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<!DOCTYPE comref SYSTEM "comref.dtd">

<comref>
  <header>
    <copyright>
      <year>1998</year><year>2021</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
      limitations under the License.

    </legalnotice>

    <title>erlsrv</title>
    <prepared>Patrik Nyblom</prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date>1998-04-29</date>
    <rev></rev>
    <file>erlsrv.xml</file>
  </header>
  <com>erlsrv</com>
  <comsummary>Run the Erlang emulator as a service on Windows</comsummary>
  <description>
    <p>This utility is specific to Windows NT/2000/XP (and later
      versions of Windows). It allows Erlang
      emulators to run as services on the Windows system, allowing embedded
      systems to start without any user needing to log on. The
      emulator started in this way can be manipulated through the
      Windows services applet in a manner similar to other services.</p>

    <p>Notice that <c>erlsrv</c> is not a general service utility for Windows,
      but designed for embedded Erlang systems.</p>

    <p><c>erlsrv</c> also provides a command-line interface for registering,
      changing, starting, and stopping services.</p>

    <p>To manipulate services, the logged on user is to have
      administrator privileges on the machine. The Erlang machine
      itself is (default) run as the local administrator. This can be
      changed with the Services applet in Windows.</p>

    <p>The processes created by the service can, as opposed to normal
      services, be "killed" with the task manager. Killing an emulator
      that is started by a service triggers the "OnFail" action
      specified for that service, which can be a reboot.</p>

