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1 | <?xml version='1.0'?> <!--*-nxml-*--> |
2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" | |
12b42c76 | 3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> |
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4 | |
5 | <!-- | |
6 | This file is part of systemd. | |
7 | ||
8 | Copyright 2010 Lennart Poettering | |
9 | ||
10 | systemd is free software; you can redistribute it and/or modify it | |
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11 | under the terms of the GNU Lesser General Public License as published by |
12 | the Free Software Foundation; either version 2.1 of the License, or | |
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13 | (at your option) any later version. |
14 | ||
15 | systemd is distributed in the hope that it will be useful, but | |
16 | WITHOUT ANY WARRANTY; without even the implied warranty of | |
17 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
5430f7f2 | 18 | Lesser General Public License for more details. |
64aba792 | 19 | |
5430f7f2 | 20 | You should have received a copy of the GNU Lesser General Public License |
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21 | along with systemd; If not, see <http://www.gnu.org/licenses/>. |
22 | --> | |
23 | ||
62adf224 | 24 | <refentry id="daemon"> |
64aba792 | 25 | |
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26 | <refentryinfo> |
27 | <title>daemon</title> | |
28 | <productname>systemd</productname> | |
29 | ||
30 | <authorgroup> | |
31 | <author> | |
32 | <contrib>Developer</contrib> | |
33 | <firstname>Lennart</firstname> | |
34 | <surname>Poettering</surname> | |
35 | <email>lennart@poettering.net</email> | |
36 | </author> | |
37 | </authorgroup> | |
38 | </refentryinfo> | |
39 | ||
40 | <refmeta> | |
41 | <refentrytitle>daemon</refentrytitle> | |
42 | <manvolnum>7</manvolnum> | |
43 | </refmeta> | |
44 | ||
45 | <refnamediv> | |
46 | <refname>daemon</refname> | |
47 | <refpurpose>Writing and packaging system daemons</refpurpose> | |
48 | </refnamediv> | |
49 | ||
50 | <refsect1> | |
51 | <title>Description</title> | |
52 | ||
53 | <para>A daemon is a service process that runs in the background | |
54 | and supervises the system or provides functionality to other | |
55 | processes. Traditionally, daemons are implemented following a | |
56 | scheme originating in SysV Unix. Modern daemons should follow a | |
57 | simpler yet more powerful scheme (here called "new-style" | |
58 | daemons), as implemented by | |
59 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>. | |
60 | This manual page covers both schemes, and in particular includes | |
61 | recommendations for daemons that shall be included in the systemd | |
62 | init system.</para> | |
63 | ||
64 | <refsect2> | |
65 | <title>SysV Daemons</title> | |
66 | ||
67 | <para>When a traditional SysV daemon starts, it should execute | |
68 | the following steps as part of the initialization. Note that | |
69 | these steps are unnecessary for new-style daemons (see below), | |
70 | and should only be implemented if compatibility with SysV is | |
71 | essential.</para> | |
72 | ||
73 | <orderedlist> | |
74 | <listitem><para>Close all open file descriptors except | |
75 | standard input, output, and error (i.e. the first three file | |
76 | descriptors 0, 1, 2). This ensures that no accidentally passed | |
77 | file descriptor stays around in the daemon process. On Linux, | |
78 | this is best implemented by iterating through | |
79 | <filename>/proc/self/fd</filename>, with a fallback of | |
80 | iterating from file descriptor 3 to the value returned by | |
81 | <function>getrlimit()</function> for | |
82 | <constant>RLIMIT_NOFILE</constant>. </para></listitem> | |
83 | ||
84 | <listitem><para>Reset all signal handlers to their default. | |
85 | This is best done by iterating through the available signals | |
86 | up to the limit of <constant>_NSIG</constant> and resetting | |
87 | them to <constant>SIG_DFL</constant>.</para></listitem> | |
88 | ||
89 | <listitem><para>Reset the signal mask | |
90 | using | |
91 | <function>sigprocmask()</function>.</para></listitem> | |
92 | ||
93 | <listitem><para>Sanitize the environment block, removing or | |
94 | resetting environment variables that might negatively impact | |
95 | daemon runtime.</para></listitem> | |
96 | ||
97 | <listitem><para>Call <function>fork()</function>, to create a | |
98 | background process.</para></listitem> | |
99 | ||
100 | <listitem><para>In the child, call | |
101 | <function>setsid()</function> to detach from any terminal and | |
102 | create an independent session.</para></listitem> | |
103 | ||
104 | <listitem><para>In the child, call <function>fork()</function> | |
105 | again, to ensure that the daemon can never re-acquire a | |
106 | terminal again.</para></listitem> | |
107 | ||
108 | <listitem><para>Call <function>exit()</function> in the first | |
109 | child, so that only the second child (the actual daemon | |
110 | process) stays around. This ensures that the daemon process is | |
