "start", "start_erl"], ErtsBinDir, BinDir,
                          [{"FINAL_ROOTDIR", RootDir}, {"EMU", "beam"}],
                          [preserve]),
        %%! Workaround for pre OTP 17.0: start.src and start_erl.src did
        %%! not have correct permissions, so the above 'preserve' option did not help
        ok = file:change_mode(filename:join(BinDir,"start"),8#0755),
        ok = file:change_mode(filename:join(BinDir,"start_erl"),8#0755),

        io:fwrite("Creating the RELEASES file ...\n"),
        create_RELEASES(RootDir, filename:join([RootDir, "releases",
    					    filename:basename(RelFileName)])).

    %% LOCALS

    %% make_script(RelFileName,Opts)
    %%
    make_script(RelFileName,Opts) ->
        systools:make_script(RelFileName, [no_module_tests,
    				       {outdir,filename:dirname(RelFileName)}
    				       |Opts]).

    %% make_tar(RelFileName,Opts)
    %%
    make_tar(RelFileName,Opts) ->
        RootDir = code:root_dir(),
        systools:make_tar(RelFileName, [{erts, RootDir},
    				    {outdir,filename:dirname(RelFileName)}
    				    |Opts]).

    %% extract_tar(TarFile, DestDir)
    %%
    extract_tar(TarFile, DestDir) ->
        erl_tar:extract(TarFile, [{cwd, DestDir}, compressed]).

    create_RELEASES(DestDir, RelFileName) ->
        release_handler:create_RELEASES(DestDir, RelFileName ++ ".rel").

    subst_src_scripts(Scripts, SrcDir, DestDir, Vars, Opts) ->
        lists:foreach(fun(Script) ->
                              subst_src_script(Script, SrcDir, DestDir,
                                               Vars, Opts)
                      end, Scripts).

    subst_src_script(Script, SrcDir, DestDir, Vars, Opts) ->
        subst_file(filename:join([SrcDir, Script ++ ".src"]),
                   filename:join([DestDir, Script]),
                   Vars, Opts).

    subst_file(Src, Dest, Vars, Opts) ->
        {ok, Conts} = read_txt_file(Src),
        NConts = subst(Conts, Vars),
        write_file(Dest, NConts),
        case lists:member(preserve, Opts) of
            true ->
                {ok, FileInfo} = file:read_file_info(Src),
                file:write_file_info(Dest, FileInfo);
            false ->
                ok
        end.

    %% subst(Str, Vars)
    %% Vars = [{Var, Val}]
    %% Var = Val = string()
    %% Substitute all occurrences of %Var% for Val in Str, using the list
    %% of variables in Vars.
    %%
    subst(Str, Vars) ->
        subst(Str, Vars, []).

    subst([$%, C| Rest], Vars, Result) when $A =< C, C =< $Z ->
        subst_var([C| Rest], Vars, Result, []);
    subst([$%, C| Rest], Vars, Result) when $a =< C, C =< $z ->
        subst_var([C| Rest], Vars, Result, []);
    subst([$%, C| Rest], Vars, Result) when  C == $_ ->
        subst_var([C| Rest], Vars, Result, []);
    subst([C| Rest], Vars, Result) ->
        subst(Rest, Vars, [C| Result]);
    subst([], _Vars, Result) ->
        lists:reverse(Result).

    subst_var([$%| Rest], Vars, Result, VarAcc) ->
        Key = lists:reverse(VarAcc),
        case lists:keysearch(Key, 1, Vars) of
            {value, {Key, Value}} ->
                subst(Rest, Vars, lists:reverse(Value, Result));
            false ->
                subst(Rest, Vars, [$%| VarAcc ++ [$%| Result]])
        end;
    subst_var([C| Rest], Vars, Result, VarAcc) ->
        subst_var(Rest, Vars, Result, [C| VarAcc]);
    subst_var([], Vars, Result, VarAcc) ->
        subst([], Vars, [VarAcc ++ [$%| Result]]).

    copy_file(Src, Dest) ->
        copy_file(Src, Dest, []).

    copy_file(Src, Dest, Opts) ->
        {ok,_} = file:copy(Src, Dest),
        case lists:member(preserve, Opts) of
            true ->
                {ok, FileInfo} = file:read_file_info(Src),
                file:write_file_info(Dest, FileInfo);
            false ->
                ok
        end.

    write_file(FName, Conts) ->
        Enc = file:native_name_encoding(),
        {ok, Fd} = file:open(FName, [write]),
        file:write(Fd, unicode:characters_to_binary(Conts,Enc,Enc)),
        file:close(Fd).

    read_txt_file(File) ->
        {ok, Bin} = file:read_file(File),
        {ok, binary_to_list(Bin)}.

    remove_dir_tree(Dir) ->
        remove_all_files(".", [Dir]).

    remove_all_files(Dir, Files) ->
        lists:foreach(fun(File) ->
                              FilePath = filename:join([Dir, File]),
                              case filelib:is_dir(FilePath) of
                                  true ->
                                      {ok, DirFiles} = file:list_dir(FilePath),
                                      remove_all_files(FilePath, DirFiles),
                                      file:del_dir(FilePath);
                                  _ ->
                                      file:delete(FilePath)
                              end
                      end, Files).

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