mirror of
https://github.com/AuxXxilium/eudev.git
synced 2024-12-12 09:35:54 +07:00
16dad32e43
As you likely know, Arch Linux is in the process of moving to systemd. So I was reading through the various systemd docs and quickly became baffled by this new abbreviation "resp.", which I've never seen before in my English-mother-tongue life. Some quick Googling turned up a reference: <http://www.transblawg.eu/index.php?/archives/870-Resp.-and-other-non-existent-English-wordsNicht-existente-englische-Woerter.html> I guess it's a literal translation of the German "Beziehungsweise", but English doesn't work the same way. The word "respectively" is used exclusively to provide an ordering connection between two lists. E.g. "the prefixes k, M, and G refer to kilo-, mega-, and giga-, respectively." It is also never abbreviated to "resp." So the sentence "Sets the default output resp. error output for all services and sockets" makes no sense to a natural English speaker. This patch removes all instances of "resp." in the man pages and replaces them with sentences which are much more clear and, hopefully, grammatically valid. In almost all instances, it was simply replacing "resp." with "or," which the original author (Lennart?) could probably just do in the future. The only other instances of "resp." are in the src/ subtree, which I don't feel privileged to correct. Signed-off-by: Andrew Eikum <aeikum@codeweavers.com>
950 lines
52 KiB
XML
950 lines
52 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
|
|
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
|
|
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
|
|
|
|
<!--
|
|
This file is part of systemd.
|
|
|
|
Copyright 2010 Lennart Poettering
|
|
|
|
systemd is free software; you can redistribute it and/or modify it
|
|
under the terms of the GNU Lesser General Public License as published by
|
|
the Free Software Foundation; either version 2.1 of the License, or
|
|
(at your option) any later version.
|
|
|
|
systemd is distributed in the hope that it will be useful, but
|
|
WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
|
|
Lesser General Public License for more details.
|
|
|
|
You should have received a copy of the GNU Lesser General Public License
|
|
along with systemd; If not, see <http://www.gnu.org/licenses/>.
|
|
-->
|
|
|
|
<refentry id="daemon">
|
|
|
|
<refentryinfo>
|
|
<title>daemon</title>
|
|
<productname>systemd</productname>
|
|
|
|
<authorgroup>
|
|
<author>
|
|
<contrib>Developer</contrib>
|
|
<firstname>Lennart</firstname>
|
|
<surname>Poettering</surname>
|
|
<email>lennart@poettering.net</email>
|
|
</author>
|
|
</authorgroup>
|
|
</refentryinfo>
|
|
|
|
<refmeta>
|
|
<refentrytitle>daemon</refentrytitle>
|
|
<manvolnum>7</manvolnum>
|
|
</refmeta>
|
|
|
|
<refnamediv>
|
|
<refname>daemon</refname>
|
|
<refpurpose>Writing and packaging system daemons</refpurpose>
|
|
</refnamediv>
|
|
|
|
<refsect1>
|
|
<title>Description</title>
|
|
|
|
<para>A daemon is a service process that runs in the
|
|
background and supervises the system or provides
|
|
functionality to other processes. Traditionally,
|
|
daemons are implemented following a scheme originating
|
|
in SysV Unix. Modern daemons should follow a simpler
|
|
yet more powerful scheme (here called "new-style"
|
|
daemons), as implemented by
|
|
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>. This
|
|
manual page covers both schemes, and in
|
|
particular includes recommendations for daemons that
|
|
shall be included in the systemd init system.</para>
|
|
|
|
<refsect2>
|
|
<title>SysV Daemons</title>
|
|
|
|
<para>When a traditional SysV daemon
|
|
starts, it should execute the following steps
|
|
as part of the initialization. Note that these
|
|
steps are unnecessary for new-style daemons (see below),
|
|
and should only be implemented if compatibility
|
|
with SysV is essential.</para>
|
|
|
|
<orderedlist>
|
|
<listitem><para>Close all open file
|
|
descriptors except STDIN, STDOUT,
|
|
STDERR (i.e. the first three file
|
|
descriptors 0, 1, 2). This ensures
|
|
that no accidentally passed file
|
|
descriptor stays around in the daemon
|
|
process. On Linux this is best
|
|
implemented by iterating through
|
|
<filename>/proc/self/fd</filename>,
|
|
with a fallback of iterating from file
|
|
descriptor 3 to the value returned by
|
|
<function>getrlimit()</function> for
|
|
RLIMIT_NOFILE.</para></listitem>
|
|
|
|