    <p>The following parameters can be specified for each Erlang service:</p>

    <taglist>
      <tag><c><![CDATA[StopAction]]></c></tag>
      <item>
        <p>Tells <c><![CDATA[erlsrv]]></c> how to stop
          the Erlang emulator. Default is to kill it (Win32
          TerminateProcess), but this action can specify any Erlang
          shell command that will be executed in the emulator to make
          it stop. The emulator is expected to stop within 30 seconds
          after the command is issued in the shell. If the emulator is
          not stopped, it reports a running state to the service
          manager.</p>
      </item>
      <tag><c><![CDATA[OnFail]]></c></tag>
      <item>
        <p>Can be one of the following:</p>
        <taglist>
          <tag><c><![CDATA[reboot]]></c></tag>
          <item>
            <p>The Windows system is rebooted whenever the emulator stops
              (a more simple form of watchdog). This can be useful for
              less critical systems, otherwise use the heart functionality
              to accomplish this.</p>
          </item>
          <tag><c><![CDATA[restart]]></c></tag>
          <item>
            <p>Makes the Erlang emulator be
              restarted (with whatever parameters are registered for the
              service at the occasion) when it stops. If the emulator
              stops again within 10 seconds, it is not restarted to avoid
              an infinite loop, which could hang the Windows system.</p>
          </item>
          <tag><c><![CDATA[restart_always]]></c></tag>
          <item>
            <p>Similar to <c><![CDATA[restart]]></c>, but does
              not try to detect cyclic restarts; it is expected that
              some other mechanism is present to avoid the problem.</p>
          </item>
          <tag><c><![CDATA[ignore]]></c> (the default)</tag>
          <item>
            <p>Reports the service as stopped to the service manager
              whenever it fails; it must be manually restarted.</p>
          </item>
        </taglist>
        <p>On a system where release handling is used,
          this is always to be set to <c><![CDATA[ignore]]></c>. Use
          <c><![CDATA[heart]]></c> to restart the service on failure
          instead.</p>
      </item>
      <tag><c><![CDATA[Machine]]></c></tag>
      <item>
        <p>The location of the Erlang emulator.
          The default is the <c><![CDATA[erl.exe]]></c> located in the same
          directory as <c>erlsrv.exe</c>. Do not specify
          <c><![CDATA[werl.exe]]></c> as this emulator, it will not work.</p>
        <p>If the system uses release handling, this is to be set to a
          program similar to <c><![CDATA[start_erl.exe]]></c>.</p>
      </item>
      <tag><c><![CDATA[Env]]></c></tag>
      <item>
        <p>Specifies an <em>extra</em> environment
          for the emulator. The environment variables specified
          here are added to the system-wide environment block that is
          normally present when a service starts up. Variables present
          in both the system-wide environment and in the service
          environment specification will be set to the value specified
          in the service.</p>
      </item>
      <tag><c><![CDATA[WorkDir]]></c></tag>
      <item>
        <p>The working directory for the Erlang emulator.
          Must be on a local drive (no network drives are mounted when a
          service starts). Default working directory for services is
          <c><![CDATA[%SystemDrive%%SystemPath%]]></c>.
          Debug log files will be placed in this directory.</p>
      </item>
      <tag><c><![CDATA[Priority]]></c></tag>
      <item>
        <p>The process priority of the emulator. Can be one of the
          following:</p>
        <taglist>
          <tag><c><![CDATA[realtime]]></c></tag>
          <item>
            <p>Not recommended, as the machine will possibly be
              inaccessible to interactive users.</p>
          </item>
          <tag><c><![CDATA[high]]></c></tag>
          <item>
            <p>Can be used if two Erlang nodes are to reside on one dedicated
              system and one is to have precedence over the other.</p>
          </item>
          <tag><c><![CDATA[low]]></c></tag>
          <item>
            <p>Can be used if interactive performance is not to be affected
              by the emulator process.</p>
          </item>
          <tag><c><![CDATA[default]]></c> (the default></tag>
          <item>
          </item>
        </taglist>
      </item>
      <tag><c><![CDATA[SName or Name]]></c></tag>
      <item>
        <p>Specifies the short or long
          node name of the Erlang emulator. The Erlang services are
          always distributed. Default is to use the service name as
          (short) nodename.</p>
      </item>
      <tag><c><![CDATA[DebugType]]></c></tag>
      <item>
        <p>Specifies that output from the Erlang shell is to be
          sent to a "debug log". The log file is named
          &lt;servicename&gt;<c><![CDATA[.debug]]></c> or
          &lt;servicename&gt;<c><![CDATA[.debug.]]></c>&lt;N&gt;,
          where &lt;N&gt; is an integer from 1 through 99.
          The log file is placed in the working directory of the
          service (as specified in <c>WorkDir</c>).</p>
        <p>Can be one of the following:</p>
        <taglist>
          <tag><c><![CDATA[new]]></c></tag>
          <item>
            <p>Uses a separate log file for every invocation of the service
              (&lt;servicename&gt;<c><![CDATA[.debug.]]></c>&lt;N&gt;).</p>
          </item>
          <tag><c><![CDATA[reuse]]></c></tag>
          <item>
            <p>Reuses the same log file
              (&lt;servicename&gt;<c><![CDATA[.debug]]></c>).</p>
          </item>
          <tag><c><![CDATA[console]]></c></tag>
          <item>
            <p>Opens an interactive Windows console window for the Erlang
              shell of the service. Automatically disables the
              <c><![CDATA[StopAction]]></c>. A service started with an
              interactive console window does not survive logouts.
              <c><![CDATA[OnFail]]></c> actions do not work with
              debug consoles either.</p>
          </item>
          <tag><c><![CDATA[none]]></c> (the default)</tag>
          <item>
            <p>The output of the Erlang shell is discarded.</p>
          </item>
        </taglist>
        <note>
          <p>The <c><![CDATA[console]]></c> option is <em>not</em> intended
            for production. It is <em>only</em> a convenient way to debug
            Erlang services during development.</p>
          <p>The <c><![CDATA[new]]></c> and <c><![CDATA[reuse]]></c> options
            might seem convenient in a production system, but consider that
            the logs grow indefinitely during the system lifetime and cannot
            be truncated, except if the service is restarted.</p>
          <p>In short, the <c><![CDATA[DebugType]]></c> is
            intended for debugging only. Logs during production are
            better produced with the standard Erlang logging facilities.</p>
        </note>
      </item>
      <tag><c><![CDATA[Args]]></c></tag>
      <item>
        <p>Passes extra arguments to the emulator startup program
          <c><![CDATA[erl.exe]]></c> (or <c><![CDATA[start_erl.exe]]></c>).
          Arguments that cannot be specified here are
          <c><![CDATA[-noinput]]></c> (<c>StopActions</c> would not work),
          <c><![CDATA[-name]]></c>, and <c><![CDATA[-sname]]></c> (they are
          specified in any way). The most common use is for specifying cookies
          and flags to be passed to <c>init:boot()</c>
          (<c><![CDATA[-s]]></c>).</p>
      </item>
      <tag><c><![CDATA[InternalServiceName]]></c></tag>
      <item>
        <p>Specifies the Windows-internal service name (not the display name,
          which is the one <c>erlsrv</c> uses to identify the service).</p>
        <p>This internal name cannot be changed, it is fixed even if the
          service is renamed. <c>erlsrv</c> generates a unique internal name
          when a service is created. It is recommended to keep to the default
          if release handling is to be used for the application.</p>
        <p>The internal service name can be seen in the Windows service
          manager if viewing <c>Properties</c> for an Erlang service.</p>
      </item>
      <tag><c><![CDATA[Comment]]></c></tag>
      <item>
        <p>A textual comment describing the service. Not mandatory, but shows
          up as the service description in the Windows service manager.</p>
      </item>
    </taglist>

