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1 | <?xml version='1.0'?> <!--*-nxml-*--> |
2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" | |
3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> | |
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"> |
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25 | |
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> | |
34511ca7 | 47 | <refpurpose>Writing and packaging system daemons</refpurpose> |
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48 | </refnamediv> |
49 | ||
50 | <refsect1> | |
51 | <title>Description</title> | |
52 | ||
53 | <para>A daemon is a service process that runs in the | |
54 | background and supervises the system or provides | |
55 | functionality to other processes. Traditionally, | |
56 | daemons are implemented following a scheme originating | |
57 | in SysV Unix. Modern daemons should follow a simpler | |
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58 | yet more powerful scheme (here called "new-style" |
59 | daemons), as implemented by | |
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60 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>. This |
61 | manual page covers both schemes, and in | |
62 | particular includes recommendations for daemons that | |
63 | shall be included in the systemd init system.</para> | |
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64 | |
65 | <refsect2> | |
66 | <title>SysV Daemons</title> | |
67 | ||
68 | <para>When a traditional SysV daemon | |
69 | starts, it should execute the following steps | |
70 | as part of the initialization. Note that these | |
62adf224 | 71 | steps are unnecessary for new-style daemons (see below), |
436c44a5 | 72 | and should only be implemented if compatibility |
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73 | with SysV is essential.</para> |
74 | ||
75 | <orderedlist> | |
76 | <listitem><para>Close all open file | |
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77 | descriptors except standard input, output, |
78 | and error (i.e. the first three file | |
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79 | descriptors 0, 1, 2). This ensures |
80 | that no accidentally passed file | |
81 | descriptor stays around in the daemon | |
409dee2e | 82 | process. On Linux, this is best |
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83 | implemented by iterating through |
84 | <filename>/proc/self/fd</filename>, | |
85 | with a fallback of iterating from file | |
86 | descriptor 3 to the value returned by | |
62adf224 | 87 | <function>getrlimit()</function> for |
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88 | RLIMIT_NOFILE.</para></listitem> |
89 | ||
90 | <listitem><para>Reset all signal | |
91 | handlers to their default. This is | |
92 | best done by iterating through the | |
93 | available signals up to the limit of | |
94 | _NSIG and resetting them to | |
05cc7267 | 95 | <constant>SIG_DFL</constant>.</para></listitem> |
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96 | |
97 | <listitem><para>Reset the signal mask | |
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98 | using |
99 | <function>sigprocmask()</function>.</para></listitem> | |
64aba792 | 100 | |
62adf224 LP |
101 | <listitem><para>Sanitize the |
102 | environment block, removing or | |
103 | resetting environment variables that | |
104 | might negatively impact daemon | |
105 | runtime.</para></listitem> | |
106 | ||
107 | <listitem><para>Call <function>fork()</function>, | |
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108 | to create a background |
109 | process.</para></listitem> | |
110 | ||
111 | <listitem><para>In the child, call | |
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112 | <function>setsid()</function> to |
113 | detach from any terminal and create an | |
114 | independent session.</para></listitem> | |
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115 | |
116 | <listitem><para>In the child, call | |
62adf224 | 117 | <function>fork()</function> again, to |
409dee2e | 118 | ensure that the daemon can never re-acquire |
62adf224 | 119 | a terminal again.</para></listitem> |
64aba792 | 120 | |
62adf224 | 121 | <listitem><para>Call <function>exit()</function> in the |
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122 | first child, so that only the second |
123 | child (the actual daemon process) | |
124 | stays around. This ensures that the | |
2ab38e24 | 125 | daemon process is re-parented to |
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126 | init/PID 1, as all daemons should |
