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<?xml version="1.0" encoding="latin1" ?>
<!DOCTYPE comref SYSTEM "comref.dtd">
<holder>Ericsson AB. All Rights Reserved.</holder>
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
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.
<prepared>Patrik Nyblom</prepared>
<comsummary>Run the Erlang emulator as a service on Windows NT&reg;</comsummary>
<p>This utility is specific to Windows NT/2000/XP&reg; (and subsequent 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 in. The
emulator started in this way can be manipulated through the
Windows&reg; services applet in a manner similar to other
<p>Note that erlsrv is not a general service utility for Windows, but designed for embedded Erlang systems.</p>
<p>As well as being the actual service, erlsrv also provides a
command line interface for registering, changing, starting and
stopping services.</p>
<p>To manipulate services, the logged in user should 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 &reg;.</p>
<p>The processes created by the service can, as opposed to normal
services, be "killed" with the task manager. Killing a emulator
that is started by a service will trigger the "OnFail" action
specified for that service, which may be a reboot.</p>
<p>The following parameters may be specified for each Erlang
<list type="bulleted">
<p><c><![CDATA[StopAction]]></c>: This 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 will report a running state to the service
<p><c><![CDATA[OnFail]]></c>: This can be either of <c><![CDATA[reboot]]></c>,
<c><![CDATA[restart]]></c>, <c><![CDATA[restart_always]]></c> or <c><![CDATA[ignore]]></c> (the
default). In case of <c><![CDATA[reboot]]></c>, the NT system is
rebooted whenever the emulator stops (a more simple form of
watchdog), this could be useful for less critical systems,
otherwise use the heart functionality to accomplish
this. The restart value 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 completely hang the NT
system. <c><![CDATA[restart_always]]></c> is similar to restart, but
does not try to detect cyclic restarts, it is expected that
some other mechanism is present to avoid the problem. The
default (ignore) just reports the service as stopped to the
service manager whenever it fails, it has to be manually
<p>On a system where release handling is
used, this should always be set to <c><![CDATA[ignore]]></c>. Use
<c><![CDATA[heart]]></c> to restart the service on failure instead.</p>
<p><c><![CDATA[Machine]]></c>: The location of the Erlang
emulator. The default is the <c><![CDATA[erl.exe]]></c> located in the
same directory as erlsrv.exe. Do not specify <c><![CDATA[werl.exe]]></c>
as this emulator, it will not work.</p>
<p>If the system
uses release handling, this should be set to a program
similar to <c><![CDATA[start_erl.exe]]></c>.</p>
<p><c><![CDATA[Env]]></c>: Specifies an <em>additional</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>
<p><c><![CDATA[WorkDir]]></c>: The working directory for the Erlang
emulator, has to be on a local drive (there are no network
drives 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>
<p><c><![CDATA[Priority]]></c>: The process priority of the emulator,
this can be one of <c><![CDATA[realtime]]></c>, <c><![CDATA[high]]></c>, <c><![CDATA[low]]></c>
or <c><![CDATA[default]]></c> (the default). Real-time priority is not
recommended, the machine will possibly be inaccessible to
interactive users. High priority could be used if two Erlang
nodes should reside on one dedicated system and one should
have precedence over the other. Low process priority may be
used if interactive performance should not be affected by
the emulator process.</p>
<p><c><![CDATA[SName or Name]]></c>: 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) node-name.</p>
<p><c><![CDATA[DebugType]]></c>: Can be one of <c><![CDATA[none]]></c> (default),
<c><![CDATA[new]]></c>, <c><![CDATA[reuse]]></c> or <c><![CDATA[console]]></c>.
Specifies that output from the Erlang shell should 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 between 1 and 99. The log-file is placed in the
working directory of the service (as specified in WorkDir). The
<c><![CDATA[reuse]]></c> option always reuses the same log file
(&lt;servicename&gt;<c><![CDATA[.debug]]></c>) and the <c><![CDATA[new]]></c> option
uses a separate log file for every invocation of the service
(&lt;servicename&gt;<c><![CDATA[.debug.]]></c>&lt;N&gt;). The <c><![CDATA[console]]></c>
option opens an interactive Windows&reg; console window for
the Erlang shell of the service. The <c><![CDATA[console]]></c> option
disables the <c><![CDATA[StopAction]]></c> and a service started with an
interactive console window will not survive logouts,
<c><![CDATA[OnFail]]></c> actions do not work with debug-consoles either.