111 | re-parented to init/PID 1, as all daemons should | |
112 | be.</para></listitem> | |
113 | ||
114 | <listitem><para>In the daemon process, connect | |
115 | <filename>/dev/null</filename> to standard input, output, and | |
116 | error.</para></listitem> | |
117 | ||
118 | <listitem><para>In the daemon process, reset the umask to 0, | |
119 | so that the file modes passed to <function>open()</function>, | |
120 | <function>mkdir()</function> and suchlike directly control the | |
121 | access mode of the created files and | |
122 | directories.</para></listitem> | |
123 | ||
124 | <listitem><para>In the daemon process, change the current | |
125 | directory to the root directory (/), in order to avoid that | |
126 | the daemon involuntarily blocks mount points from being | |
127 | unmounted.</para></listitem> | |
128 | ||
129 | <listitem><para>In the daemon process, write the daemon PID | |
130 | (as returned by <function>getpid()</function>) to a PID file, | |
131 | for example <filename>/run/foobar.pid</filename> (for a | |
132 | hypothetical daemon "foobar") to ensure that the daemon cannot | |
133 | be started more than once. This must be implemented in | |
134 | race-free fashion so that the PID file is only updated when it | |
135 | is verified at the same time that the PID previously stored in | |
136 | the PID file no longer exists or belongs to a foreign | |
137 | process.</para></listitem> | |
138 | ||
139 | <listitem><para>In the daemon process, drop privileges, if | |
140 | possible and applicable.</para></listitem> | |
141 | ||
142 | <listitem><para>From the daemon process, notify the original | |
143 | process started that initialization is complete. This can be | |
144 | implemented via an unnamed pipe or similar communication | |
145 | channel that is created before the first | |
146 | <function>fork()</function> and hence available in both the | |
147 | original and the daemon process.</para></listitem> | |
148 | ||
149 | <listitem><para>Call <function>exit()</function> in the | |
150 | original process. The process that invoked the daemon must be | |
151 | able to rely on that this <function>exit()</function> happens | |
152 | after initialization is complete and all external | |
153 | communication channels are established and | |
154 | accessible.</para></listitem> | |
155 | </orderedlist> | |
156 | ||
157 | <para>The BSD <function>daemon()</function> function should not | |
158 | be used, as it implements only a subset of these steps.</para> | |
159 | ||
160 | <para>A daemon that needs to provide compatibility with SysV | |
161 | systems should implement the scheme pointed out above. However, | |
162 | it is recommended to make this behavior optional and | |
163 | configurable via a command line argument to ease debugging as | |
164 | well as to simplify integration into systems using | |
165 | systemd.</para> | |
166 | </refsect2> | |
167 | ||
168 | <refsect2> | |
169 | <title>New-Style Daemons</title> | |
170 | ||
171 | <para>Modern services for Linux should be implemented as | |
172 | new-style daemons. This makes it easier to supervise and control | |
173 | them at runtime and simplifies their implementation.</para> | |
174 | ||
175 | <para>For developing a new-style daemon, none of the | |
176 | initialization steps recommended for SysV daemons need to be | |
177 | implemented. New-style init systems such as systemd make all of | |
178 | them redundant. Moreover, since some of these steps interfere | |
179 | with process monitoring, file descriptor passing and other | |
180 | functionality of the init system, it is recommended not to | |
181 | execute them when run as new-style service.</para> | |
182 | ||
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183 | <para>Note that new-style init systems guarantee execution of daemon processes in a clean process context: it is |
184 | guaranteed that the environment block is sanitized, that the signal handlers and mask is reset and that no | |
185 | left-over file descriptors are passed. Daemons will be executed in their own session, with standard input | |
186 | connected to <filename>/dev/null</filename> and standard output/error connected to the | |
187 | <citerefentry><refentrytitle>systemd-journald.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
188 | logging service, unless otherwise configured. The umask is reset. | |
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189 | </para> |
190 | ||
191 | <para>It is recommended for new-style daemons to implement the | |
192 | following:</para> | |
193 | ||
194 | <orderedlist> | |
195 | <listitem><para>If <constant>SIGTERM</constant> is received, | |
196 | shut down the daemon and exit cleanly.</para></listitem> | |
197 | ||
198 | <listitem><para>If <constant>SIGHUP</constant> is received, | |
199 | reload the configuration files, if this | |
200 | applies.</para></listitem> | |
201 | ||
202 | <listitem><para>Provide a correct exit code from the main | |
203 | daemon process, as this is used by the init system to detect | |
204 | service errors and problems. It is recommended to follow the | |