127 | be.</para></listitem> | |
128 | ||
129 | <listitem><para>In the daemon process, | |
130 | connect <filename>/dev/null</filename> | |
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131 | to standard input, output, and error. |
132 | </para></listitem> | |
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133 | |
134 | <listitem><para>In the daemon process, | |
135 | reset the umask to 0, so that the file | |
62adf224 | 136 | modes passed to <function>open()</function>, <function>mkdir()</function> and |
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137 | suchlike directly control the access |
138 | mode of the created files and | |
139 | directories.</para></listitem> | |
140 | ||
141 | <listitem><para>In the daemon process, | |
142 | change the current directory to the | |
143 | root directory (/), in order to avoid | |
144 | that the daemon involuntarily | |
145 | blocks mount points from being | |
146 | unmounted.</para></listitem> | |
147 | ||
62adf224 LP |
148 | <listitem><para>In the daemon process, |
149 | write the daemon PID (as returned by | |
150 | <function>getpid()</function>) to a | |
151 | PID file, for example | |
41a55c46 | 152 | <filename>/run/foobar.pid</filename> |
409dee2e | 153 | (for a hypothetical daemon "foobar") |
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154 | to ensure that the daemon cannot be |
155 | started more than once. This must be | |
156 | implemented in race-free fashion so | |
157 | that the PID file is only updated when | |
409dee2e | 158 | it is verified at the same time that |
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159 | the PID previously stored in the PID |
160 | file no longer exists or belongs to a | |
c4b834a4 | 161 | foreign process.</para></listitem> |
62adf224 | 162 | |
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163 | <listitem><para>In the daemon process, |
164 | drop privileges, if possible and | |
165 | applicable.</para></listitem> | |
166 | ||
167 | <listitem><para>From the daemon | |
409dee2e | 168 | process, notify the original process |
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169 | started that initialization is |
170 | complete. This can be implemented via | |
171 | an unnamed pipe or similar | |
172 | communication channel that is created | |
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173 | before the first |
174 | <function>fork()</function> and hence | |
175 | available in both the original and the | |
176 | daemon process.</para></listitem> | |
64aba792 | 177 | |
62adf224 LP |
178 | <listitem><para>Call |
179 | <function>exit()</function> in the | |
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180 | original process. The process that |
181 | invoked the daemon must be able to | |
c5315881 | 182 | rely on that this |
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183 | <function>exit()</function> happens |
184 | after initialization is complete and | |
185 | all external communication channels | |
c5315881 | 186 | are established and |
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187 | accessible.</para></listitem> |
188 | </orderedlist> | |
189 | ||
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190 | <para>The BSD <function>daemon()</function> function should not be |
191 | used, as it implements only a subset of these steps.</para> | |
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192 | |
193 | <para>A daemon that needs to provide | |
194 | compatibility with SysV systems should | |
195 | implement the scheme pointed out | |
196 | above. However, it is recommended to make this | |
c5315881 | 197 | behavior optional and configurable via a |
409dee2e | 198 | command line argument to ease debugging as |
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199 | well as to simplify integration into systems |
200 | using systemd.</para> | |
201 | </refsect2> | |
202 | ||
203 | <refsect2> | |
204 | <title>New-Style Daemons</title> | |
205 | ||
206 | <para>Modern services for Linux should be | |
207 | implemented as new-style daemons. This makes it | |
208 | easier to supervise and control them at | |
209 | runtime and simplifies their | |
210 | implementation.</para> | |
211 | ||
409dee2e | 212 | <para>For developing a new-style daemon, none |
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213 | of the initialization steps recommended for |
214 | SysV daemons need to be implemented. New-style | |
215 | init systems such as systemd make all of them | |
216 | redundant. Moreover, since some of these steps | |
