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<chapter>
  <header>
    <copyright>
      <year>2002</year><year>2020</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
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    </legalnotice>

    <title>Creating and Upgrading a 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>
  <marker id="creating upgrading target system"></marker>

    <p>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.</p>
    <p>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.</p>
    <p>This chapter is about creating such a system, which is called a
      <em>target system</em>.</p>
    <p>The following sections deal with 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 runtime can be performed.</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>Here is only considered the case when Erlang/OTP is running on a
      UNIX system.</p>
    <p>The <c>sasl</c> application includes the example Erlang
      module <c>target_system.erl</c>, 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 <seeguide marker="#listing of target system">
      Listing of target_system.erl</seeguide></p>

  <section>
    <marker id="create"/>
    <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> Create a <c>.rel</c> file (see the
      <seefile marker="sasl:rel">rel(4)</seefile> 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 <c>mysystem.rel</c> file:</p>
    <code type="none">
%% mysystem.rel
{release,
 {"MYSYSTEM", "FIRST"},
 {erts, "5.10.4"},
 [{kernel, "2.16.4"},
  {stdlib, "1.19.4"},
  {sasl, "2.3.4"},
  {pea, "1.0"}]}.</code>
    <p>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 (<c>pea</c>)).</p>
    <p><em>Step 2.</em> Start Erlang/OTP from the directory where
      the <c>mysystem.rel</c> file resides:</p>
    <pre>
os> <input>erl -pa /home/user/target_system/myapps/pea-1.0/ebin</input></pre>
    <p>Here also the path to the <c>pea-1.0</c> ebin directory is
      provided.</p>
    <p><em>Step 3.</em> Create the target system:</p>
    <pre>
1> <input>target_system:create("mysystem").</input></pre>
    <p>The function <c>target_system:create/1</c> performs the
      following:</p>
    <list type="ordered">
      <item>Reads the file <c>mysystem.rel</c> and creates a new file
      <c>plain.rel</c> that is identical to the former, except that it
       only lists the Kernel and STDLIB applications.</item>
      <item>From the files <c>mysystem.rel</c> and <c>plain.rel</c>
       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-5.10.4/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/</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 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> (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.</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 files <c>erl</c> and <c>start</c> from
      <c>tmp/erts-5.10.4/bin</c>. These files are 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-5.10.4/bin</c> to
       the directory <c>tmp/bin</c>.</item>
      <item>Creates the directory <c>tmp/log</c>, which is used
       if the system is started as embedded with the <c>bin/start</c>
       script.</item>
      <item>Creates the file <c>tmp/releases/start_erl.data</c> with
       the contents "5.10.4 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> performs 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> to find the Erlang runtime system
       version ("5.10.4").</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-5.10.4/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 file <c>releases/mysystem.rel</c>.</item>
    </list>
  </section>

  <section>
    <marker id="start"/>
    <title>Starting a Target System</title>
    <p>Now we have a target system that can be started in various ways.
      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>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:</p>
      <list type="ordered">
	<item><c>bin/erl</c> (obtained from
	<c>erts-5.10.4/bin/erl.src</c>)</item>
	<item><c>bin/start.boot</c> (a copy of
	<c>plain.boot</c>)</item>
      </list>
    <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 flag <c>-boot</c> 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 runtime to work.</p>
    <p>To start an <em>embedded target system</em>, the shell script
      <c>bin/start</c> is used. The 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>, 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.</p>
    <p><c>run_erl</c> 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 (<c>to_erl</c>).</p>
    <p><c>start_erl</c> requires:</p>
     <list type="ordered">
	<item>The root directory (<c>"/usr/local/erl-target"</c>)</item>
	<item>The releases directory
	(<c>"/usr/local/erl-target/releases"</c></item>
	<item>The location of the file <c>start_erl.data</c></item>
     </list>
     <p>It performs the following:</p>
     <list type="ordered">
	<item>Reads the runtime system version (<c>"5.10.4"</c>) and
	release version (<c>"FIRST"</c>) from the file
	<c>start_erl.data</c>.</item>
	<item>Starts the runtime system of the version found.</item>
	<item>Provides the flag <c>-boot</c> specifying the boot
	file of the release version found
	(<c>"releases/FIRST/start.boot"</c>).</item>
     </list>
    <p><c>start_erl</c> also assumes that there is <c>sys.config</c>
      in the release version directory (<c>"releases/FIRST/sys.config"</c>).
      That is the topic of the next section.</p>
    <p>The <c>start_erl</c> shell script is normally not to be
      altered by the user.</p>
  </section>

  <section>
    <title>System Configuration Parameters</title>
    <p>As was mentioned in the previous section, <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
      file, the system start fails. Such a file must therefore
      also be added.</p>
    <p>If you have system configuration data that is neither
      file-location-dependent nor site-dependent, it can be convenient
      to create <c>sys.config</c> early, so it becomes part of
      the target system tar file created by
      <c>target_system:create/1</c>. In fact, if you in the
      current directory create not only the file <c>mysystem.rel</c>,
      but also file <c>sys.config</c>, the latter file is tacitly
      put in the appropriate directory.</p>
    <p>However, it can also be convenient to replace variables in
       within a <c>sys.config</c> on the target after unpacking but
       before running the release. If you have a <c>sys.config.src</c>
       it will be included and is not required to be a valid Erlang term
       file like <c>sys.config</c>. Before running the release you must
       have a valid <c>sys.config</c> in the same directory, so using
       <c>sys.config.src</c> requires having some tool to populate what is
       needed and write <c>sys.config</c> to disk before booting the release.</p>
  </section>

