path: root/system/doc/system_principles/create_target.xmlsrc

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<chapter>
  <header>
    <copyright>
      <year>2002</year><year>2011</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.

      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.

    </legalnotice>

    <title>Creating a First Target System</title>
    <prepared>Peter H&ouml;gfeldt</prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date>2002-09-17</date>
    <rev>A</rev>
    <file>create_target.xml</file>
  </header>

  <section>
    <title>Introduction</title>
    <p>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.</p>
    <p>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 are included. Documentation and
      source code is irrelevant and is therefore not included in the
      new system.</p>
    <p>This chapter is about creating such a system, which we call a
      <em>target system</em>.</p>
    <p>In the following sections we consider creating target systems with
      different requirements of functionality:</p>
    <list type="bulleted">
      <item>a <em>basic target system</em> that can be started by
       calling the ordinary <c>erl</c> script, </item>
      <item>a <em>simple target system</em> where also code
       replacement in run-time can be performed, and</item>
      <item>an <em>embedded target system</em> 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. </item>
    </list>
    <p>We only consider the case when Erlang/OTP is running on a UNIX
      system.</p>
    <p>In the <c>sasl</c> application there is an example Erlang
      module <c>target_system.erl</c> that contains functions for
      creating and installing a target system.  This module is used in
      the examples below, and the source code of the module is listed
      at the end of this chapter.</p>
  </section>

  <section>
    <title>Creating a Target System</title>
    <p>It is assumed that you have a working Erlang/OTP system structured
      according to the OTP Design Principles.</p>
    <p><em>Step 1.</em> First create a <c>.rel</c> file (see
      <c>rel(4)</c>) that specifies the <c>erts</c> version
      and lists all applications that should be included in the new
      basic target system. An example is the following
      <c>mysystem.rel</c> file:</p>
    <code type="none">
%% mysystem.rel
{release,
 {"MYSYSTEM", "FIRST"},
 {erts, "5.1"},
 [{kernel, "2.7"},
  {stdlib, "1.10"},
  {sasl, "1.9.3"},
  {pea, "1.0"}]}.    </code>
    <p>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
      <c>pea</c>). </p>
    <p><em>Step 2.</em> From the directory where the <c>mysystem.rel</c>
      file reside, start the Erlang/OTP system:</p>
    <pre>
os> <input>erl -pa /home/user/target_system/myapps/pea-1.0/ebin</input></pre>
    <p>where also the path to the <c>pea-1.0</c> ebin directory is
      provided. </p>
    <p><em>Step 3.</em> Now create the target system: </p>
    <pre>
1> <input>target_system:create("mysystem").</input></pre>
    <p>The <c>target_system:create/1</c> function does the following:</p>
    <list type="ordered">
      <item>Reads the <c>mysystem.rel</c> file, and creates a new file
      <c>plain.rel</c> which is identical to former, except that it
       only lists the <c>kernel</c> and <c>stdlib</c> applications. </item>
      <item>From the <c>mysystem.rel</c> and <c>plain.rel</c> files
       creates the files <c>mysystem.script</c>,
      <c>mysystem.boot</c>, <c>plain.script</c>, and
      <c>plain.boot</c> through a call to
      <c>systools:make_script/2</c>.</item>
      <item>
        <p>Creates the file <c>mysystem.tar.gz</c> by a call to
          <c>systools:make_tar/2</c>. That file has the following
          contents:</p>
        <code type="none">
erts-5.1/bin/
releases/FIRST/start.boot
releases/FIRST/mysystem.rel
releases/mysystem.rel
lib/kernel-2.7/
lib/stdlib-1.10/
lib/sasl-1.9.3/
lib/pea-1.0/        </code>
        <p>The file <c>releases/FIRST/start.boot</c> is a copy of our
          <c>mysystem.boot</c></p>
	<p>The release resource file <c>mysystem.rel</c> is duplicated
          in the tar file. Originally, this file was only stored in
          the <c>releases</c> directory in order to make it possible
          for the <c>release_handler</c> to extract this file
          separately. After unpacking the tar
          file, <c>release_handler</c> would automatically copy the
          file to <c>releases/FIRST</c>. However, sometimes the tar
          file is unpacked without involving
          the <c>release_handler</c> (e.g. when unpacking the first
          target system) and therefore the file is now instead
          duplicated in the tar file so no manual copying is
          necessary.</p>
      </item>
      <item>Creates the temporary directory <c>tmp</c> and extracts the tar file
      <c>mysystem.tar.gz</c> into that directory. </item>
      <item>Deletes the <c>erl</c> and <c>start</c> files from
      <c>tmp/erts-5.1/bin</c>. These files will be created again from
      source when installing the release.</item>
      <item>Creates the directory <c>tmp/bin</c>.</item>
      <item>Copies the previously created file <c>plain.boot</c> to
      <c>tmp/bin/start.boot</c>.</item>
      <item>Copies the files <c>epmd</c>, <c>run_erl</c>, and
      <c>to_erl</c> from the directory <c>tmp/erts-5.1/bin</c> to
       the directory <c>tmp/bin</c>.</item>
      <item>Creates the file <c>tmp/releases/start_erl.data</c> with
       the contents "5.1 FIRST". This file is to be passed as data
       file to the <c>start_erl</c> script.
      </item>
      <item>Recreates the file <c>mysystem.tar.gz</c> from the directories
       in the directory <c>tmp</c>, and removes <c>tmp</c>.</item>
    </list>
  </section>