217 | interfere with process monitoring, file | |
218 | descriptor passing and other functionality of | |
409dee2e | 219 | the init system, it is recommended not to |
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220 | execute them when run as new-style |
221 | service.</para> | |
222 | ||
62adf224 | 223 | <para>Note that new-style init systems |
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224 | guarantee execution of daemon processes in a |
225 | clean process context: it is guaranteed that | |
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226 | the environment block is sanitized, that the |
227 | signal handlers and mask is reset and that no | |
228 | left-over file descriptors are passed. Daemons | |
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229 | will be executed in their own session, with |
230 | standard input/output/error connected to | |
62adf224 | 231 | <filename>/dev/null</filename> unless |
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232 | otherwise configured. The umask is reset. |
233 | </para> | |
62adf224 | 234 | |
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235 | <para>It is recommended for new-style daemons |
236 | to implement the following:</para> | |
237 | ||
238 | <orderedlist> | |
05cc7267 | 239 | <listitem><para>If <constant>SIGTERM</constant> is |
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240 | received, shut down the daemon and |
241 | exit cleanly.</para></listitem> | |
242 | ||
05cc7267 | 243 | <listitem><para>If <constant>SIGHUP</constant> is received, |
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244 | reload the configuration files, if |
245 | this applies.</para></listitem> | |
246 | ||
247 | <listitem><para>Provide a correct exit | |
248 | code from the main daemon process, as | |
249 | this is used by the init system to | |
250 | detect service errors and problems. It | |
251 | is recommended to follow the exit code | |
62adf224 | 252 | scheme as defined in the <ulink |
27d14fb3 | 253 | url="http://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html">LSB |
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254 | recommendations for SysV init |
255 | scripts</ulink>.</para></listitem> | |
64aba792 | 256 | |
64aba792 | 257 | <listitem><para>If possible and |
409dee2e | 258 | applicable, expose the daemon's control |
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259 | interface via the D-Bus IPC system and |
260 | grab a bus name as last step of | |
261 | initialization.</para></listitem> | |
262 | ||
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263 | <listitem><para>For integration in |
264 | systemd, provide a | |
265 | <filename>.service</filename> unit | |
266 | file that carries information about | |
267 | starting, stopping and otherwise | |
268 | maintaining the daemon. See | |
269 | <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
270 | for details.</para></listitem> | |
271 | ||
272 | <listitem><para>As much as possible, | |
c5315881 | 273 | rely on the init system's |
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274 | functionality to limit the access of |
275 | the daemon to files, services and | |
409dee2e | 276 | other resources, i.e. in the case of |
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277 | systemd, rely on systemd's resource |
278 | limit control instead of implementing | |
279 | your own, rely on systemd's privilege | |
280 | dropping code instead of implementing | |
281 | it in the daemon, and similar. See | |
282 | <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
283 | for the available | |
284 | controls.</para></listitem> | |
285 | ||
64aba792 | 286 | <listitem><para>If D-Bus is used, make |
409dee2e | 287 | your daemon bus-activatable by |
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288 | supplying a D-Bus service activation |
289 | configuration file. This has multiple | |
290 | advantages: your daemon may be started | |
291 | lazily on-demand; it may be started in | |
292 | parallel to other daemons requiring it | |
293 | -- which maximizes parallelization and | |
294 | boot-up speed; your daemon can be | |
409dee2e | 295 | restarted on failure without losing |
64aba792 | 296 | any bus requests, as the bus queues |
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297 | requests for activatable services. See |
298 | below for details.</para></listitem> | |
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299 | |
300 | <listitem><para>If your daemon | |
301 | provides services to other local | |
302 | processes or remote clients via a | |