    <p><marker id="001"></marker>
      The naming of the service in a system that
      uses release handling must follow the convention
      <em>NodeName</em>_<em>Release</em>, where <em>NodeName</em> is
      the first part of the Erlang node name (up to, but not including
      the "@") and <em>Release</em> is the current release of the
      application.</p>
  </description>

  <funcs>
    <func>
      <name>erlsrv {set | add} &lt;service-name> [&lt;service options>]</name>
      <fsummary>Add or modify an Erlang service.</fsummary>
      <desc>
        <p>The <c>set</c> and <c>add</c> commands modifies or adds an Erlang
          service, respectively. The simplest form of an <c>add</c> command is
          without any options in which case all default values
          (described above) apply. The service name is mandatory.</p>
        <p>Every option can be specified without parameters, the
          default value is then applied. Values to the options are
          supplied <em>only</em> when the default is not to be used.
          For example, <c><![CDATA[erlsrv set myservice -prio -arg]]></c>
          sets the default priority and removes all arguments.</p>
        <p>Service options:</p>
        <taglist>
          <tag><c>-st[opaction] [&lt;erlang shell command&gt;]</c></tag>
          <item>
            <p>Defines the <c><![CDATA[StopAction]]></c>, the command given
              to the Erlang shell when the service is stopped.
              Default is none.</p>
          </item>
          <tag><c>-on[fail] [{reboot | restart | restart_always}]</c></tag>
          <item>
            <p>The action to take when the Erlang emulator
              stops unexpectedly. Default is to ignore.</p>
          </item>
          <tag><c>-m[achine] [&lt;erl-command&gt;]</c></tag>
          <item>
            <p>The complete path to the Erlang emulator. Never use the
              <c>werl</c> program for this. Defaults to the
              <c><![CDATA[erl.exe]]></c> in the same directory as
              <c><![CDATA[erlsrv.exe]]></c>. When release handling
              is used, this is to be set to a program similar to
              <c><![CDATA[start_erl.exe]]></c>.</p>
          </item>
          <tag><c>-e[nv] [&lt;variable&gt;[=&lt;value&gt;]] ...</c></tag>
          <item>
            <p>Edits the environment block for the service. Every
              environment variable specified is added to the system
              environment block. If a variable specified here has the same
              name as a system-wide environment variable, the specified
              value overrides the system-wide. Environment variables are
              added to this list by specifying
              &lt;variable&gt;=&lt;value&gt; and deleted from the list by
              specifying &lt;variable&gt; alone. The environment block is
              automatically sorted. Any number of <c><![CDATA[-env]]></c>
              options can be specified in one command. Default is to use the
              system environment block unmodified (except for two additions,
              see section <seecom marker="#002">Environment</seecom>
              below).</p>
          </item>
          <tag><c>-w[orkdir] [&lt;directory&gt;]</c></tag>
          <item>
            <p>The initial working directory of the Erlang
              emulator. Defaults to the system directory.</p>
          </item>
          <tag><c>-p[riority] [{low|high|realtime}]</c></tag>
          <item>
            <p>The priority of the Erlang emulator. Default to the
              Windows default priority.</p>
          </item>
          <tag><c>{-sn[ame] | -n[ame]} [&lt;node-name&gt;]</c></tag>
          <item>
            <p>The node name of the Erlang machine. Distribution is mandatory.
              Defaults to <c><![CDATA[-sname <service name>]]></c>.</p>
          </item>
          <tag><c>-d[ebugtype] [{new|reuse|console}]</c></tag>
          <item>
            <p>Specifies where shell output is to be sent.
              Default is that shell output is discarded.
              To be used only for debugging.</p>
          </item>
          <tag><c>-ar[gs] [&lt;limited erl arguments&gt;]</c></tag>
          <item>
            <p>Extra arguments to the Erlang emulator. Avoid
              <c><![CDATA[-noinput]]></c>, <c><![CDATA[-noshell]]></c>, and
              <c><![CDATA[-sname]]></c>/<c><![CDATA[-name]]></c>. Default is
              no extra arguments. Remember that the services cookie file is not
              necessarily the same as the interactive users. The service
              runs as the local administrator. Specify all arguments
              together in one string, use double quotes (") to specify an
              argument string containing spaces, and use quoted quotes (\")
              to specify a quote within the argument string if necessary.</p>
          </item>
          <tag><c>-i[nternalservicename] [&lt;internal name&gt;]</c></tag>
          <item>
            <p><em>Only</em> allowed for <c>add</c>. Specifies a
              Windows-internal service name for the service, which by
              default is set to something unique (prefixed with the
              original service name) by <c>erlsrv</c> when adding a new
              service. Specifying this is a purely cosmethic action and is
              <em>not</em> recommended if release handling is to be
              performed. The internal service name cannot be changed once
              the service is created. The internal name is <em>not</em> to
              be confused with the ordinary service name, which is the name
              used to identify a service to <c>erlsrv</c>.</p>
          </item>
          <tag><c>-c[omment] [&lt;short description&gt;]</c></tag>
          <item>
            <p>Specifies a textual comment describing the
              service. This comment shows up as the service description
              in the Windows service manager.</p>
          </item>
        </taglist>
      </desc>
    </func>