<listitem><para>Reset all signal
|
|
handlers to their default. This is
|
|
best done by iterating through the
|
|
available signals up to the limit of
|
|
_NSIG and resetting them to
|
|
SIG_DFL.</para></listitem>
|
|
|
|
<listitem><para>Reset the signal mask
|
|
using
|
|
<function>sigprocmask()</function>.</para></listitem>
|
|
|
|
<listitem><para>Sanitize the
|
|
environment block, removing or
|
|
resetting environment variables that
|
|
might negatively impact daemon
|
|
runtime.</para></listitem>
|
|
|
|
<listitem><para>Call <function>fork()</function>,
|
|
to create a background
|
|
process.</para></listitem>
|
|
|
|
<listitem><para>In the child, call
|
|
<function>setsid()</function> to
|
|
detach from any terminal and create an
|
|
independent session.</para></listitem>
|
|
|
|
<listitem><para>In the child, call
|
|
<function>fork()</function> again, to
|
|
ensure the daemon can never re-acquire
|
|
a terminal again.</para></listitem>
|
|
|
|
<listitem><para>Call <function>exit()</function> in the
|
|
first child, so that only the second
|
|
child (the actual daemon process)
|
|
stays around. This ensures that the
|
|
daemon process is reparented to
|
|
init/PID 1, as all daemons should
|
|
be.</para></listitem>
|
|
|
|
<listitem><para>In the daemon process,
|
|
connect <filename>/dev/null</filename>
|
|
to STDIN, STDOUT,
|
|
STDERR.</para></listitem>
|
|
|
|
<listitem><para>In the daemon process,
|
|
reset the umask to 0, so that the file
|
|
modes passed to <function>open()</function>, <function>mkdir()</function> and
|
|
suchlike directly control the access
|
|
mode of the created files and
|
|
directories.</para></listitem>
|
|
|
|
<listitem><para>In the daemon process,
|
|
change the current directory to the
|
|
root directory (/), in order to avoid
|
|
that the daemon involuntarily
|
|
blocks mount points from being
|
|
unmounted.</para></listitem>
|
|
|
|
<listitem><para>In the daemon process,
|
|
write the daemon PID (as returned by
|
|
<function>getpid()</function>) to a
|
|
PID file, for example
|
|
<filename>/var/run/foobar.pid</filename>
|
|
(for a hypothetical daemon "foobar"),
|
|
to ensure that the daemon cannot be
|
|
started more than once. This must be
|
|
implemented in race-free fashion so
|
|
that the PID file is only updated when
|
|
at the same time it is verified that
|
|
the PID previously stored in the PID
|
|
file no longer exists or belongs to a
|
|
foreign process. Commonly some kind of
|
|
file locking is employed to implement
|
|
this logic.</para></listitem>
|
|
|
|
<listitem><para>In the daemon process,
|
|
drop privileges, if possible and
|
|
applicable.</para></listitem>
|
|
|
|
<listitem><para>From the daemon
|
|
process notify the original process
|
|
started that initialization is
|
|
complete. This can be implemented via
|
|
an unnamed pipe or similar
|
|
communication channel that is created
|
|
before the first
|
|
<function>fork()</function> and hence
|
|
available in both the original and the
|
|
daemon process.</para></listitem>
|
|
|
|
<listitem><para>Call
|
|
<function>exit()</function> in the
|
|
original process. The process that
|
|
invoked the daemon must be able to
|
|
rely on that this
|
|
<function>exit()</function> happens
|
|
after initialization is complete and
|
|
all external communication channels
|
|
are established and
|
|
accessible.</para></listitem>
|
|
</orderedlist>
|
|
|
|
<para>The BSD <function>daemon()</function> function should not be
|
|
used, as it implements only a subset of these steps.</para>
|
|
|
|
<para>A daemon that needs to provide
|
|
compatibility with SysV systems should
|
|
implement the scheme pointed out
|
|
above. However, it is recommended to make this
|
|
behavior optional and configurable via a
|
|
command line argument, to ease debugging as
|
|
well as to simplify integration into systems
|
|
using systemd.</para>
|
|
</refsect2>
|
|
|
|
<refsect2>
|
|
<title>New-Style Daemons</title>
|
|
|
|
<para>Modern services for Linux should be
|
|
implemented as new-style daemons. This makes it
|
|
easier to supervise and control them at
|
|
runtime and simplifies their
|
|
implementation.</para>
|
|
|
|
<para>For developing a new-style daemon none
|
|
of the initialization steps recommended for
|
|
SysV daemons need to be implemented. New-style
|
|
init systems such as systemd make all of them
|
|
redundant. Moreover, since some of these steps
|
|
interfere with process monitoring, file
|
|