303 | socket, it should be made | |
304 | socket-activatable following the | |
305 | scheme pointed out below. Like D-Bus | |
409dee2e | 306 | activation, this enables on-demand |
64aba792 | 307 | starting of services as well as it |
436c44a5 | 308 | allows improved parallelization of |
64aba792 | 309 | service start-up. Also, for state-less |
409dee2e | 310 | protocols (such as syslog, DNS), a |
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311 | daemon implementing socket-based |
312 | activation can be restarted without | |
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313 | losing a single request. See below for |
314 | details.</para></listitem> | |
64aba792 | 315 | |
409dee2e | 316 | <listitem><para>If applicable, a daemon |
64aba792 | 317 | should notify the init system about |
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318 | startup completion or status updates |
319 | via the | |
320 | <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
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321 | interface.</para></listitem> |
322 | ||
323 | <listitem><para>Instead of using the | |
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324 | <function>syslog()</function> call to |
325 | log directly to the system syslog | |
326 | service, a new-style daemon may choose | |
327 | to simply log to standard error via | |
328 | <function>fprintf()</function>, which | |
329 | is then forwarded to syslog by the | |
330 | init system. If log priorities are | |
331 | necessary, these can be encoded by | |
332 | prefixing individual log lines with | |
333 | strings like "<4>" (for log | |
334 | priority 4 "WARNING" in the syslog | |
335 | priority scheme), following a similar | |
336 | style as the Linux kernel's | |
337 | <function>printk()</function> priority | |
338 | system. For details, see | |
cb07866b | 339 | <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry> |
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340 | and |
341 | <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem> | |
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342 | |
343 | </orderedlist> | |
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344 | |
345 | <para>These recommendations are similar but | |
346 | not identical to the <ulink | |
0afedd30 | 347 | url="https://developer.apple.com/library/mac/documentation/MacOSX/Conceptual/BPSystemStartup/Chapters/CreatingLaunchdJobs.html">Apple |
62adf224 | 348 | MacOS X Daemon Requirements</ulink>.</para> |
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349 | </refsect2> |
350 | ||
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351 | </refsect1> |
352 | <refsect1> | |
353 | <title>Activation</title> | |
354 | ||
355 | <para>New-style init systems provide multiple | |
356 | additional mechanisms to activate services, as | |
357 | detailed below. It is common that services are | |
358 | configured to be activated via more than one mechanism | |
359 | at the same time. An example for systemd: | |
360 | <filename>bluetoothd.service</filename> might get | |
361 | activated either when Bluetooth hardware is plugged | |
362 | in, or when an application accesses its programming | |
363 | interfaces via D-Bus. Or, a print server daemon might | |
364 | get activated when traffic arrives at an IPP port, or | |
365 | when a printer is plugged in, or when a file is queued | |
366 | in the printer spool directory. Even for services that | |
367 | are intended to be started on system bootup | |
409dee2e | 368 | unconditionally, it is a good idea to implement some of |
99ffae46 | 369 | the various activation schemes outlined below, in |
409dee2e | 370 | order to maximize parallelization. If a daemon |
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371 | implements a D-Bus service or listening socket, |
372 | implementing the full bus and socket activation scheme | |
373 | allows starting of the daemon with its clients in | |
374 | parallel (which speeds up boot-up), since all its | |
375 | communication channels are established already, and no | |
376 | request is lost because client requests will be queued | |
377 | by the bus system (in case of D-Bus) or the kernel (in | |
409dee2e | 378 | case of sockets) until the activation is |
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379 | completed.</para> |
380 | ||
381 | <refsect2> | |
382 | <title>Activation on Boot</title> | |
383 | ||
384 | <para>Old-style daemons are usually activated | |
385 | exclusively on boot (and manually by the | |
386 | administrator) via SysV init scripts, as | |