205 | exit code scheme as defined in the <ulink | |
206 | url="http://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html">LSB | |
207 | recommendations for SysV init | |
208 | scripts</ulink>.</para></listitem> | |
209 | ||
210 | <listitem><para>If possible and applicable, expose the | |
211 | daemon's control interface via the D-Bus IPC system and grab a | |
212 | bus name as last step of initialization.</para></listitem> | |
213 | ||
214 | <listitem><para>For integration in systemd, provide a | |
215 | <filename>.service</filename> unit file that carries | |
216 | information about starting, stopping and otherwise maintaining | |
217 | the daemon. See | |
218 | <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
219 | for details.</para></listitem> | |
220 | ||
221 | <listitem><para>As much as possible, rely on the init system's | |
222 | functionality to limit the access of the daemon to files, | |
223 | services and other resources, i.e. in the case of systemd, | |
224 | rely on systemd's resource limit control instead of | |
225 | implementing your own, rely on systemd's privilege dropping | |
226 | code instead of implementing it in the daemon, and similar. | |
227 | See | |
228 | <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
229 | for the available controls.</para></listitem> | |
230 | ||
231 | <listitem><para>If D-Bus is used, make your daemon | |
232 | bus-activatable by supplying a D-Bus service activation | |
233 | configuration file. This has multiple advantages: your daemon | |
234 | may be started lazily on-demand; it may be started in parallel | |
ccddd104 | 235 | to other daemons requiring it — which maximizes |
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236 | parallelization and boot-up speed; your daemon can be |
237 | restarted on failure without losing any bus requests, as the | |
238 | bus queues requests for activatable services. See below for | |
239 | details.</para></listitem> | |
240 | ||
241 | <listitem><para>If your daemon provides services to other | |
242 | local processes or remote clients via a socket, it should be | |
243 | made socket-activatable following the scheme pointed out | |
244 | below. Like D-Bus activation, this enables on-demand starting | |
245 | of services as well as it allows improved parallelization of | |
246 | service start-up. Also, for state-less protocols (such as | |
247 | syslog, DNS), a daemon implementing socket-based activation | |
248 | can be restarted without losing a single request. See below | |
249 | for details.</para></listitem> | |
250 | ||
251 | <listitem><para>If applicable, a daemon should notify the init | |
252 | system about startup completion or status updates via the | |
253 | <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
254 | interface.</para></listitem> | |
255 | ||
256 | <listitem><para>Instead of using the | |
257 | <function>syslog()</function> call to log directly to the | |
258 | system syslog service, a new-style daemon may choose to simply | |
259 | log to standard error via <function>fprintf()</function>, | |
260 | which is then forwarded to syslog by the init system. If log | |
261 | levels are necessary, these can be encoded by prefixing | |
262 | individual log lines with strings like | |
263 | <literal><4></literal> (for log level 4 "WARNING" in the | |
264 | syslog priority scheme), following a similar style as the | |
265 | Linux kernel's <function>printk()</function> level system. For | |
266 | details, see | |
267 | <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
268 | and | |
269 | <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem> | |
270 | ||
271 | </orderedlist> | |
272 | ||
273 | <para>These recommendations are similar but not identical to the | |
274 | <ulink | |
275 | url="https://developer.apple.com/library/mac/documentation/MacOSX/Conceptual/BPSystemStartup/Chapters/CreatingLaunchdJobs.html">Apple | |
276 | MacOS X Daemon Requirements</ulink>.</para> | |
277 | </refsect2> | |
278 | ||
279 | </refsect1> | |
280 | <refsect1> | |
281 | <title>Activation</title> | |
282 | ||
283 | <para>New-style init systems provide multiple additional | |
284 | mechanisms to activate services, as detailed below. It is common | |
285 | that services are configured to be activated via more than one | |
286 | mechanism at the same time. An example for systemd: | |
287 | <filename>bluetoothd.service</filename> might get activated either | |
288 | when Bluetooth hardware is plugged in, or when an application | |
289 | accesses its programming interfaces via D-Bus. Or, a print server | |
290 | daemon might get activated when traffic arrives at an IPP port, or | |
291 | when a printer is plugged in, or when a file is queued in the | |
292 | printer spool directory. Even for services that are intended to be | |
293 | started on system bootup unconditionally, it is a good idea to | |
294 | implement some of the various activation schemes outlined below, | |
295 | in order to maximize parallelization. If a daemon implements a | |
296 | D-Bus service or listening socket, implementing the full bus and | |