descriptor passing and other functionality of
|
|
the init system it is recommended not to
|
|
execute them when run as new-style
|
|
service.</para>
|
|
|
|
<para>Note that new-style init systems
|
|
guarantee execution of daemon processes in
|
|
clean process contexts: it is guaranteed that
|
|
the environment block is sanitized, that the
|
|
signal handlers and mask is reset and that no
|
|
left-over file descriptors are passed. Daemons
|
|
will be executed in their own session, and
|
|
STDIN/STDOUT/STDERR connected to
|
|
<filename>/dev/null</filename> unless
|
|
otherwise configured. The umask is reset.</para>
|
|
|
|
<para>It is recommended for new-style daemons
|
|
to implement the following:</para>
|
|
|
|
<orderedlist>
|
|
<listitem><para>If SIGTERM is
|
|
received, shut down the daemon and
|
|
exit cleanly.</para></listitem>
|
|
|
|
<listitem><para>If SIGHUP is received,
|
|
reload the configuration files, if
|
|
this applies.</para></listitem>
|
|
|
|
<listitem><para>Provide a correct exit
|
|
code from the main daemon process, as
|
|
this is used by the init system to
|
|
detect service errors and problems. It
|
|
is recommended to follow the exit code
|
|
scheme as defined in the <ulink
|
|
url="http://refspecs.freestandards.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html">LSB
|
|
recommendations for SysV init
|
|
scripts</ulink>.</para></listitem>
|
|
|
|
<listitem><para>If possible and
|
|
applicable expose the daemon's control
|
|
interface via the D-Bus IPC system and
|
|
grab a bus name as last step of
|
|
initialization.</para></listitem>
|
|
|
|
<listitem><para>For integration in
|
|
systemd, provide a
|
|
<filename>.service</filename> unit
|
|
file that carries information about
|
|
starting, stopping and otherwise
|
|
maintaining the daemon. See
|
|
<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
|
|
for details.</para></listitem>
|
|
|
|
<listitem><para>As much as possible,
|
|
rely on the init system's
|
|
functionality to limit the access of
|
|
the daemon to files, services and
|
|
other resources. i.e. in the case of
|
|
systemd, rely on systemd's resource
|
|
limit control instead of implementing
|
|
your own, rely on systemd's privilege
|
|
dropping code instead of implementing
|
|
it in the daemon, and similar. See
|
|
<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
|
|
for the available
|
|
controls.</para></listitem>
|
|
|
|
<listitem><para>If D-Bus is used, make
|
|
your daemon bus-activatable, via
|
|
supplying a D-Bus service activation
|
|
configuration file. This has multiple
|
|
advantages: your daemon may be started
|
|
lazily on-demand; it may be started in
|
|
parallel to other daemons requiring it
|
|
-- which maximizes parallelization and
|
|
boot-up speed; your daemon can be
|
|
restarted on failure, without losing
|
|
any bus requests, as the bus queues
|
|
requests for activatable services. See
|
|
below for details.</para></listitem>
|
|
|
|
<listitem><para>If your daemon
|
|
provides services to other local
|
|
processes or remote clients via a
|
|
socket, it should be made
|
|
socket-activatable following the
|
|
scheme pointed out below. Like D-Bus
|
|
activation this enables on-demand
|
|
starting of services as well as it
|
|
allows improved parallelization of
|
|
service start-up. Also, for state-less
|
|
protocols (such as syslog, DNS) a
|
|
daemon implementing socket-based
|
|
activation can be restarted without
|
|
losing a single request. See below for
|
|
details.</para></listitem>
|
|
|
|
<listitem><para>If applicable a daemon
|
|
should notify the init system about
|
|
startup completion or status updates
|
|
via the
|
|
<citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
|
|
interface.</para></listitem>
|
|
|
|
<listitem><para>Instead of using the
|
|
<function>syslog()</function> call to log directly to the
|
|
system syslog service, a new-style daemon may
|
|
choose to simply log to STDERR via
|
|
<function>fprintf()</function>, which is then forwarded to
|
|
syslog by the init system. If log
|
|
priorities are necessary these can be
|
|
encoded by prefixing individual log
|
|
lines with strings like "<4>"
|
|
(for log priority 4 "WARNING" in the
|
|
syslog priority scheme), following a
|
|
similar style as the Linux kernel's
|
|
<function>printk()</function> priority system. In fact,
|
|
using this style of logging also
|
|
enables the init system to optionally
|
|