If no <c><![CDATA[DebugType]]></c> is specified (<c><![CDATA[none]]></c>), the
output of the Erlang shell is discarded.</p>
<p>The <c><![CDATA[console]]></c><c><![CDATA[DebugType]]></c> is <em>not in any way</em>
intended for production. It is <em>only</em> a convenient way to
debug Erlang services during development. The <c><![CDATA[new]]></c> and
<c><![CDATA[reuse]]></c> options might seem convenient to have in a
production system, but one has to take into account that the
logs will grow indefinitely during the systems lifetime and
there is no way, short of restarting the service, to
truncate those logs. In short, the <c><![CDATA[DebugType]]></c> is
intended for debugging only. Logs during production are
better produced with the standard Erlang logging
<p><c><![CDATA[Args]]></c>: Additional arguments passed 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> (StopActions 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 init:boot() (<c><![CDATA[-s]]></c>).</p>
<p><c><![CDATA[InternalServiceName]]></c>: Specifies the Windows&reg; internal service name (not the display name, which is the one <c>erlsrv</c> uses to identify the service).</p>
<p>This internal name can not 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 defaut if release-handling is to be used for the application.</p>
<p>The internal service name can be seen in the Windows&reg; service manager if viewing <c>Properties</c> for an erlang service.</p>
<p><c><![CDATA[Comment]]></c>: A textual comment describing the service. Not mandatory, but shows up as the service description in the Windows&reg; service manager.</p>
<p> <marker id="001"></marker>
The naming of the service in a system that
uses release handling has to follow the convention
<em>NodeName</em>_<em>Release</em>, where <em>NodeName</em> is
the first part of the Erlang nodename (up to, but not including
the "@") and <em>Release</em> is the current release of the
<name>erlsrv {set | add} &lt;service-name> [&lt;service options>]</name>
<fsummary>Add or modify an Erlang service</fsummary>
<p>The set and add commands adds or modifies a Erlang service
respectively. The simplest form of an add command would be
completely without options in which case all default values
(described above) apply. The service name is mandatory.</p>
<p>Every option can be given without parameters, in which case
the default value is applied. Values to the options are
supplied <em>only</em> when the default should not be used
(i.e. <c><![CDATA[erlsrv set myservice -prio -arg]]></c> sets the
default priority and removes all arguments).</p>
<p>The following service options are currently available:</p>
<tag>-st[opaction] [&lt;erlang shell command&gt;]</tag>
<item>Defines the StopAction, the command given to the Erlang
shell when the service is stopped. Default is none.</item>
<tag>-on[fail] [{reboot | restart | restart_always}]</tag>
<item>Specifies the action to take when the Erlang emulator
stops unexpectedly. Default is to ignore.</item>
<tag>-m[achine] [&lt;erl-command&gt;]</tag>
<item>The complete path to the Erlang emulator, never use the
werl program for this. Default is the <c><![CDATA[erl.exe]]></c> in the
same directory as <c><![CDATA[erlsrv.exe]]></c>. When release handling
is used, this should be set to a program similar to
<tag>-e[nv] [&lt;variable&gt;[=&lt;value&gt;]] ...</tag>
<item>Edits the environment block for the service. Every
environment variable specified will add 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
<seealso marker="#002">below</seealso>).</item>
<tag>-w[orkdir] [&lt;directory&gt;]</tag>
<item>The initial working directory of the Erlang
emulator. Default is the system directory.</item>
<tag>-p[riority] [{low|high|realtime}]</tag>
<item>The priority of the Erlang emulator. The default is the
Windows&reg; default priority.</item>
<tag>{-sn[ame] | -n[ame]} [&lt;node-name&gt;]</tag>
<item>The node-name of the Erlang machine, distribution is
mandatory. Default is <c><![CDATA[-sname <service name>]]></c>.