297 | socket activation scheme allows starting of the daemon with its | |
298 | clients in parallel (which speeds up boot-up), since all its | |
299 | communication channels are established already, and no request is | |
300 | lost because client requests will be queued by the bus system (in | |
301 | case of D-Bus) or the kernel (in case of sockets) until the | |
302 | activation is completed.</para> | |
303 | ||
304 | <refsect2> | |
305 | <title>Activation on Boot</title> | |
306 | ||
307 | <para>Old-style daemons are usually activated exclusively on | |
308 | boot (and manually by the administrator) via SysV init scripts, | |
309 | as detailed in the <ulink | |
310 | url="http://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html">LSB | |
311 | Linux Standard Base Core Specification</ulink>. This method of | |
312 | activation is supported ubiquitously on Linux init systems, both | |
313 | old-style and new-style systems. Among other issues, SysV init | |
314 | scripts have the disadvantage of involving shell scripts in the | |
315 | boot process. New-style init systems generally employ updated | |
316 | versions of activation, both during boot-up and during runtime | |
317 | and using more minimal service description files.</para> | |
318 | ||
319 | <para>In systemd, if the developer or administrator wants to | |
320 | make sure that a service or other unit is activated | |
321 | automatically on boot, it is recommended to place a symlink to | |
322 | the unit file in the <filename>.wants/</filename> directory of | |
323 | either <filename>multi-user.target</filename> or | |
324 | <filename>graphical.target</filename>, which are normally used | |
325 | as boot targets at system startup. See | |
326 | <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
327 | for details about the <filename>.wants/</filename> directories, | |
328 | and | |
329 | <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry> | |
330 | for details about the two boot targets.</para> | |
331 | ||
332 | </refsect2> | |
333 | ||
334 | <refsect2> | |
335 | <title>Socket-Based Activation</title> | |
336 | ||
337 | <para>In order to maximize the possible parallelization and | |
338 | robustness and simplify configuration and development, it is | |
339 | recommended for all new-style daemons that communicate via | |
340 | listening sockets to employ socket-based activation. In a | |
341 | socket-based activation scheme, the creation and binding of the | |
342 | listening socket as primary communication channel of daemons to | |
343 | local (and sometimes remote) clients is moved out of the daemon | |
344 | code and into the init system. Based on per-daemon | |
345 | configuration, the init system installs the sockets and then | |
346 | hands them off to the spawned process as soon as the respective | |
347 | daemon is to be started. Optionally, activation of the service | |
348 | can be delayed until the first inbound traffic arrives at the | |
349 | socket to implement on-demand activation of daemons. However, | |
350 | the primary advantage of this scheme is that all providers and | |
351 | all consumers of the sockets can be started in parallel as soon | |
352 | as all sockets are established. In addition to that, daemons can | |
353 | be restarted with losing only a minimal number of client | |
354 | transactions, or even any client request at all (the latter is | |
355 | particularly true for state-less protocols, such as DNS or | |
356 | syslog), because the socket stays bound and accessible during | |
357 | the restart, and all requests are queued while the daemon cannot | |
358 | process them.</para> | |
359 | ||
360 | <para>New-style daemons which support socket activation must be | |
361 | able to receive their sockets from the init system instead of | |
362 | creating and binding them themselves. For details about the | |
363 | programming interfaces for this scheme provided by systemd, see | |
364 | <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
365 | and | |
366 | <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>. | |
367 | For details about porting existing daemons to socket-based | |
368 | activation, see below. With minimal effort, it is possible to | |
369 | implement socket-based activation in addition to traditional | |
370 | internal socket creation in the same codebase in order to | |
371 | support both new-style and old-style init systems from the same | |
372 | daemon binary.</para> | |
373 | ||
374 | <para>systemd implements socket-based activation via | |
375 | <filename>.socket</filename> units, which are described in | |
376 | <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>. | |
377 | When configuring socket units for socket-based activation, it is | |
378 | essential that all listening sockets are pulled in by the | |
379 | special target unit <filename>sockets.target</filename>. It is | |
380 | recommended to place a | |
381 | <varname>WantedBy=sockets.target</varname> directive in the | |
382 | <literal>[Install]</literal> section to automatically add such a | |