direct all application logging to the
|
|
kernel log buffer (kmsg), as
|
|
accessible via
|
|
<citerefentry><refentrytitle>dmesg</refentrytitle><manvolnum>1</manvolnum></citerefentry>. This
|
|
kind of logging may be enabled by
|
|
setting
|
|
<varname>StandardError=syslog</varname>
|
|
in the service unit file. For details
|
|
see
|
|
<citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>
|
|
and
|
|
<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
|
|
|
|
</orderedlist>
|
|
|
|
<para>These recommendations are similar but
|
|
not identical to the <ulink
|
|
url="http://developer.apple.com/mac/library/documentation/MacOSX/Conceptual/BPSystemStartup/Articles/LaunchOnDemandDaemons.html#//apple_ref/doc/uid/TP40001762-104738">Apple
|
|
MacOS X Daemon Requirements</ulink>.</para>
|
|
</refsect2>
|
|
|
|
</refsect1>
|
|
<refsect1>
|
|
<title>Activation</title>
|
|
|
|
<para>New-style init systems provide multiple
|
|
additional mechanisms to activate services, as
|
|
detailed below. It is common that services are
|
|
configured to be activated via more than one mechanism
|
|
at the same time. An example for systemd:
|
|
<filename>bluetoothd.service</filename> might get
|
|
activated either when Bluetooth hardware is plugged
|
|
in, or when an application accesses its programming
|
|
interfaces via D-Bus. Or, a print server daemon might
|
|
get activated when traffic arrives at an IPP port, or
|
|
when a printer is plugged in, or when a file is queued
|
|
in the printer spool directory. Even for services that
|
|
are intended to be started on system bootup
|
|
unconditionally it is a good idea to implement some of
|
|
the various activation schemes outlined below, in
|
|
order to maximize parallelization: if a daemon
|
|
implements a D-Bus service or listening socket,
|
|
implementing the full bus and socket activation scheme
|
|
allows starting of the daemon with its clients in
|
|
parallel (which speeds up boot-up), since all its
|
|
communication channels are established already, and no
|
|
request is lost because client requests will be queued
|
|
by the bus system (in case of D-Bus) or the kernel (in
|
|
case of sockets), until the activation is
|
|
completed.</para>
|
|
|
|
<refsect2>
|
|
<title>Activation on Boot</title>
|
|
|
|
<para>Old-style daemons are usually activated
|
|
exclusively on boot (and manually by the
|
|
administrator) via SysV init scripts, as
|
|
detailed in the <ulink
|
|
url="http://refspecs.freestandards.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html">LSB
|
|
Linux Standard Base Core
|
|
Specification</ulink>. This method of
|
|
activation is supported ubiquitously on Linux
|
|
init systems, both old-style and new-style
|
|
systems. Among other issues SysV init scripts
|
|
have the disadvantage of involving shell
|
|
scripts in the boot process. New-style init
|
|
systems generally employ updated versions of
|
|
activation, both during boot-up and during
|
|
runtime and using more minimal service
|
|
description files.</para>
|
|
|
|
<para>In systemd, if the developer or
|
|
administrator wants to make sure a service or
|
|
other unit is activated automatically on boot
|
|
it is recommended to place a symlink to the
|
|
unit file in the <filename>.wants/</filename>
|
|
directory of either
|
|
<filename>multi-user.target</filename> or
|
|
<filename>graphical.target</filename>, which
|
|
are normally used as boot targets at system
|
|
startup. See
|
|
<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
|
|
for details about the
|
|
<filename>.wants/</filename> directories, and
|
|
<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
|
|
for details about the two boot targets.</para>
|
|
|
|
</refsect2>
|
|
|
|
<refsect2>
|
|
<title>Socket-Based Activation</title>
|
|
|
|
<para>In order to maximize the possible
|
|
parallelization and robustness and simplify
|
|
configuration and development, it is
|
|
recommended for all new-style daemons that
|
|
communicate via listening sockets to employ
|
|
socket-based activation. In a socket-based
|
|
activation scheme the creation and binding of
|
|
the listening socket as primary communication
|
|
channel of daemons to local (and sometimes
|
|
remote) clients is moved out of the daemon
|
|
code and into the init system. Based on
|
|
per-daemon configuration the init system
|
|
installs the sockets and then hands them off
|
|
to the spawned process as soon as the
|
|
respective daemon is to be started.