    <func>
      <name>erlsrv {start | start_disabled | stop | disable |
        enable} &lt;service-name></name>
      <fsummary>Manipulate the current service status.</fsummary>
      <desc>
        <p>These commands are only added for convenience, the normal
          way to manipulate the state of a service is through the
          control panels services applet.</p>
        <p>The <c><![CDATA[start]]></c> and
          <c><![CDATA[stop]]></c> commands communicates
          with the service manager for starting and stopping a
          service. The commands wait until the service is
          started or stopped. When disabling a service, it is not
          stopped, the disabled state does not take effect until the
          service is stopped. Enabling a service sets it in
          automatic mode, which is started at boot. This command cannot
          set the service to manual.</p>
        <p>The <c>start_disabled</c> command operates on a service
          regardless of if it is enabled/disabled or started/stopped. It
          does this by first enabling it (regardless of if it is enabled
          or not), then starting it (if not already started), and
          then disabling it. The result is a disabled but started
          service, regardless of its earlier state. This is useful for
          starting services temporarily during a release upgrade. The
          difference between using <c>start_disabled</c> and the
          sequence <c>enable</c>, <c>start</c>, and <c>disable</c> is
          that all other <c>erlsrv</c> commands are locked out during
          the sequence of operations in <c>start_disable</c>, making the
          operation atomic from an <c>erlsrv</c> user's point of view.</p>
      </desc>
    </func>

    <func>
      <name>erlsrv remove &lt;service-name&gt;</name>
      <fsummary>Remove the service.</fsummary>
      <desc>
        <p>Removes the service completely with all its registered
          options. It is stopped before it is removed.</p>
      </desc>
    </func>

    <func>
      <name>erlsrv list [&lt;service-name&gt;]</name>
      <fsummary>List all Erlang services or all options for one service.
      </fsummary>
      <desc>
        <p>If no service name is specified, a brief listing of all Erlang
          services is presented. If a service name is supplied, all options
          for that service are presented.</p>
      </desc>
    </func>