383 | dependency on installation of a socket unit. Unless | |
384 | <varname>DefaultDependencies=no</varname> is set, the necessary | |
385 | ordering dependencies are implicitly created for all socket | |
386 | units. For more information about | |
387 | <filename>sockets.target</filename>, see | |
388 | <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>. | |
389 | It is not necessary or recommended to place any additional | |
390 | dependencies on socket units (for example from | |
391 | <filename>multi-user.target</filename> or suchlike) when one is | |
392 | installed in <filename>sockets.target</filename>.</para> | |
393 | </refsect2> | |
394 | ||
395 | <refsect2> | |
396 | <title>Bus-Based Activation</title> | |
397 | ||
398 | <para>When the D-Bus IPC system is used for communication with | |
399 | clients, new-style daemons should employ bus activation so that | |
400 | they are automatically activated when a client application | |
401 | accesses their IPC interfaces. This is configured in D-Bus | |
402 | service files (not to be confused with systemd service unit | |
403 | files!). To ensure that D-Bus uses systemd to start-up and | |
404 | maintain the daemon, use the <varname>SystemdService=</varname> | |
405 | directive in these service files to configure the matching | |
406 | systemd service for a D-Bus service. e.g.: For a D-Bus service | |
407 | whose D-Bus activation file is named | |
408 | <filename>org.freedesktop.RealtimeKit.service</filename>, make | |
409 | sure to set | |
410 | <varname>SystemdService=rtkit-daemon.service</varname> in that | |
411 | file to bind it to the systemd service | |
412 | <filename>rtkit-daemon.service</filename>. This is needed to | |
413 | make sure that the daemon is started in a race-free fashion when | |
414 | activated via multiple mechanisms simultaneously.</para> | |
415 | </refsect2> | |
416 | ||
417 | <refsect2> | |
418 | <title>Device-Based Activation</title> | |
419 | ||
420 | <para>Often, daemons that manage a particular type of hardware | |
421 | should be activated only when the hardware of the respective | |
422 | kind is plugged in or otherwise becomes available. In a | |
423 | new-style init system, it is possible to bind activation to | |
424 | hardware plug/unplug events. In systemd, kernel devices | |
425 | appearing in the sysfs/udev device tree can be exposed as units | |
426 | if they are tagged with the string <literal>systemd</literal>. | |
427 | Like any other kind of unit, they may then pull in other units | |
428 | when activated (i.e. plugged in) and thus implement device-based | |
429 | activation. systemd dependencies may be encoded in the udev | |
430 | database via the <varname>SYSTEMD_WANTS=</varname> property. See | |
431 | <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
432 | for details. Often, it is nicer to pull in services from devices | |
433 | only indirectly via dedicated targets. Example: Instead of | |
434 | pulling in <filename>bluetoothd.service</filename> from all the | |
435 | various bluetooth dongles and other hardware available, pull in | |
436 | bluetooth.target from them and | |
437 | <filename>bluetoothd.service</filename> from that target. This | |
438 | provides for nicer abstraction and gives administrators the | |
439 | option to enable <filename>bluetoothd.service</filename> via | |
440 | controlling a <filename>bluetooth.target.wants/</filename> | |
441 | symlink uniformly with a command like <command>enable</command> | |
442 | of | |
443 | <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
444 | instead of manipulating the udev ruleset.</para> | |
445 | </refsect2> | |
446 | ||
447 | <refsect2> | |
448 | <title>Path-Based Activation</title> | |
449 | ||
450 | <para>Often, runtime of daemons processing spool files or | |
451 | directories (such as a printing system) can be delayed until | |
452 | these file system objects change state, or become non-empty. | |
453 | New-style init systems provide a way to bind service activation | |
454 | to file system changes. systemd implements this scheme via | |
455 | path-based activation configured in <filename>.path</filename> | |
456 | units, as outlined in | |
457 | <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para> | |
458 | </refsect2> | |
459 | ||
460 | <refsect2> | |
461 | <title>Timer-Based Activation</title> | |
462 | ||
463 | <para>Some daemons that implement clean-up jobs that are | |
464 | intended to be executed in regular intervals benefit from | |
465 | timer-based activation. In systemd, this is implemented via | |
466 | <filename>.timer</filename> units, as described in | |
467 | <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para> | |
468 | </refsect2> | |
469 | ||
470 | <refsect2> | |
471 | <title>Other Forms of Activation</title> | |
472 | ||
473 | <para>Other forms of activation have been suggested and | |
474 | implemented in some systems. However, there are often simpler or | |
475 | better alternatives, or they can be put together of combinations | |
476 | of the schemes above. Example: Sometimes, it appears useful to | |