|
|
Optionally activation of the service can be
|
|
delayed until the first inbound traffic
|
|
arrives at the socket, to implement on-demand
|
|
activation of daemons. However, the primary
|
|
advantage of this scheme is that all providers
|
|
and all consumers of the sockets can be
|
|
started in parallel as soon as all sockets
|
|
are established. In addition to that daemons
|
|
can be restarted with losing only a minimal
|
|
number of client transactions or even any
|
|
client request at all (the latter is
|
|
particularly true for state-less protocols,
|
|
such as DNS or syslog), because the socket
|
|
stays bound and accessible during the restart,
|
|
and all requests are queued while the daemon
|
|
cannot process them.</para>
|
|
|
|
<para>New-style daemons which support socket
|
|
activation must be able to receive their
|
|
sockets from the init system, instead of
|
|
creating and binding them themselves. For
|
|
details about the programming interfaces for
|
|
this scheme provided by systemd see
|
|
<citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
|
|
and
|
|
<citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>. For
|
|
details about porting existing daemons to
|
|
socket-based activation see below. With
|
|
minimal effort it is possible to implement
|
|
socket-based activation in addition to
|
|
traditional internal socket creation in the
|
|
same codebase in order to support both
|
|
new-style and old-style init systems from the
|
|
same daemon binary.</para>
|
|
|
|
<para>systemd implements socket-based
|
|
activation via <filename>.socket</filename>
|
|
units, which are described in
|
|
<citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>. When
|
|
configuring socket units for socket-based
|
|
activation it is essential that all listening
|
|
sockets are pulled in by the special target
|
|
unit <filename>sockets.target</filename>. It
|
|
is recommended to place a
|
|
<varname>WantedBy=sockets.target</varname>
|
|
directive in the <literal>[Install]</literal>
|
|
section, to automatically add such a
|
|
dependency on installation of a socket
|
|
unit. Unless
|
|
<varname>DefaultDependencies=no</varname> is
|
|
set the necessary ordering dependencies are
|
|
implicitly created for all socket units. For
|
|
more information about
|
|
<filename>sockets.target</filename> see
|
|
<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>. It
|
|
is not necessary or recommended to place any
|
|
additional dependencies on socket units (for
|
|
example from
|
|
<filename>multi-user.target</filename> or
|
|
suchlike) when one is installed in
|
|
<filename>sockets.target</filename>.</para>
|
|
</refsect2>
|
|
|
|
<refsect2>
|
|
<title>Bus-Based Activation</title>
|
|
|
|
<para>When the D-Bus IPC system is used for
|
|
communication with clients, new-style daemons
|
|
should employ bus activation so that they are
|
|
automatically activated when a client
|
|
application accesses their IPC
|
|
interfaces. This is configured in D-Bus
|
|
service files (not to be confused with systemd
|
|
service unit files!). To ensure that D-Bus
|
|
uses systemd to start-up and maintain the
|
|
daemon use the
|
|
<varname>SystemdService=</varname> directive
|
|
in these service files, to configure the
|
|
matching systemd service for a D-Bus
|
|
service. e.g.: for a D-Bus service whose D-Bus
|
|
activation file is named
|
|
<filename>org.freedesktop.RealtimeKit.service</filename>,
|
|
make sure to set
|
|
<varname>SystemdService=rtkit-daemon.service</varname>
|
|
in that file, to bind it to the systemd
|
|
service
|
|
<filename>rtkit-daemon.service</filename>. This
|
|
is needed to make sure that the daemon is
|
|
started in a race-free fashion when activated
|
|
via multiple mechanisms simultaneously.</para>
|
|
</refsect2>
|
|
|
|
<refsect2>
|
|
<title>Device-Based Activation</title>
|
|
|
|
<para>Often, daemons that manage a particular
|
|
type of hardware should be activated only when
|
|
the hardware of the respective kind is plugged
|
|
in or otherwise becomes available. In a
|
|
new-style init system it is possible to bind
|
|
activation to hardware plug/unplug events. In
|
|
systemd, kernel devices appearing in the
|
|
sysfs/udev device tree can be exposed as units
|
|
if they are tagged with the string
|
|
"<literal>systemd</literal>". Like any other
|
|
kind of unit they may then pull in other units
|
|
when activated (i.e. Plugged in) and thus
|
|
implement device-based activation. Systemd
|
|
dependencies may be encoded in the udev
|
|
database via the
|
|
<varname>SYSTEMD_WANTS=</varname>
|
|
property. See
|
|
<citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>
|
|
for details. Often it is nicer to pull in
|
|
services from devices only indirectly via