387 | detailed in the <ulink | |
27d14fb3 | 388 | url="http://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html">LSB |
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389 | Linux Standard Base Core |
390 | Specification</ulink>. This method of | |
8f28cbcd | 391 | activation is supported ubiquitously on Linux |
99ffae46 | 392 | init systems, both old-style and new-style |
409dee2e | 393 | systems. Among other issues, SysV init scripts |
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394 | have the disadvantage of involving shell |
395 | scripts in the boot process. New-style init | |
396 | systems generally employ updated versions of | |
397 | activation, both during boot-up and during | |
398 | runtime and using more minimal service | |
399 | description files.</para> | |
400 | ||
401 | <para>In systemd, if the developer or | |
e0e009c0 | 402 | administrator wants to make sure that a service or |
409dee2e | 403 | other unit is activated automatically on boot, |
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404 | it is recommended to place a symlink to the |
405 | unit file in the <filename>.wants/</filename> | |
406 | directory of either | |
407 | <filename>multi-user.target</filename> or | |
408 | <filename>graphical.target</filename>, which | |
409 | are normally used as boot targets at system | |
410 | startup. See | |
411 | <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
412 | for details about the | |
413 | <filename>.wants/</filename> directories, and | |
414 | <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry> | |
415 | for details about the two boot targets.</para> | |
416 | ||
417 | </refsect2> | |
418 | ||
64aba792 | 419 | <refsect2> |
62adf224 | 420 | <title>Socket-Based Activation</title> |
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421 | |
422 | <para>In order to maximize the possible | |
423 | parallelization and robustness and simplify | |
424 | configuration and development, it is | |
425 | recommended for all new-style daemons that | |
426 | communicate via listening sockets to employ | |
427 | socket-based activation. In a socket-based | |
409dee2e | 428 | activation scheme, the creation and binding of |
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429 | the listening socket as primary communication |
430 | channel of daemons to local (and sometimes | |
431 | remote) clients is moved out of the daemon | |
432 | code and into the init system. Based on | |
409dee2e | 433 | per-daemon configuration, the init system |
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434 | installs the sockets and then hands them off |
435 | to the spawned process as soon as the | |
436 | respective daemon is to be started. | |
409dee2e | 437 | Optionally, activation of the service can be |
99ffae46 | 438 | delayed until the first inbound traffic |
409dee2e | 439 | arrives at the socket to implement on-demand |
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440 | activation of daemons. However, the primary |
441 | advantage of this scheme is that all providers | |
442 | and all consumers of the sockets can be | |
20604ebc | 443 | started in parallel as soon as all sockets |
409dee2e | 444 | are established. In addition to that, daemons |
99ffae46 | 445 | can be restarted with losing only a minimal |
409dee2e | 446 | number of client transactions, or even any |
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447 | client request at all (the latter is |
448 | particularly true for state-less protocols, | |
449 | such as DNS or syslog), because the socket | |
450 | stays bound and accessible during the restart, | |
451 | and all requests are queued while the daemon | |
452 | cannot process them.</para> | |
453 | ||
454 | <para>New-style daemons which support socket | |
455 | activation must be able to receive their | |
409dee2e | 456 | sockets from the init system instead of |
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457 | creating and binding them themselves. For |
458 | details about the programming interfaces for | |
409dee2e | 459 | this scheme provided by systemd, see |
99ffae46 LP |
460 | <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry> |
461 | and | |
cb07866b | 462 | <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>. For |
99ffae46 | 463 | details about porting existing daemons to |
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464 | socket-based activation, see below. With |
465 | minimal effort, it is possible to implement | |
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466 | socket-based activation in addition to |
