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<chapter>
<header>
<copyright>
<year>2002</year><year>2013</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
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Software distributed under the License is distributed on an "AS IS"
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<title>Creating a First Target System</title>
<prepared>Peter Högfeldt</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>