  <section>
    <title>Differences From the Install Script</title>
    <p>The previous <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>Creating the Next Version</title>
    <p>In this example the Pea application has been changed, and
      so are the applications ERTS, Kernel, STDLIB
      and SASL.</p>
    <p><em>Step 1.</em> Create the file <c>.rel</c>:</p>
    <code type="none">
%% mysystem2.rel
{release,
 {"MYSYSTEM", "SECOND"},
 {erts, "6.0"},
 [{kernel, "3.0"},
  {stdlib, "2.0"},
  {sasl, "2.4"},
  {pea, "2.0"}]}.</code>
    <p><em>Step 2.</em> Create the application upgrade file (see the
      <seefile marker="sasl:appup">appup(4)</seefile> manual page in
      SASL) for Pea, for example:</p>
    <code type="none">
%% pea.appup
{"2.0",
 [{"1.0",[{load_module,pea_lib}]}],
 [{"1.0",[{load_module,pea_lib}]}]}.</code>
    <p><em>Step 3.</em> From the directory where the file
      <c>mysystem2.rel</c> resides, start the Erlang/OTP system,
      giving the path to the new version of Pea:</p>
    <pre>
os> <input>erl -pa /home/user/target_system/myapps/pea-2.0/ebin</input></pre>
    <p><em>Step 4.</em> Create the release upgrade file (see the
      <seefile marker="sasl:relup">relup(4)</seefile> manual page in
      SASL):</p>
    <pre>
1> <input>systools:make_relup("mysystem2",["mysystem"],["mysystem"],
    [{path,["/home/user/target_system/myapps/pea-1.0/ebin",
    "/my/old/erlang/lib/*/ebin"]}]).</input></pre>
    <p>Here <c>"mysystem"</c> is the base release and
      <c>"mysystem2"</c> is the release to upgrade to.</p>
    <p>The <c>path</c> 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.)</p>
    <p><em>Step 5.</em> Create the new release:</p>
    <pre>
2> <input>target_system:create("mysystem2").</input></pre>
    <p>Given that the file <c>relup</c> generated in Step 4 is
      now located in the current directory, it is automatically
      included in the release package.</p>
  </section>

  <section>
    <title>Upgrading the Target System</title>
    <p>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
      <c>-heart</c> option, allowing automatic restart of the node.
      For more information, see  <seeguide marker="#start">
      Starting a Target System</seeguide>.</p>
    <p>We add <c>-heart</c> to <c>bin/start</c>:</p>
    <code type="none">
#!/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"</code>
    <p>We use the simplest possible <c>sys.config</c>, which we
      store in <c>releases/FIRST</c>:</p>
    <code type="none">
%% sys.config
[].</code>
    <p>Finally, to prepare the upgrade, we must put the new
      release package in the <c>releases</c> directory of the first
      target system:</p>
    <pre>
os> <input>cp mysystem2.tar.gz /usr/local/erl-target/releases</input></pre>
    <p>Assuming that the node has been started as follows:</p>
    <pre>
os> <input>/usr/local/erl-target/bin/start</input></pre>
    <p>It can be accessed as follows:</p>
    <pre>
os> <input>/usr/local/erl-target/bin/to_erl /tmp/erlang.pipe.1</input></pre>
    <p>Logs can be found in
      <c>/usr/local/erl-target/log</c>. This directory is specified as
      an argument to <c>run_erl</c>in the start script listed above.</p>
    <p>
      <em>Step 1.</em> Unpack the release:
    </p>
    <pre>
1> <input>{ok,Vsn} = release_handler:unpack_release("mysystem2").</input></pre>
    <p>
      <em>Step 2.</em> Install the release:
    </p>
    <pre>
2> <input>release_handler:install_release(Vsn).</input>
{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]</pre>
    <p>
      The above return value and output after the call to
      <c>release_handler:install_release/1</c> means that the
      <c>release_handler</c> has restarted the node by using
      <c>heart</c>. This is always done when the upgrade involves
      a change of the applications ERTS, Kernel,
      STDLIB, or SASL. For more information, see
      <seeguide marker="upgrade">
      Upgrade when Erlang/OTP has Changed</seeguide>.
    </p>
    <p>
      The node is accessible through a new pipe:
    </p>
    <pre>
os> <input>/usr/local/erl-target/bin/to_erl /tmp/erlang.pipe.2</input></pre>
    <p>
      Check which releases there are in the system:
    </p>
    <pre>
1> <input>release_handler:which_releases().</input>
[{"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}]</pre>
    <p>
      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.
    </p>
    <p>
      <em>Step 3.</em> Make the new release permanent:
    </p>
    <pre>
2> <input>release_handler:make_permanent("SECOND").</input></pre>
    <p>
      Check the releases again:
    </p>
    <pre>
3> <input>release_handler:which_releases().</input>
[{"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}]</pre>
  <p>
    We see that the new release version is <c>permanent</c>, so
    it would be safe to restart the node.</p>
  </section>

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