467 | traditional internal socket creation in the | |
468 | same codebase in order to support both | |
469 | new-style and old-style init systems from the | |
470 | same daemon binary.</para> | |
471 | ||
472 | <para>systemd implements socket-based | |
473 | activation via <filename>.socket</filename> | |
474 | units, which are described in | |
475 | <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>. When | |
476 | configuring socket units for socket-based | |
409dee2e | 477 | activation, it is essential that all listening |
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478 | sockets are pulled in by the special target |
479 | unit <filename>sockets.target</filename>. It | |
480 | is recommended to place a | |
481 | <varname>WantedBy=sockets.target</varname> | |
482 | directive in the <literal>[Install]</literal> | |
409dee2e | 483 | section to automatically add such a |
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484 | dependency on installation of a socket |
485 | unit. Unless | |
486 | <varname>DefaultDependencies=no</varname> is | |
409dee2e | 487 | set, the necessary ordering dependencies are |
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488 | implicitly created for all socket units. For |
489 | more information about | |
409dee2e | 490 | <filename>sockets.target</filename>, see |
99ffae46 LP |
491 | <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>. It |
492 | is not necessary or recommended to place any | |
493 | additional dependencies on socket units (for | |
494 | example from | |
495 | <filename>multi-user.target</filename> or | |
496 | suchlike) when one is installed in | |
497 | <filename>sockets.target</filename>.</para> | |
64aba792 LP |
498 | </refsect2> |
499 | ||
500 | <refsect2> | |
62adf224 | 501 | <title>Bus-Based Activation</title> |
99ffae46 LP |
502 | |
503 | <para>When the D-Bus IPC system is used for | |
504 | communication with clients, new-style daemons | |
505 | should employ bus activation so that they are | |
506 | automatically activated when a client | |
507 | application accesses their IPC | |
508 | interfaces. This is configured in D-Bus | |
509 | service files (not to be confused with systemd | |
510 | service unit files!). To ensure that D-Bus | |
511 | uses systemd to start-up and maintain the | |
409dee2e | 512 | daemon, use the |
99ffae46 | 513 | <varname>SystemdService=</varname> directive |
409dee2e | 514 | in these service files to configure the |
99ffae46 | 515 | matching systemd service for a D-Bus |
409dee2e | 516 | service. e.g.: For a D-Bus service whose D-Bus |
99ffae46 LP |
517 | activation file is named |
518 | <filename>org.freedesktop.RealtimeKit.service</filename>, | |
519 | make sure to set | |
520 | <varname>SystemdService=rtkit-daemon.service</varname> | |
409dee2e | 521 | in that file to bind it to the systemd |
99ffae46 LP |
522 | service |
523 | <filename>rtkit-daemon.service</filename>. This | |
524 | is needed to make sure that the daemon is | |
525 | started in a race-free fashion when activated | |
526 | via multiple mechanisms simultaneously.</para> | |
527 | </refsect2> | |
528 | ||
529 | <refsect2> | |
530 | <title>Device-Based Activation</title> | |
531 | ||
532 | <para>Often, daemons that manage a particular | |
533 | type of hardware should be activated only when | |
534 | the hardware of the respective kind is plugged | |
535 | in or otherwise becomes available. In a | |
409dee2e | 536 | new-style init system, it is possible to bind |
ee5762e3 LP |
537 | activation to hardware plug/unplug events. In |
538 | systemd, kernel devices appearing in the | |
539 | sysfs/udev device tree can be exposed as units | |
540 | if they are tagged with the string | |
909f413d | 541 | <literal>systemd</literal>. Like any other |
409dee2e JE |
542 | kind of unit, they may then pull in other units |
543 | when activated (i.e. plugged in) and thus | |
544 | implement device-based activation. systemd | |
99ffae46 LP |
545 | dependencies may be encoded in the udev |
546 | database via the | |
547 | <varname>SYSTEMD_WANTS=</varname> | |
548 | property. See | |
549 | <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
409dee2e | 550 | for details. Often, it is nicer to pull in |
99ffae46 | 551 | services from devices only indirectly via |
409dee2e | 552 | dedicated targets. Example: Instead of pulling |
99ffae46 LP |
553 | in <filename>bluetoothd.service</filename> |
554 | from all the various bluetooth dongles and | |
555 | other hardware available, pull in | |
556 | bluetooth.target from them and | |