477 | start daemons or <filename>.socket</filename> units when a | |
478 | specific IP address is configured on a network interface, | |
479 | because network sockets shall be bound to the address. However, | |
480 | an alternative to implement this is by utilizing the Linux | |
481 | <constant>IP_FREEBIND</constant> socket option, as accessible | |
482 | via <varname>FreeBind=yes</varname> in systemd socket files (see | |
483 | <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
484 | for details). This option, when enabled, allows sockets to be | |
485 | bound to a non-local, not configured IP address, and hence | |
486 | allows bindings to a particular IP address before it actually | |
487 | becomes available, making such an explicit dependency to the | |
488 | configured address redundant. Another often suggested trigger | |
489 | for service activation is low system load. However, here too, a | |
490 | more convincing approach might be to make proper use of features | |
b938cb90 | 491 | of the operating system, in particular, the CPU or I/O scheduler |
798d3a52 ZJS |
492 | of Linux. Instead of scheduling jobs from userspace based on |
493 | monitoring the OS scheduler, it is advisable to leave the | |
494 | scheduling of processes to the OS scheduler itself. systemd | |
b938cb90 | 495 | provides fine-grained access to the CPU and I/O schedulers. If a |
798d3a52 | 496 | process executed by the init system shall not negatively impact |
b938cb90 | 497 | the amount of CPU or I/O bandwidth available to other processes, |
798d3a52 ZJS |
498 | it should be configured with |
499 | <varname>CPUSchedulingPolicy=idle</varname> and/or | |
500 | <varname>IOSchedulingClass=idle</varname>. Optionally, this may | |
501 | be combined with timer-based activation to schedule background | |
502 | jobs during runtime and with minimal impact on the system, and | |
503 | remove it from the boot phase itself.</para> | |
504 | </refsect2> | |
505 | ||
506 | </refsect1> | |
507 | <refsect1> | |
508 | <title>Integration with Systemd</title> | |
509 | ||
510 | <refsect2> | |
511 | <title>Writing Systemd Unit Files</title> | |
512 | ||
513 | <para>When writing systemd unit files, it is recommended to | |
514 | consider the following suggestions:</para> | |
515 | ||
516 | <orderedlist> | |
517 | <listitem><para>If possible, do not use the | |
518 | <varname>Type=forking</varname> setting in service files. But | |
519 | if you do, make sure to set the PID file path using | |
520 | <varname>PIDFile=</varname>. See | |
521 | <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
522 | for details.</para></listitem> | |
523 | ||
524 | <listitem><para>If your daemon registers a D-Bus name on the | |
525 | bus, make sure to use <varname>Type=dbus</varname> in the | |
526 | service file if possible.</para></listitem> | |
527 | ||
528 | <listitem><para>Make sure to set a good human-readable | |
529 | description string with | |
530 | <varname>Description=</varname>.</para></listitem> | |
531 | ||
532 | <listitem><para>Do not disable | |
533 | <varname>DefaultDependencies=</varname>, unless you really | |
534 | know what you do and your unit is involved in early boot or | |
535 | late system shutdown.</para></listitem> | |
536 | ||
537 | <listitem><para>Normally, little if any dependencies should | |
538 | need to be defined explicitly. However, if you do configure | |
539 | explicit dependencies, only refer to unit names listed on | |
540 | <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry> | |
541 | or names introduced by your own package to keep the unit file | |
542 | operating system-independent.</para></listitem> | |
543 | ||
544 | <listitem><para>Make sure to include an | |
545 | <literal>[Install]</literal> section including installation | |
546 | information for the unit file. See | |
547 | <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
548 | for details. To activate your service on boot, make sure to | |
549 | add a <varname>WantedBy=multi-user.target</varname> or | |
550 | <varname>WantedBy=graphical.target</varname> directive. To | |
551 | activate your socket on boot, make sure to add | |
552 | <varname>WantedBy=sockets.target</varname>. Usually, you also | |
553 | want to make sure that when your service is installed, your | |
554 | socket is installed too, hence add | |
555 | <varname>Also=foo.socket</varname> in your service file | |
556 | <filename>foo.service</filename>, for a hypothetical program | |
557 | <filename>foo</filename>.</para></listitem> | |
558 | ||
559 | </orderedlist> | |
560 | </refsect2> | |
561 | ||
562 | <refsect2> | |
563 | <title>Installing Systemd Service Files</title> | |
564 | ||
565 | <para>At the build installation time (e.g. <command>make | |
566 | install</command> during package build), packages are | |
567 | recommended to install their systemd unit files in the directory | |
568 | returned by <command>pkg-config systemd | |
569 | --variable=systemdsystemunitdir</command> (for system services) | |
570 | or <command>pkg-config systemd | |
