3 Creating and Upgrading a Target System

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 where also code replacement in runtime can be performed.
  • 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.

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

3.1  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
 {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:

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

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

Step 3. Create the target system:

1> target_system:create("mysystem").

The function target_system:create/1 performs the following:

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


    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). The file is therefore now instead duplicated in the tar file so no manual copying is needed.

  • Creates the temporary directory tmp and extracts the tar file mysystem.tar.gz into that directory.
  • Deletes the files erl and start from tmp/erts-5.10.4/bin. These files are created again from source when installing the release.
  • Creates the directory tmp/bin.
  • Copies the previously created file plain.boot to tmp/bin/start.boot.
  • Copies the files epmd, run_erl, and to_erl from the directory tmp/erts-5.10.4/bin to the directory tmp/bin.
  • Creates the directory tmp/log, which is used if the system is started as embedded with the bin/start script.
  • 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.
  • Recreates the file mysystem.tar.gz from the directories in the directory tmp and removes tmp.

3.2  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:

  • Extracts the tar file mysystem.tar.gz into the target directory /usr/local/erl-target.
  • In the target directory reads the file releases/start_erl.data to find the Erlang runtime system version ("5.10.4").
  • 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.
  • Finally the target releases/RELEASES file is created from data in the file releases/mysystem.rel.

3.3  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

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:

  • bin/erl (obtained from erts-5.10.4/bin/erl.src)
  • 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:

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 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 only an example. 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 runtime 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"
  • The location of the file start_erl.data

It performs the following:

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

start_erl also assumes that there is 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.

3.4  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 in 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.

3.5  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 leave to the install/2 procedure to finish by only considering location-dependent files.

3.6  Creating the Next Version

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

Step 1. Create the file .rel:

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

Step 2. Create the application upgrade file (see the appup(4) manual page in SASL) for Pea, for example:

%% pea.appup

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:

os> erl -pa /home/user/target_system/myapps/pea-2.0/ebin

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

1> systools:make_relup("mysystem2",["mysystem"],["mysystem"],

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.

3.7  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:


if [ -z "$RELDIR" ]


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

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:

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

Assuming that the node has been started as follows:

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

It can be accessed as follows:

os> /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_erlin 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).
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.

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:

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

Check which releases there are in the system:

1> release_handler:which_releases().

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().

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

3.8  Listing of target_system.erl

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

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

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

%% 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: ~tp ...~n", [RelFile]),
    {ok, [RelSpec]} = file:consult(RelFile),
    io:fwrite("Creating file: ~tp from ~tp ...~n",
              [PlainRelFile, RelFile]),
     {RelName, RelVsn},
     {erts, ErtsVsn},
     AppVsns} = RelSpec,
    PlainRelSpec = {release, 
                    {RelName, RelVsn},
                    {erts, ErtsVsn},
                    lists:filter(fun({kernel, _}) -> 
                                    ({stdlib, _}) ->
                                    (_) ->
                                 end, AppVsns)
    {ok, Fd} = file:open(PlainRelFile, [write]),
    io:fwrite(Fd, "~p.~n", [PlainRelSpec]),

    io:fwrite("Making \"~ts.script\" and \"~ts.boot\" files ...~n",

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

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

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

    io:fwrite("Extracting ~tp into directory ~tp ...~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 ~tp ...~n",
    file:delete(filename:join([ErtsBinDir, "erl"])),
    file:delete(filename:join([ErtsBinDir, "start"])),

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

    io:fwrite("Copying file \"~ts.boot\" to ~tp ...~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"
              "~tp to ~tp ...~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 ~tp ...~n", [TmpLogDir]),
    ok = file:make_dir(TmpLogDir),

    StartErlDataFile = filename:join([TmpDir, "releases", "start_erl.data"]),
    io:fwrite("Creating ~tp ...~n", [StartErlDataFile]),
    StartErlData = io_lib:fwrite("~s ~s~n", [ErtsVsn, RelVsn]),
    write_file(StartErlDataFile, StartErlData),
    io:fwrite("Recreating tar file ~tp from contents in directory ~tp ...~n",
    {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", []),
    %% file:set_cwd(Cwd),
    io:fwrite("Removing directory ~tp ...~n",[TmpDir]),

install(RelFileName, RootDir) ->
    TarFile = RelFileName ++ ".tar.gz", 
    io:fwrite("Extracting ~tp ...~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"}],
    %%! 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",


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

%% make_tar(RelFileName,Opts)
make_tar(RelFileName,Opts) ->
    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 ->

%% 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) ->

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 ++ [$%| Result]])
subst_var([C| Rest], Vars, Result, VarAcc) ->
    subst_var(Rest, Vars, Result, [C| VarAcc]);
subst_var([], Vars, Result, VarAcc) ->
    subst([], Vars, [VarAcc ++ [$%| 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 ->
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)),

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),
                              _ ->
                  end, Files).