erlsrv - Run the Erlang emulator as a service on Windows
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.
Notice that erlsrv is not a general service utility for
Windows, but designed for embedded Erlang systems.
erlsrv also provides a command-line interface for
registering, changing, starting, and stopping services.
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.
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.
The following parameters can be specified for each Erlang
service:
- StopAction:
- Tells erlsrv 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.
- OnFail:
- Can be one of the following:
- reboot:
- 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.
- restart:
- 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.
- restart_always:
- Similar to restart, but does not try to detect cyclic restarts; it
is expected that some other mechanism is present to avoid the
problem.
- ignore (the
default):
- Reports the service as stopped to the service manager whenever it fails;
it must be manually restarted.
On a system where release handling is used, this is always to be
set to ignore. Use heart to restart the service on failure
instead.
- Machine:
- The location of the Erlang emulator. The default is the erl.exe
located in the same directory as erlsrv.exe.
If the system uses release handling, this is to be set to a
program similar to start_erl.exe.
- Env:
- Specifies an extra 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.
- WorkDir:
- 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 %SystemDrive%%SystemPath%. Debug log
files will be placed in this directory.
- Priority:
- The process priority of the emulator. Can be one of the following:
- realtime:
- Not recommended, as the machine will possibly be inaccessible to
interactive users.
- high:
- Can be used if two Erlang nodes are to reside on one dedicated system and
one is to have precedence over the other.
- low:
- Can be used if interactive performance is not to be affected by the
emulator process.
- default (the
default>:
-
- SName or
Name:
- 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.
- DebugType:
- Specifies that output from the Erlang shell is to be sent to a "debug
log". The log file is named <servicename>.debug or
<servicename>.debug.<N>, where <N> is an integer
from 1 through 99. The log file is placed in the working directory of the
service (as specified in WorkDir).
Can be one of the following:
- new:
- Uses a separate log file for every invocation of the service
(<servicename>.debug.<N>).
- reuse:
- Reuses the same log file (<servicename>.debug).
- console:
- Opens an interactive Windows console window for the Erlang shell of the
service. Automatically disables the StopAction. A service started
with an interactive console window does not survive logouts. OnFail
actions do not work with debug consoles either.
- none (the
default):
- The output of the Erlang shell is discarded.
Note:
The
console option is
not intended for production. It is
only a convenient way to debug Erlang services during development.
The new and reuse 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.
In short, the DebugType is intended for debugging only.
Logs during production are better produced with the standard Erlang logging
facilities.
- Args:
- Passes extra arguments to the emulator startup program erl.exe (or
start_erl.exe). Arguments that cannot be specified here are
-noinput (StopActions would not work), -name, and
-sname (they are specified in any way). The most common use is for
specifying cookies and flags to be passed to init:boot()
(-s).
- InternalServiceName:
- Specifies the Windows-internal service name (not the display name, which
is the one erlsrv uses to identify the service).
This internal name cannot be changed, it is fixed even if the
service is renamed. erlsrv 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.
The internal service name can be seen in the Windows service
manager if viewing Properties for an Erlang service.
- A textual comment describing the service. Not mandatory, but shows up as
the service description in the Windows service manager.
The naming of the service in a system that uses release handling
must follow the convention NodeName_Release, where
NodeName is the first part of the Erlang node name (up to, but not
including the "@") and Release is the current release of
the application.
erlsrv {set | add} <service-name> [<service
options>]
The set and add commands modifies or adds an Erlang
service, respectively. The simplest form of an add command is without
any options in which case all default values (described above) apply. The
service name is mandatory.
Every option can be specified without parameters, the default
value is then applied. Values to the options are supplied only when
the default is not to be used. For example, erlsrv set myservice -prio
-arg sets the default priority and removes all arguments.
Service options:
- -st[opaction]
[<erlang shell command>]:
- Defines the StopAction, the command given to the Erlang shell when
the service is stopped. Default is none.
- -on[fail]
[{reboot | restart | restart_always}]:
- The action to take when the Erlang emulator stops unexpectedly. Default is
to ignore.
- -m[achine]
[<erl-command>]:
- The complete path to the Erlang emulator. Never use the werl
program for this. Defaults to the erl.exe in the same directory as
erlsrv.exe. When release handling is used, this is to be set to a
program similar to start_erl.exe.
- -e[nv]
[<variable>[=<value>]] ...:
- 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 <variable>=<value> and
deleted from the list by specifying <variable> alone. The
environment block is automatically sorted. Any number of -env
options can be specified in one command. Default is to use the system
environment block unmodified (except for two additions, see section
Environment below).
- -w[orkdir]
[<directory>]:
- The initial working directory of the Erlang emulator. Defaults to the
system directory.
- -p[riority]
[{low|high|realtime}]:
- The priority of the Erlang emulator. Default to the Windows default
priority.
- {-sn[ame] | -n[ame]} [<node-name>]:
- The node name of the Erlang machine. Distribution is mandatory. Defaults
to -sname <service name>.
- -d[ebugtype]
[{new|reuse|console}]:
- Specifies where shell output is to be sent. Default is that shell output
is discarded. To be used only for debugging.
- -ar[gs] [<limited
erl arguments>]:
- Extra arguments to the Erlang emulator. Avoid -noinput,
-noshell, and -sname/-name. 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.
- -i[nternalservicename]
[<internal name>]:
- Only allowed for add. Specifies a Windows-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
not recommended if release handling is to be performed. The
internal service name cannot be changed once the service is created. The
internal name is not to be confused with the ordinary service name,
which is the name used to identify a service to erlsrv.
- -c[omment]
[<short description>]:
- Specifies a textual comment describing the service. This comment shows up
as the service description in the Windows service manager.
erlsrv {start | start_disabled | stop | disable | enable}
<service-name>
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 start and stop 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.
The start_disabled 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 start_disabled and the sequence enable,
start, and disable is that all other erlsrv commands
are locked out during the sequence of operations in start_disable,
making the operation atomic from an erlsrv user's point of view.
erlsrv remove <service-name>
Removes the service completely with all its registered options. It
is stopped before it is removed.
erlsrv list [<service-name>]
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.
erlsrv help
Displays a brief help text.
The environment of an Erlang machine started as a service contains
two special variables:
- ERLSRV_SERVICE_NAME:
- The name of the service that started the machine.
- ERLSRV_EXECUTABLE:
- The full path to the erlsrv.exe, which can be used to manipulate
the service. This comes in handy when defining a heart command for your
service.
A command file for restarting a service looks as follows:
@echo off
%ERLSRV_EXECUTABLE% stop %ERLSRV_SERVICE_NAME%
%ERLSRV_EXECUTABLE% start %ERLSRV_SERVICE_NAME%
This command file is then set as heart command.
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).
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 SetConsoleCtrlHandler to
register a control handler that returns true in answer to the
CTRL_LOGOFF_EVENT and CTRL_SHUTDOWN_EVENT events. Other
applications only forward WM_ENDSESSION and WM_QUERYENDSESSION
to the default window procedure.
A brief example in C of how to set the console control
handler:
#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);
}
}
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.
Notice that the program resides in the emulator's bin
directory, not in the bin 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.
To manipulate the Erlang services easily, put the
<erlang_root>\erts-<version>\bin directory in the path
instead of <erlang_root>\bin. The erlsrv program can be
found from inside Erlang by using the os:find_executable/1 Erlang
function.
For release handling to work, use start_erl as the Erlang
machine. As stated above, the service name is significant.
start_erl(1), release_handler(3)