diff options
Diffstat (limited to 'man/daemon.xml')
| -rw-r--r-- | man/daemon.xml | 130 |
1 files changed, 65 insertions, 65 deletions
diff --git a/man/daemon.xml b/man/daemon.xml index 76ae832a42..7790420c6e 100644 --- a/man/daemon.xml +++ b/man/daemon.xml @@ -79,7 +79,7 @@ descriptors 0, 1, 2). This ensures that no accidentally passed file descriptor stays around in the daemon - process. On Linux this is best + process. On Linux, this is best implemented by iterating through <filename>/proc/self/fd</filename>, with a fallback of iterating from file @@ -115,7 +115,7 @@ <listitem><para>In the child, call <function>fork()</function> again, to - ensure the daemon can never re-acquire + ensure that the daemon can never re-acquire a terminal again.</para></listitem> <listitem><para>Call <function>exit()</function> in the @@ -150,15 +150,15 @@ <function>getpid()</function>) to a PID file, for example <filename>/var/run/foobar.pid</filename> - (for a hypothetical daemon "foobar"), + (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 + it is verified at the same time that the PID previously stored in the PID file no longer exists or belongs to a - foreign process. Commonly some kind of + foreign process. Commonly, some kind of file locking is employed to implement this logic.</para></listitem> @@ -167,7 +167,7 @@ applicable.</para></listitem> <listitem><para>From the daemon - process notify the original process + process, notify the original process started that initialization is complete. This can be implemented via an unnamed pipe or similar @@ -197,7 +197,7 @@ 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 + command line argument to ease debugging as well as to simplify integration into systems using systemd.</para> </refsect2> @@ -211,20 +211,20 @@ runtime and simplifies their implementation.</para> - <para>For developing a new-style daemon none + <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 + 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 + a clean process context: 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 @@ -256,7 +256,7 @@ scripts</ulink>.</para></listitem> <listitem><para>If possible and - applicable expose the daemon's control + 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> @@ -274,7 +274,7 @@ 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 + 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 @@ -285,7 +285,7 @@ controls.</para></listitem> <listitem><para>If D-Bus is used, make - your daemon bus-activatable, via + your daemon bus-activatable by supplying a D-Bus service activation configuration file. This has multiple advantages: your daemon may be started @@ -293,7 +293,7 @@ parallel to other daemons requiring it -- which maximizes parallelization and boot-up speed; your daemon can be - restarted on failure, without losing + restarted on failure without losing any bus requests, as the bus queues requests for activatable services. See below for details.</para></listitem> @@ -304,17 +304,17 @@ socket, it should be made socket-activatable following the scheme pointed out below. Like D-Bus - activation this enables on-demand + 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 + 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 + <listitem><para>If applicable, a daemon should notify the init system about startup completion or status updates via the @@ -327,7 +327,7 @@ 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 + priorities are necessary, these can be encoded by prefixing individual log lines with strings like "<4>" (for log priority 4 "WARNING" in the @@ -343,7 +343,7 @@ kind of logging may be enabled by setting <varname>StandardError=syslog</varname> - in the service unit file. For details + in the service unit file. For details, see <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry> and @@ -374,9 +374,9 @@ 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 + unconditionally, it is a good idea to implement some of the various activation schemes outlined below, in - order to maximize parallelization: if a daemon + 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 @@ -384,7 +384,7 @@ 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 + case of sockets) until the activation is completed.</para> <refsect2> @@ -399,7 +399,7 @@ 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 + 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 @@ -409,7 +409,7 @@ <para>In systemd, if the developer or administrator wants to make sure a service or - other unit is activated automatically on boot + 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 @@ -434,25 +434,25 @@ 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 + 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 + 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 + Optionally, activation of the service can be delayed until the first inbound traffic - arrives at the socket, to implement on-demand + 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 + are established. In addition to that, daemons can be restarted with losing only a minimal - number of client transactions or even any + 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 @@ -462,16 +462,16 @@ <para>New-style daemons which support socket activation must be able to receive their - sockets from the init system, instead of + 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 + 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, 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 @@ -483,20 +483,20 @@ 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 + 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 + 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 + set, the necessary ordering dependencies are implicitly created for all socket units. For more information about - <filename>sockets.target</filename> see + <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 @@ -518,16 +518,16 @@ 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 + daemon, use the <varname>SystemdService=</varname> directive - in these service files, to configure the + 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 + 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 + 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 @@ -542,23 +542,23 @@ 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 + 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 + 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 + for details. Often, it is nicer to pull in services from devices only indirectly via - dedicated targets. Example: instead of pulling + dedicated targets. Example: Instead of pulling in <filename>bluetoothd.service</filename> from all the various bluetooth dongles and other hardware available, pull in @@ -610,10 +610,10 @@ <para>Other forms of activation have been suggested and implemented in some - systems. However, often there are simpler or + systems. However, there are often simpler or better alternatives, or they can be put together of combinations of the schemes - above. Example: sometimes it appears useful to + 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 @@ -634,7 +634,7 @@ 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 + 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 @@ -668,7 +668,7 @@ suggestions:</para> <orderedlist> - <listitem><para>If possible do not use + <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 @@ -711,15 +711,15 @@ 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 + 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 + <varname>WantedBy=sockets.target</varname>. Usually, you also want to make sure that when - your service is installed your socket + your service is installed, your socket is installed too, hence add <varname>Also=foo.socket</varname> in your service file @@ -735,7 +735,7 @@ <para>At the build installation time (e.g. <command>make install</command> during - package build) packages are recommended to + package build), packages are recommended to install their systemd unit files in the directory returned by <command>pkg-config systemd @@ -748,12 +748,12 @@ request but not activate them automatically during boot. Optionally, during package installation (e.g. <command>rpm -i</command> - by the administrator) symlinks should be + 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 + tool to activate them automatically on boot.</para> <para>Packages using @@ -801,7 +801,7 @@ endif</programlisting> <para>In the <citerefentry><refentrytitle>rpm</refentrytitle><manvolnum>8</manvolnum></citerefentry> - <filename>.spec</filename> file use snippets + <filename>.spec</filename> file, use snippets like the following to enable/disable the service during installation/deinstallation. This makes use of @@ -827,7 +827,7 @@ endif</programlisting> %systemd_postun</programlisting> <para>If the service shall be restarted during - upgrades replace the + upgrades, replace the <literal>%postun</literal> scriptlet above with the following:</para> @@ -859,7 +859,7 @@ 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 + 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 @@ -875,13 +875,13 @@ fi</programlisting> <title>Porting Existing Daemons</title> <para>Since new-style init systems such as systemd are - compatible with traditional SysV init systems it is + compatible with traditional SysV init systems, it is not strictly necessary to port existing daemons to the - new style. However doing so offers additional + 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 + <para>To port an existing SysV compatible daemon, the following steps are recommended:</para> <orderedlist> @@ -896,7 +896,7 @@ fi</programlisting> interfaces to other software running on the local system via local <constant>AF_UNIX</constant> sockets, consider implementing socket-based activation - (see above). Usually a minimal patch is + (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> |