|
|
dedicated targets. Example: instead of pulling
|
|
in <filename>bluetoothd.service</filename>
|
|
from all the various bluetooth dongles and
|
|
other hardware available, pull in
|
|
bluetooth.target from them and
|
|
<filename>bluetoothd.service</filename> from
|
|
that target. This provides for nicer
|
|
abstraction and gives administrators the
|
|
option to enable
|
|
<filename>bluetoothd.service</filename> via
|
|
controlling a
|
|
<filename>bluetooth.target.wants/</filename>
|
|
symlink uniformly with a command like
|
|
<command>enable</command> of
|
|
<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
|
|
instead of manipulating the udev
|
|
ruleset.</para>
|
|
</refsect2>
|
|
|
|
<refsect2>
|
|
<title>Path-Based Activation</title>
|
|
|
|
<para>Often, runtime of daemons processing
|
|
spool files or directories (such as a printing
|
|
system) can be delayed until these file system
|
|
objects change state, or become
|
|
non-empty. New-style init systems provide a
|
|
way to bind service activation to file system
|
|
changes. systemd implements this scheme via
|
|
path-based activation configured in
|
|
<filename>.path</filename> units, as outlined
|
|
in
|
|
<citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
|
|
</refsect2>
|
|
|
|
<refsect2>
|
|
<title>Timer-Based Activation</title>
|
|
|
|
<para>Some daemons that implement clean-up
|
|
jobs that are intended to be executed in
|
|
regular intervals benefit from timer-based
|
|
activation. In systemd, this is implemented
|
|
via <filename>.timer</filename> units, as
|
|
described in
|
|
<citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
|
|
</refsect2>
|
|
|
|
<refsect2>
|
|
<title>Other Forms of Activation</title>
|
|
|
|
<para>Other forms of activation have been
|
|
suggested and implemented in some
|
|
systems. However, often there are simpler or
|
|
better alternatives, or they can be put
|
|
together of combinations of the schemes
|
|
above. Example: sometimes it appears useful to
|
|
start daemons or <filename>.socket</filename>
|
|
units when a specific IP address is configured
|
|
on a network interface, because network
|
|
sockets shall be bound to the
|
|
address. However, an alternative to implement
|
|
this is by utilizing the Linux IP_FREEBIND
|
|
socket option, as accessible via
|
|
<varname>FreeBind=yes</varname> in systemd
|
|
socket files (see
|
|
<citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
|
|
for details). This option, when enabled,
|
|
allows sockets to be bound to a non-local, not
|
|
configured IP address, and hence allows
|
|
bindings to a particular IP address before it
|
|
actually becomes available, making such an
|
|
explicit dependency to the configured address
|
|
redundant. Another often suggested trigger for
|
|
service activation is low system
|
|
load. However, here too, a more convincing
|
|
approach might be to make proper use of
|
|
features of the operating system: in
|
|
particular, the CPU or IO scheduler of
|
|
Linux. Instead of scheduling jobs from
|
|
userspace based on monitoring the OS
|
|
scheduler, it is advisable to leave the
|
|
scheduling of processes to the OS scheduler
|
|
itself. systemd provides fine-grained access
|
|
to the CPU and IO schedulers. If a process
|
|
executed by the init system shall not
|
|
negatively impact the amount of CPU or IO
|
|
bandwidth available to other processes, it
|
|
should be configured with
|
|
<varname>CPUSchedulingPolicy=idle</varname>
|
|
and/or
|
|
<varname>IOSchedulingClass=idle</varname>. Optionally,
|
|
this may be combined with timer-based
|
|
activation to schedule background jobs during
|
|
runtime and with minimal impact on the system,
|
|
and remove it from the boot phase
|
|
itself.</para>
|
|
</refsect2>
|
|
|
|
</refsect1>
|
|
<refsect1>
|
|
<title>Integration with Systemd</title>
|
|
|
|
<refsect2>
|
|
<title>Writing Systemd Unit Files</title>
|
|
|
|
<para>When writing systemd unit files, it is
|
|
recommended to consider the following
|
|
suggestions:</para>
|
|
|
|
<orderedlist>
|
|
<listitem><para>If possible do not use
|
|
the <varname>Type=forking</varname>
|
|
setting in service files. But if you
|
|
do, make sure to set the PID file path
|
|
using <varname>PIDFile=</varname>. See
|
|
<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
|
|
for details.</para></listitem>
|
|
|
|
<listitem><para>If your daemon
|
|
registers a D-Bus name on the bus,
|
|
make sure to use
|
|
<varname>Type=dbus</varname> in the
|
|
service file if
|
|
possible.</para></listitem>
|
|
|
|
<listitem><para>Make sure to set a
|
|
good human-readable description string
|
|
with
|
|
<varname>Description=</varname>.</para></listitem>
|
|
|
|
<listitem><para>Do not disable
|
|