<tag>-d[ebugtype] [{new|reuse|console}]</tag>
<item>Specifies where shell output should be sent,
default is that shell output is discarded.
To be used only for debugging.</item>
<tag>-ar[gs] [&lt;limited erl arguments&gt;]</tag>
<item>Additional 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 additional
arguments. Remember that the services cookie file is not
necessarily the same as the interactive users. The service
runs as the local administrator. All arguments should be given
together in one string, use double quotes (") to give an
argument string containing spaces and use quoted quotes (\")
to give an quote within the argument string if
<tag>-i[nternalservicename] [&lt;internal name&gt;]</tag>
<item><em>Only</em> allowed for <c>add</c>. Specifies a
Windows&reg; internal service name for the service, which by
default is set to something unique (prefixed with the
original service name) by erlsrv 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 erlsrv.</item>
<tag>-c[omment] [&lt;short description&gt;]</tag>
<item>Specifies a textual comment describing the
service. This comment will show upp as the service description
in the Windows&reg; service manager.</item>
<name>erlsrv {start | start_disabled | stop | disable | enable} &lt;service-name></name>
<fsummary>Manipulate the current service status.</fsummary>
<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. The <c><![CDATA[start]]></c> and
<c><![CDATA[stop]]></c> commands communicates
with the service manager for stopping and starting a
service. The commands wait until the service is actually
stopped or started. When disabling a service, it is not
stopped, the disabled state will not take effect until the
service actually is stopped. Enabling a service sets it in
automatic mode, that 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's enabled/disabled or started/stopped. It
does this by first enabling it (regardless of if it's enabled
or not), then starting it (if it's not already started) and
then disabling it. The result will be 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
<name>erlsrv remove &lt;service-name&gt;</name>
<fsummary>Remove the service.</fsummary>
<p>This command removes the service completely with all its registered
options. It will be stopped before it is removed.</p>
<name>erlsrv list [&lt;service-name&gt;]</name>
<fsummary>List all Erlang services or all options for one service.</fsummary>
<p>If no service name is supplied, a brief listing of all Erlang services
is presented. If a service-name is supplied, all options for that
service are presented.</p>
<name>erlsrv help</name>
<fsummary>Display a brief help text</fsummary>
<p> <marker id="002"></marker>
The environment of an Erlang machine started
as a service will contain two special variables,
<c><![CDATA[ERLSRV_SERVICE_NAME]]></c>, which is the name of the service that
started the machine and <c><![CDATA[ERLSRV_EXECUTABLE]]></c> which is the
full path to the <c><![CDATA[erlsrv.exe]]></c> that can be used to manipulate
the service. This will come in handy when defining a heart command for
your service. A command file for restarting a service will
simply look like this:</p>
<code type="none"><![CDATA[
@echo off
<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 below).</p>
<title>PORT PROGRAMS</title>
<p>When a program runs in
the service context, it has to handle the control events that is
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 which runs in the console subsystem
(normal for port programs) uses the win32 function
<c><![CDATA[SetConsoleCtrlHandler]]></c> to a control handler that returns
TRUE in answer to the <c><![CDATA[CTRL_LOGOFF_EVENT]]></c>. Other
applications just forward <c><![CDATA[WM_ENDSESSION]]></c> and
<c><![CDATA[WM_QUERYENDSESSION]]></c> to the default window procedure. Here
is a brief example in C of how to set the console control
<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){
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
(DWORD) 2)){
** Actually set the control handler
SetConsoleCtrlHandler(&service_aware_handler, TRUE);
} ]]></code>
<p>Even though the options are described in a Unix-like format, the case of
the options or commands is not relevant, and the "/" character for options
can be used as well as the "-" character. </p>
<p>Note that the program resides in the emulators
<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 easily manipulate the Erlang services, put
the <c><![CDATA[<erlang_root>\erts-<version>\bin]]></c> directory in
the path instead of <c><![CDATA[<erlang_root>\bin]]></c>. The erlsrv 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. It is also worth mentioning again that the name of the
service is significant (see <seealso marker="#001">above</seealso>).</p>
<title>SEE ALSO</title>
<p>start_erl(1), release_handler(3)</p>
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