## 3 Creating a First Target System

### 3.1 Introduction

When creating a system using Erlang/OTP, the most simple 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.

Often it is not desirable to use an Erlang/OTP system as is. A developer may create new Erlang/OTP compliant applications for a particular purpose, and several original Erlang/OTP applications may 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 a set of new applications that are included in the new system. Documentation and source code is irrelevant and is therefore not included in the new system.

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

In the following sections we consider creating 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 where also code replacement in run-time can be performed, and
- an embedded target system where there is also support for logging output from the system to file for later inspection, and where the system can be started automatically at boot time.

We only consider the case when Erlang/OTP is running on a UNIX system.

There is an example Erlang module target_system.erl that contains functions for creating and installing a target system. That module is used in the examples below. The source code of the module is listed at the end of this chapter.

### 3.2 Creating a Target System

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

Step 1. First create a .rel file (see rel(4)) that specifies the erts version and lists all applications that should be included in the new basic target system. An example is the following mysystem.rel file:

    %% mysystem.rel
    {release,
     {"MYSYSTEM", "FIRST"},
     {erts, "5.1"},
     [{kernel, "2.7"},
      {stdlib, "1.10"},
      {sasl, "1.9.3"},
      {pea, "1.0"}]}.

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

Step 2. From the directory where the mysystem.rel file reside, start the Erlang/OTP system:

    os> erl -pa /home/user/target_system/myapps/pea-1.0/ebin

where also the path to the pea-1.0 ebin directory is provided.

Step 3. Now create the target system:

    1> target_system:create("mysystem").

The target_system:create/1 function does the following:

1. Reads the mysystem.rel file, and creates a new file plain.rel which is identical to former, except that it only lists the kernel and stdlib applications.
2. From the mysystem.rel and plain.rel files creates the files mysystem.script, mysystem.boot, plain.script, and plain.boot through a call to systools:make_script/2.
3. Creates the file mysystem.tar.gz by a call to systools:make_tar/2. That file has the following contents:

       erts-5.1/bin/
       releases/FIRST/start.boot
       releases/mysystem.rel
       lib/kernel-2.7/
       lib/stdlib-1.10/
       lib/sasl-1.9.3/
       lib/pea-1.0/

   The file releases/FIRST/start.boot is a copy of our mysystem.boot, and a copy of the original mysystem.rel has been put in the releases directory.
4. Creates the temporary directory tmp and extracts the tar file mysystem.tar.gz into that directory.
5. Deletes the erl and start files from tmp/erts-5.1/bin. XXX Why.
6. Creates the directory tmp/bin.
7. Copies the previously creates file plain.boot to tmp/bin/start.boot.
8. Copies the files epmd, run_erl, and to_erl from the directory tmp/erts-5.1/bin to the directory tmp/bin.
9. Creates the file tmp/releases/start_erl.data with the contents "5.1 FIRST".
10. Recreates the file mysystem.tar.gz from the directories in the directory tmp, and removes tmp.

### 3.3 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 does 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 in order to find the Erlang runtime system version ("5.1").
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-5.1/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 releases/mysystem.rel file.

### 3.4 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

    os> /usr/local/erl-target/bin/erl

where only the kernel and stdlib applications are started, i.e. the system is started as an ordinary development system. There are only two files needed for all this to work: bin/erl file (obtained from erts-5.1/bin/erl.src) and the bin/start.boot file (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 the -boot flag as follows:

    os> /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 run-time to work.

To start an embedded target system the shell script bin/start is used. That shell 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 is only an example. You should edit it to suite your needs. Typically it is executed when the UNIX system boots.

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

start_erl requires the root directory ("/usr/local/erl-target"), the releases directory ("/usr/local/erl-target/releases"), and the location of the start_erl.data file. It reads the run-time system version ("5.1") and release version ("FIRST") from the start_erl.data file, starts the run-time system of the version found, and provides -boot flag specifying the boot file of the release version found ("releases/FIRST/start.boot").

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

The start_erl shell script should normally not be altered by the user.

### 3.5 System Configuration Parameters

As was pointed out above start_erl requires a sys.config in the release version directory ("releases/FIRST/sys.config"). If there is no such a file, the system start will fail. Hence such a file has to added as well.

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

### 3.6 Differences from the Install Script

The above 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 leave to the install/2 procedure to finish by only considering location dependent files.

### 3.7 Listing of target_system.erl

    -module(target_system).
    -include_lib("kernel/include/file.hrl").
    -export([create/1, install/2]).
    -define(BUFSIZE, 8192).

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

    %% create(RelFileName)
    %%
    create(RelFileName) ->
        RelFile = RelFileName ++ ".rel",
        io:fwrite("Reading file: \"~s\" ...~n", [RelFile]),
        {ok, [RelSpec]} = file:consult(RelFile),
        io:fwrite("Creating file: \"~s\" from \"~s\" ...~n",
                  ["plain.rel", 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("plain.rel", [write]),
        io:fwrite(Fd, "~p.~n", [PlainRelSpec]),
        file:close(Fd),

        io:fwrite("Making \"plain.script\" and \"plain.boot\" files ...~n"),
        make_script("plain"),

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

        TarFileName = io_lib:fwrite("~s.tar.gz", [RelFileName]),
        io:fwrite("Creating tar file \"~s\" ...~n", [TarFileName]),
        make_tar(RelFileName),

        io:fwrite("Creating directory \"tmp\" ...~n"),
        file:make_dir("tmp"),

        io:fwrite("Extracting \"~s\" into directory \"tmp\" ...~n", [TarFileName]),
        extract_tar(TarFileName, "tmp"),

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

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

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

        io:fwrite("Copying files \"epmd\", \"run_erl\" and \"to_erl\" from \n"
                  "\"~s\" to \"~s\" ...~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]),

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

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

    install(RelFileName, RootDir) ->
        TarFile = RelFileName ++ ".tar.gz",
        io:fwrite("Extracting ~s ...~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\n"
                  "form erl, start and start_erl ...\n"),
        subst_src_scripts(["erl", "start", "start_erl"], ErtsBinDir, BinDir,
                          [{"FINAL_ROOTDIR", RootDir}, {"EMU", "beam"}],
                          [preserve]),
        io:fwrite("Creating the RELEASES file ...\n"),
        create_RELEASES(RootDir,
                        filename:join([RootDir, "releases", RelFileName])).

    %% LOCALS

    %% make_script(RelFileName)
    %%
    make_script(RelFileName) ->
        Opts = [no_module_tests],
        systools:make_script(RelFileName, Opts).

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

    %% 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, InFd} = file:open(Src, [raw, binary, read]),
        {ok, OutFd} = file:open(Dest, [raw, binary, write]),
        do_copy_file(InFd, OutFd),
        file:close(InFd),
        file:close(OutFd),
        case lists:member(preserve, Opts) of
            true ->
                {ok, FileInfo} = file:read_file_info(Src),
                file:write_file_info(Dest, FileInfo);
            false ->
                ok
        end.

    do_copy_file(InFd, OutFd) ->
        case file:read(InFd, ?BUFSIZE) of
            {ok, Bin} ->
                file:write(OutFd, Bin),
                do_copy_file(InFd, OutFd);
            eof  ->
                ok
        end.

    write_file(FName, Conts) ->
        {ok, Fd} = file:open(FName, [write]),
        file:write(Fd, Conts),
        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]),
                              {ok, FileInfo} = file:read_file_info(FilePath),
                              case FileInfo#file_info.type of
                                  directory ->
                                      {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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