<varname>DefaultDependencies=</varname>,
|
|
unless you really know what you do and
|
|
your unit is involved in early boot or
|
|
late system shutdown.</para></listitem>
|
|
|
|
<listitem><para>Normally, little if
|
|
any dependencies should need to
|
|
be defined explicitly. However, if you
|
|
do configure explicit dependencies, only refer to
|
|
unit names listed on
|
|
<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
|
|
or names introduced by your own
|
|
package to keep the unit file
|
|
operating
|
|
system-independent.</para></listitem>
|
|
|
|
<listitem><para>Make sure to include
|
|
an <literal>[Install]</literal>
|
|
section including installation
|
|
information for the unit file. See
|
|
<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
|
|
for details. To activate your service
|
|
on boot make sure to add a
|
|
<varname>WantedBy=multi-user.target</varname>
|
|
or
|
|
<varname>WantedBy=graphical.target</varname>
|
|
directive. To activate your socket on
|
|
boot, make sure to add
|
|
<varname>WantedBy=sockets.target</varname>. Usually
|
|
you also want to make sure that when
|
|
your service is installed your socket
|
|
is installed too, hence add
|
|
<varname>Also=foo.socket</varname> in
|
|
your service file
|
|
<filename>foo.service</filename>, for
|
|
a hypothetical program
|
|
<filename>foo</filename>.</para></listitem>
|
|
|
|
</orderedlist>
|
|
</refsect2>
|
|
|
|
<refsect2>
|
|
<title>Installing Systemd Service Files</title>
|
|
|
|
<para>At the build installation time
|
|
(e.g. <command>make install</command> during
|
|
package build) packages are recommended to
|
|
install their systemd unit files in the
|
|
directory returned by <command>pkg-config
|
|
systemd
|
|
--variable=systemdsystemunitdir</command> (for
|
|
system services) or <command>pkg-config
|
|
systemd
|
|
--variable=systemduserunitdir</command>
|
|
(for user services). This will make the
|
|
services available in the system on explicit
|
|
request but not activate them automatically
|
|
during boot. Optionally, during package
|
|
installation (e.g. <command>rpm -i</command>
|
|
by the administrator) symlinks should be
|
|
created in the systemd configuration
|
|
directories via the <command>enable</command>
|
|
command of the
|
|
<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
|
|
tool, to activate them automatically on
|
|
boot.</para>
|
|
|
|
<para>Packages using
|
|
<citerefentry><refentrytitle>autoconf</refentrytitle><manvolnum>1</manvolnum></citerefentry>
|
|
are recommended to use a configure script
|
|
excerpt like the following to determine the
|
|
unit installation path during source
|
|
configuration:</para>
|
|
|
|
<programlisting>PKG_PROG_PKG_CONFIG
|
|
AC_ARG_WITH([systemdsystemunitdir],
|
|
AS_HELP_STRING([--with-systemdsystemunitdir=DIR], [Directory for systemd service files]),
|
|
[], [with_systemdsystemunitdir=$($PKG_CONFIG --variable=systemdsystemunitdir systemd)])
|
|
if test "x$with_systemdsystemunitdir" != xno; then
|
|
AC_SUBST([systemdsystemunitdir], [$with_systemdsystemunitdir])
|
|
fi
|
|
AM_CONDITIONAL(HAVE_SYSTEMD, [test -n "$with_systemdsystemunitdir" -a "x$with_systemdsystemunitdir" != xno ])</programlisting>
|
|
|
|
<para>This snippet allows automatic
|
|
installation of the unit files on systemd
|
|
machines, and optionally allows their
|
|
installation even on machines lacking
|
|
systemd. (Modification of this snippet for the
|
|
user unit directory is left as an exercise for the
|
|
reader.)</para>
|
|
|
|
<para>Additionally, to ensure that
|
|
<command>make distcheck</command> continues to
|
|
work, it is recommended to add the following
|
|
to the top-level <filename>Makefile.am</filename>
|
|
file in
|
|
<citerefentry><refentrytitle>automake</refentrytitle><manvolnum>1</manvolnum></citerefentry>-based
|
|
projects:</para>
|
|
|
|
<programlisting>DISTCHECK_CONFIGURE_FLAGS = \
|
|
--with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)</programlisting>
|
|
|
|
<para>Finally, unit files should be installed in the system with an automake excerpt like the following:</para>
|
|
|
|
<programlisting>if HAVE_SYSTEMD
|
|
systemdsystemunit_DATA = \
|
|
foobar.socket \
|
|
foobar.service
|
|
endif</programlisting>
|
|
|
|
<para>In the
|
|
<citerefentry><refentrytitle>rpm</refentrytitle><manvolnum>8</manvolnum></citerefentry>
|
|
<filename>.spec</filename> file use snippets
|
|
like the following to enable/disable the
|
|
service during
|
|
installation/deinstallation. This makes use of
|
|
the RPM macros shipped along systemd. Consult
|
|
the packaging guidelines of your distribution
|
|
for details and the equivalent for other
|
|
package managers.</para>
|
|
|
|
<para>At the top of the file:</para>
|
|
|
|