    <func>
      <name>erlsrv help</name>
      <fsummary>Display a brief help text.</fsummary>
      <desc>
        <p>Displays a brief help text.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Environment</title>
    <p><marker id="002"></marker>
      The environment of an Erlang machine started
      as a service contains two special variables:</p>

    <taglist>
      <tag><c><![CDATA[ERLSRV_SERVICE_NAME]]></c></tag>
      <item>The name of the service that started the machine.</item>
      <tag><c><![CDATA[ERLSRV_EXECUTABLE]]></c></tag>
      <item>The full path to the <c><![CDATA[erlsrv.exe]]></c>, which can be
        used to manipulate the service. This comes in handy when defining a
        heart command for your service.</item>
    </taglist>

    <p>A command file for restarting a service looks as follows:</p>

    <code type="none"><![CDATA[
@echo off
%ERLSRV_EXECUTABLE% stop %ERLSRV_SERVICE_NAME%
%ERLSRV_EXECUTABLE% start %ERLSRV_SERVICE_NAME%    ]]></code>

    <p>This command file is then set as heart command.</p>

    <p>The environment variables can also be used to detect that we
      are running as a service and make port programs react correctly
      to the control events generated on logout (see the next section).</p>
  </section>

  <section>
    <title>Port Programs</title>
    <p>When a program runs in
      the service context, it must handle the control events that are
      sent to every program in the system when the interactive user
      logs off. This is done in different ways for programs running in
      the console subsystem and programs running as window
      applications. An application running in the console subsystem
      (normal for port programs) uses the win32 function
      <c><![CDATA[SetConsoleCtrlHandler]]></c> to register a control handler
      that returns <c>true</c> in answer to the
      <c><![CDATA[CTRL_LOGOFF_EVENT]]></c>
      and <c><![CDATA[CTRL_SHUTDOWN_EVENT]]></c> events. Other applications
      only forward <c><![CDATA[WM_ENDSESSION]]></c> and
      <c><![CDATA[WM_QUERYENDSESSION]]></c> to the default window procedure.</p>

    <p>A brief example in C of how to set the console control handler:</p>

    <code type="none"><![CDATA[
#include <windows.h>
/*
** A Console control handler that ignores the log off events,
** and lets the default handler take care of other events.
*/
BOOL WINAPI service_aware_handler(DWORD ctrl){
    if(ctrl == CTRL_LOGOFF_EVENT)
        return TRUE;
    if(ctrl == CTRL_SHUTDOWN_EVENT)
        return TRUE;
    return FALSE;
}

void initialize_handler(void){
    char buffer[2];
    /*
     * We assume we are running as a service if this
     * environment variable is defined.
     */
    if(GetEnvironmentVariable("ERLSRV_SERVICE_NAME",buffer,
                              (DWORD) 2)){
        /*
        ** Actually set the control handler
        */
        SetConsoleCtrlHandler(&service_aware_handler, TRUE);
    }
}    ]]></code>
  </section>

  <section>
    <title>Notes</title>
    <p>Although the options are described in a Unix-like format, the case of
      the options or commands is not relevant, and both character "/" and "-"
      can be used for options.</p>

    <p>Notice that the program resides in the emulator's <c><![CDATA[bin]]></c>
      directory, not in the <c><![CDATA[bin]]></c> directory directly under
      the Erlang root. The reasons for this are the subtle problem of
      upgrading the emulator on a running system, where a new version of
      the runtime system should not need to overwrite existing (and probably
      used) executables.</p>

    <p>To manipulate the Erlang services easily, put
      the <c><![CDATA[<erlang_root>\erts-<version>\bin]]></c> directory in
      the path instead of <c><![CDATA[<erlang_root>\bin]]></c>. The
      <c>erlsrv</c> program can be found from inside Erlang by using the
      <c><![CDATA[os:find_executable/1]]></c> Erlang function.</p>

    <p>For release handling to work, use <c><![CDATA[start_erl]]></c> as the
      Erlang machine. As stated <seecom marker="#001">above</seecom>,
      the service name is significant.</p>
  </section>

  <section>
    <title>See Also</title>
    <p><seecom marker="start_erl"><c>start_erl(1)</c></seecom>,
      <seeerl marker="sasl:release_handler">
      <c>release_handler(3)</c></seeerl></p>
  </section>
</comref>