  <section>
    <title>Installing a Target System</title>
    <p><em>Step 4.</em> Install the created target system in a
      suitable directory. </p>
    <pre>
2> <input>target_system:install("mysystem", "/usr/local/erl-target").</input></pre>
    <p>The function <c>target_system:install/2</c> does the following:
      </p>
    <list type="ordered">
      <item>Extracts the tar file <c>mysystem.tar.gz</c> into the target
       directory <c>/usr/local/erl-target</c>.</item>
      <item>In the target directory reads the file <c>releases/start_erl.data</c>
       in order to find the Erlang runtime system version ("5.1").</item>
      <item>Substitutes <c>%FINAL_ROOTDIR%</c> and <c>%EMU%</c> for
      <c>/usr/local/erl-target</c> and <c>beam</c>, respectively, in
       the files <c>erl.src</c>, <c>start.src</c>, and
      <c>start_erl.src</c> of the target <c>erts-5.1/bin</c>
       directory, and puts the resulting files <c>erl</c>,
      <c>start</c>, and <c>run_erl</c> in the target <c>bin</c>
       directory.</item>
      <item>Finally the target <c>releases/RELEASES</c> file is created
       from data in the <c>releases/mysystem.rel</c> file.</item>
    </list>
  </section>

  <section>
    <title>Starting a Target System</title>
    <p>Now we have a target system that can be started in various ways.</p>
    <p>We start it as a <em>basic target system</em> by invoking</p>
    <pre>
os> <input>/usr/local/erl-target/bin/erl</input></pre>
    <p>where only the <c>kernel</c> and <c>stdlib</c> 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:
      <c>bin/erl</c> file (obtained from <c>erts-5.1/bin/erl.src</c>)
      and the <c>bin/start.boot</c> file (a copy of <c>plain.boot</c>).</p>
    <p>We can also start a distributed system (requires <c>bin/epmd</c>).</p>
    <p>To start all applications specified in the original
      <c>mysystem.rel</c> file, use the <c>-boot</c> flag as follows:</p>
    <pre>
os> <input>/usr/local/erl-target/bin/erl -boot /usr/local/erl-target/releases/FIRST/start</input></pre>
    <p>We start a <em>simple target system</em> as above. The only difference
      is that also the file <c>releases/RELEASES</c> is present for
      code replacement in run-time to work.</p>
    <p>To start an <em>embedded target system</em> the shell script
      <c>bin/start</c> is used. That shell script calls
      <c>bin/run_erl</c>, which in turn calls <c>bin/start_erl</c>
      (roughly, <c>start_erl</c> is an embedded variant of
      <c>erl</c>). </p>
    <p>The shell script <c>start</c> is only an example. You should
      edit it to suite your needs. Typically it is executed when the
      UNIX system boots.</p>
    <p><c>run_erl</c> 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 (<c>to_erl</c>).</p>
    <p><c>start_erl</c> requires the root directory
      (<c>"/usr/local/erl-target"</c>), the releases directory
      (<c>"/usr/local/erl-target/releases"</c>), and the location of
      the <c>start_erl.data</c> file. It reads the run-time system
      version (<c>"5.1"</c>) and release version (<c>"FIRST"</c>) from
      the <c>start_erl.data</c> file, starts the run-time system of the
      version found, and provides <c>-boot</c> flag specifying the boot
      file of the release version found
      (<c>"releases/FIRST/start.boot"</c>).</p>
    <p><c>start_erl</c> also assumes that there is <c>sys.config</c> in
      release version directory (<c>"releases/FIRST/sys.config"</c>). That
      is the topic of the next section (see below).</p>
    <p>The <c>start_erl</c> shell script should normally not be
      altered by the user.</p>
  </section>

  <section>
    <title>System Configuration Parameters</title>
    <p>As was pointed out above <c>start_erl</c> requires a
      <c>sys.config</c> in the release version directory
      (<c>"releases/FIRST/sys.config"</c>). If there is no such a
      file, the system start will fail. Hence such a file has to
      be added as well.</p>
    <p></p>
    <p>If you have system configuration data that are neither file
      location dependent nor site dependent, it may be convenient to
      create the <c>sys.config</c> early, so that it becomes a part of
      the target system tar file created by
      <c>target_system:create/1</c>. In fact, if you create, in the
      current directory, not only the <c>mysystem.rel</c> file, but
      also a <c>sys.config</c> file, that latter file will be tacitly
      put in the apropriate directory.</p>
  </section>

  <section>
    <title>Differences from the Install Script</title>
    <p>The above <c>install/2</c> procedure differs somewhat from that
      of the ordinary <c>Install</c> shell script. In fact, <c>create/1</c>
      makes the release package as complete as possible, and leave to the
      <c>install/2</c> procedure to finish by only considering location
      dependent files.</p>
  </section>

  <section>
    <title>Listing of target_system.erl</title>
    <p>This module can also be found in the <c>examples</c> directory
      of the <c>sasl</c> application.</p>
    <codeinclude file="../../../lib/sasl/examples/src/target_system.erl" tag="%module" type="erl"></codeinclude>

  </section>
</chapter>