571 | --variable=systemduserunitdir</command> (for user services). | |
572 | This will make the services available in the system on explicit | |
573 | request but not activate them automatically during boot. | |
574 | Optionally, during package installation (e.g. <command>rpm | |
575 | -i</command> by the administrator), symlinks should be created | |
576 | in the systemd configuration directories via the | |
577 | <command>enable</command> command of the | |
578 | <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
579 | tool to activate them automatically on boot.</para> | |
580 | ||
581 | <para>Packages using | |
582 | <citerefentry project='die-net'><refentrytitle>autoconf</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
583 | are recommended to use a configure script | |
584 | excerpt like the following to determine the | |
585 | unit installation path during source | |
586 | configuration:</para> | |
587 | ||
588 | <programlisting>PKG_PROG_PKG_CONFIG | |
62adf224 | 589 | AC_ARG_WITH([systemdsystemunitdir], |
fc9acf25 | 590 | [AS_HELP_STRING([--with-systemdsystemunitdir=DIR], [Directory for systemd service files])],, |
5486855f MG |
591 | [with_systemdsystemunitdir=auto]) |
592 | AS_IF([test "x$with_systemdsystemunitdir" = "xyes" -o "x$with_systemdsystemunitdir" = "xauto"], [ | |
593 | def_systemdsystemunitdir=$($PKG_CONFIG --variable=systemdsystemunitdir systemd) | |
594 | ||
595 | AS_IF([test "x$def_systemdsystemunitdir" = "x"], | |
798d3a52 ZJS |
596 | [AS_IF([test "x$with_systemdsystemunitdir" = "xyes"], |
597 | [AC_MSG_ERROR([systemd support requested but pkg-config unable to query systemd package])]) | |
598 | with_systemdsystemunitdir=no], | |
599 | [with_systemdsystemunitdir="$def_systemdsystemunitdir"])]) | |
5486855f MG |
600 | AS_IF([test "x$with_systemdsystemunitdir" != "xno"], |
601 | [AC_SUBST([systemdsystemunitdir], [$with_systemdsystemunitdir])]) | |
fc9acf25 | 602 | AM_CONDITIONAL([HAVE_SYSTEMD], [test "x$with_systemdsystemunitdir" != "xno"])</programlisting> |
62adf224 | 603 | |
798d3a52 ZJS |
604 | <para>This snippet allows automatic |
605 | installation of the unit files on systemd | |
606 | machines, and optionally allows their | |
607 | installation even on machines lacking | |
608 | systemd. (Modification of this snippet for the | |
609 | user unit directory is left as an exercise for the | |
610 | reader.)</para> | |
62adf224 | 611 | |
798d3a52 ZJS |
612 | <para>Additionally, to ensure that |
613 | <command>make distcheck</command> continues to | |
614 | work, it is recommended to add the following | |
615 | to the top-level <filename>Makefile.am</filename> | |
616 | file in | |
617 | <citerefentry project='die-net'><refentrytitle>automake</refentrytitle><manvolnum>1</manvolnum></citerefentry>-based | |
618 | projects:</para> | |
62adf224 | 619 | |
798d3a52 ZJS |
620 | <programlisting>DISTCHECK_CONFIGURE_FLAGS = \ |
621 | --with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)</programlisting> | |
62adf224 | 622 | |
798d3a52 | 623 | <para>Finally, unit files should be installed in the system with an automake excerpt like the following:</para> |
62adf224 | 624 | |
798d3a52 | 625 | <programlisting>if HAVE_SYSTEMD |
62adf224 | 626 | systemdsystemunit_DATA = \ |
798d3a52 ZJS |
627 | foobar.socket \ |
628 | foobar.service | |
62adf224 LP |
629 | endif</programlisting> |
630 | ||
798d3a52 ZJS |
631 | <para>In the |
632 | <citerefentry project='die-net'><refentrytitle>rpm</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
633 | <filename>.spec</filename> file, use snippets like the following | |
634 | to enable/disable the service during | |
635 | installation/deinstallation. This makes use of the RPM macros | |
636 | shipped along systemd. Consult the packaging guidelines of your | |
637 | distribution for details and the equivalent for other package | |
638 | managers.</para> | |
8a422bb2 | 639 | |
798d3a52 | 640 | <para>At the top of the file:</para> |
8a422bb2 | 641 | |
798d3a52 | 642 | <programlisting>BuildRequires: systemd |
8a422bb2 LP |
643 | %{?systemd_requires}</programlisting> |
644 | ||
798d3a52 | 645 | <para>And as scriptlets, further down:</para> |
62adf224 | 646 | |
798d3a52 | 647 | <programlisting>%post |
8a422bb2 | 648 | %systemd_post foobar.service foobar.socket |
62adf224 LP |
649 | |
650 | %preun | |
8a422bb2 | 651 | %systemd_preun foobar.service foobar.socket |
ee5762e3 LP |
652 | |
653 | %postun | |
8a422bb2 LP |
654 | %systemd_postun</programlisting> |
655 | ||
798d3a52 ZJS |
656 | <para>If the service shall be restarted during upgrades, replace |
657 | the <literal>%postun</literal> scriptlet above with the | |
658 | following:</para> | |
8a422bb2 | 659 | |
798d3a52 | 660 | <programlisting>%postun |
8a422bb2 LP |
661 | %systemd_postun_with_restart foobar.service</programlisting> |
662 | ||
798d3a52 ZJS |
663 | <para>Note that <literal>%systemd_post</literal> and |
664 | <literal>%systemd_preun</literal> expect the names of all units | |
665 | that are installed/removed as arguments, separated by spaces. | |
666 | <literal>%systemd_postun</literal> expects no arguments. | |