<programlisting>BuildRequires: systemd
|
|
%{?systemd_requires}</programlisting>
|
|
|
|
<para>And as scriptlets, further down:</para>
|
|
|
|
<programlisting>%post
|
|
%systemd_post foobar.service foobar.socket
|
|
|
|
%preun
|
|
%systemd_preun foobar.service foobar.socket
|
|
|
|
%postun
|
|
%systemd_postun</programlisting>
|
|
|
|
<para>If the service shall be restarted during
|
|
upgrades replace the
|
|
<literal>%postun</literal> scriptlet above
|
|
with the following:</para>
|
|
|
|
<programlisting>%postun
|
|
%systemd_postun_with_restart foobar.service</programlisting>
|
|
|
|
<para>Note that
|
|
<literal>%systemd_post</literal> and
|
|
<literal>%systemd_preun</literal> expect the
|
|
names of all units that are installed/removed
|
|
as arguments, separated by
|
|
spaces. <literal>%systemd_postun</literal>
|
|
expects no
|
|
arguments. <literal>%systemd_postun_with_restart</literal>
|
|
expects the units to restart as
|
|
arguments.</para>
|
|
|
|
<para>To facilitate upgrades from a package
|
|
version that shipped only SysV init scripts to
|
|
a package version that ships both a SysV init
|
|
script and a native systemd service file, use
|
|
a fragment like the following:</para>
|
|
|
|
<programlisting>%triggerun -- foobar < 0.47.11-1
|
|
if /sbin/chkconfig --level 5 foobar ; then
|
|
/bin/systemctl --no-reload enable foobar.service foobar.socket >/dev/null 2>&1 || :
|
|
fi</programlisting>
|
|
|
|
<para>Where 0.47.11-1 is the first package
|
|
version that includes the native unit
|
|
file. This fragment will ensure that the first
|
|
time the unit file is installed it will be
|
|
enabled if and only if the SysV init script is
|
|
enabled, thus making sure that the enable
|
|
status is not changed. Note that
|
|
<command>chkconfig</command> is a command
|
|
specific to Fedora which can be used to check
|
|
whether a SysV init script is enabled. Other
|
|
operating systems will have to use different
|
|
commands here.</para>
|
|
</refsect2>
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
<title>Porting Existing Daemons</title>
|
|
|
|
<para>Since new-style init systems such as systemd are
|
|
compatible with traditional SysV init systems it is
|
|
not strictly necessary to port existing daemons to the
|
|
new style. However doing so offers additional
|
|
functionality to the daemons as well as simplifying
|
|
integration into new-style init systems.</para>
|
|
|
|
<para>To port an existing SysV compatible daemon the
|
|
following steps are recommended:</para>
|
|
|
|
<orderedlist>
|
|
<listitem><para>If not already implemented,
|
|
add an optional command line switch to the
|
|
daemon to disable daemonization. This is
|
|
useful not only for using the daemon in
|
|
new-style init systems, but also to ease
|
|
debugging.</para></listitem>
|
|
|
|
<listitem><para>If the daemon offers
|
|
interfaces to other software running on the
|
|
local system via local AF_UNIX sockets,
|
|
consider implementing socket-based activation
|
|
(see above). Usually a minimal patch is
|
|
sufficient to implement this: Extend the
|
|
socket creation in the daemon code so that
|
|
<citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
|
|
is checked for already passed sockets
|
|
first. If sockets are passed (i.e. when
|
|
<function>sd_listen_fds()</function> returns a
|
|
positive value), skip the socket creation step
|
|
and use the passed sockets. Secondly, ensure
|
|
that the file-system socket nodes for local
|
|
AF_UNIX sockets used in the socket-based
|
|
activation are not removed when the daemon
|
|
shuts down, if sockets have been
|
|
passed. Third, if the daemon normally closes
|
|
all remaining open file descriptors as part of
|
|
its initialization, the sockets passed from
|
|
the init system must be spared. Since
|
|
new-style init systems guarantee that no
|
|
left-over file descriptors are passed to
|
|
executed processes, it might be a good choice
|
|
to simply skip the closing of all remaining
|
|
open file descriptors if sockets are
|
|
passed.</para></listitem>
|
|
|
|
<listitem><para>Write and install a systemd
|
|
unit file for the service (and the sockets if
|
|
socket-based activation is used, as well as a
|
|
path unit file, if the daemon processes a
|
|
spool directory), see above for
|
|
details.</para></listitem>
|
|
|
|
<listitem><para>If the daemon exposes
|
|
interfaces via D-Bus, write and install a
|
|
D-Bus activation file for the service, see
|
|
above for details.</para></listitem>
|
|
</orderedlist>
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
<title>See Also</title>
|
|
<para>
|
|
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
|
|
<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
|
|
</para>
|
|
</refsect1>
|
|
|
|
</refentry>
|