667 | <literal>%systemd_postun_with_restart</literal> expects the | |
668 | units to restart as arguments.</para> | |
669 | ||
670 | <para>To facilitate upgrades from a package version that shipped | |
671 | only SysV init scripts to a package version that ships both a | |
672 | SysV init script and a native systemd service file, use a | |
673 | fragment like the following:</para> | |
674 | ||
675 | <programlisting>%triggerun -- foobar < 0.47.11-1 | |
63415a2d | 676 | if /sbin/chkconfig --level 5 foobar ; then |
798d3a52 | 677 | /bin/systemctl --no-reload enable foobar.service foobar.socket >/dev/null 2>&1 || : |
6908d384 LP |
678 | fi</programlisting> |
679 | ||
798d3a52 ZJS |
680 | <para>Where 0.47.11-1 is the first package version that includes |
681 | the native unit file. This fragment will ensure that the first | |
682 | time the unit file is installed, it will be enabled if and only | |
683 | if the SysV init script is enabled, thus making sure that the | |
684 | enable status is not changed. Note that | |
685 | <command>chkconfig</command> is a command specific to Fedora | |
686 | which can be used to check whether a SysV init script is | |
687 | enabled. Other operating systems will have to use different | |
688 | commands here.</para> | |
689 | </refsect2> | |
690 | </refsect1> | |
691 | ||
692 | <refsect1> | |
693 | <title>Porting Existing Daemons</title> | |
694 | ||
695 | <para>Since new-style init systems such as systemd are compatible | |
696 | with traditional SysV init systems, it is not strictly necessary | |
697 | to port existing daemons to the new style. However, doing so | |
698 | offers additional functionality to the daemons as well as | |
699 | simplifying integration into new-style init systems.</para> | |
700 | ||
701 | <para>To port an existing SysV compatible daemon, the following | |
702 | steps are recommended:</para> | |
703 | ||
704 | <orderedlist> | |
705 | <listitem><para>If not already implemented, add an optional | |
706 | command line switch to the daemon to disable daemonization. This | |
707 | is useful not only for using the daemon in new-style init | |
708 | systems, but also to ease debugging.</para></listitem> | |
709 | ||
710 | <listitem><para>If the daemon offers interfaces to other | |
711 | software running on the local system via local | |
712 | <constant>AF_UNIX</constant> sockets, consider implementing | |
713 | socket-based activation (see above). Usually, a minimal patch is | |
714 | sufficient to implement this: Extend the socket creation in the | |
715 | daemon code so that | |
716 | <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
717 | is checked for already passed sockets first. If sockets are | |
718 | passed (i.e. when <function>sd_listen_fds()</function> returns a | |
719 | positive value), skip the socket creation step and use the | |
720 | passed sockets. Secondly, ensure that the file system socket | |
721 | nodes for local <constant>AF_UNIX</constant> sockets used in the | |
722 | socket-based activation are not removed when the daemon shuts | |
723 | down, if sockets have been passed. Third, if the daemon normally | |
724 | closes all remaining open file descriptors as part of its | |
725 | initialization, the sockets passed from the init system must be | |
726 | spared. Since new-style init systems guarantee that no left-over | |
727 | file descriptors are passed to executed processes, it might be a | |
728 | good choice to simply skip the closing of all remaining open | |
729 | file descriptors if sockets are passed.</para></listitem> | |
730 | ||
731 | <listitem><para>Write and install a systemd unit file for the | |
732 | service (and the sockets if socket-based activation is used, as | |
733 | well as a path unit file, if the daemon processes a spool | |
734 | directory), see above for details.</para></listitem> | |
735 | ||
736 | <listitem><para>If the daemon exposes interfaces via D-Bus, | |
737 | write and install a D-Bus activation file for the service, see | |
738 | above for details.</para></listitem> | |
739 | </orderedlist> | |
740 | </refsect1> | |
741 | ||
742 | <refsect1> | |
743 | <title>Placing Daemon Data</title> | |
744 | ||
745 | <para>It is recommended to follow the general guidelines for | |
746 | placing package files, as discussed in | |
747 | <citerefentry><refentrytitle>file-hierarchy</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para> | |
748 | </refsect1> | |
749 | ||
750 | <refsect1> | |
751 | <title>See Also</title> | |
752 | <para> | |
753 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
754 | <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
755 | <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
756 | <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
757 | <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
758 | <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>, | |
759 | <citerefentry><refentrytitle>file-hierarchy</refentrytitle><manvolnum>7</manvolnum></citerefentry> | |
760 | </para> | |
761 | </refsect1> | |
64aba792 LP |
762 | |
763 | </refentry> |