557 | <filename>bluetoothd.service</filename> from | |
558 | that target. This provides for nicer | |
559 | abstraction and gives administrators the | |
560 | option to enable | |
561 | <filename>bluetoothd.service</filename> via | |
562 | controlling a | |
563 | <filename>bluetooth.target.wants/</filename> | |
ee5762e3 LP |
564 | symlink uniformly with a command like |
565 | <command>enable</command> of | |
566 | <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
99ffae46 LP |
567 | instead of manipulating the udev |
568 | ruleset.</para> | |
64aba792 LP |
569 | </refsect2> |
570 | ||
571 | <refsect2> | |
62adf224 | 572 | <title>Path-Based Activation</title> |
99ffae46 LP |
573 | |
574 | <para>Often, runtime of daemons processing | |
575 | spool files or directories (such as a printing | |
576 | system) can be delayed until these file system | |
577 | objects change state, or become | |
578 | non-empty. New-style init systems provide a | |
579 | way to bind service activation to file system | |
580 | changes. systemd implements this scheme via | |
581 | path-based activation configured in | |
582 | <filename>.path</filename> units, as outlined | |
583 | in | |
584 | <citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para> | |
585 | </refsect2> | |
586 | ||
587 | <refsect2> | |
588 | <title>Timer-Based Activation</title> | |
589 | ||
590 | <para>Some daemons that implement clean-up | |
591 | jobs that are intended to be executed in | |
592 | regular intervals benefit from timer-based | |
593 | activation. In systemd, this is implemented | |
594 | via <filename>.timer</filename> units, as | |
595 | described in | |
596 | <citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para> | |
62adf224 LP |
597 | </refsect2> |
598 | ||
99ffae46 LP |
599 | <refsect2> |
600 | <title>Other Forms of Activation</title> | |
601 | ||
602 | <para>Other forms of activation have been | |
603 | suggested and implemented in some | |
409dee2e | 604 | systems. However, there are often simpler or |
99ffae46 LP |
605 | better alternatives, or they can be put |
606 | together of combinations of the schemes | |
409dee2e | 607 | above. Example: Sometimes, it appears useful to |
99ffae46 LP |
608 | start daemons or <filename>.socket</filename> |
609 | units when a specific IP address is configured | |
610 | on a network interface, because network | |
611 | sockets shall be bound to the | |
612 | address. However, an alternative to implement | |
613 | this is by utilizing the Linux IP_FREEBIND | |
614 | socket option, as accessible via | |
615 | <varname>FreeBind=yes</varname> in systemd | |
616 | socket files (see | |
617 | <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
618 | for details). This option, when enabled, | |
619 | allows sockets to be bound to a non-local, not | |
620 | configured IP address, and hence allows | |
621 | bindings to a particular IP address before it | |
622 | actually becomes available, making such an | |
623 | explicit dependency to the configured address | |
624 | redundant. Another often suggested trigger for | |
625 | service activation is low system | |
626 | load. However, here too, a more convincing | |
627 | approach might be to make proper use of | |
409dee2e | 628 | features of the operating system, in |
99ffae46 LP |
629 | particular, the CPU or IO scheduler of |
630 | Linux. Instead of scheduling jobs from | |
631 | userspace based on monitoring the OS | |
632 | scheduler, it is advisable to leave the | |
633 | scheduling of processes to the OS scheduler | |
634 | itself. systemd provides fine-grained access | |
635 | to the CPU and IO schedulers. If a process | |
636 | executed by the init system shall not | |
637 | negatively impact the amount of CPU or IO | |
2dddca4a | 638 | bandwidth available to other processes, it |
99ffae46 LP |
639 | should be configured with |
640 | <varname>CPUSchedulingPolicy=idle</varname> | |
641 | and/or | |
642 | <varname>IOSchedulingClass=idle</varname>. Optionally, | |
643 | this may be combined with timer-based | |
644 | activation to schedule background jobs during | |
645 | runtime and with minimal impact on the system, | |
646 | and remove it from the boot phase | |
647 | itself.</para> | |
648 | </refsect2> | |
649 | ||
650 | </refsect1> | |
651 | <refsect1> | |
652 | <title>Integration with Systemd</title> | |
653 | ||
62adf224 LP |
654 | <refsect2> |
655 | <title>Writing Systemd Unit Files</title> | |
656 | ||
657 | <para>When writing systemd unit files, it is | |
658 | recommended to consider the following | |
659 | suggestions:</para> | |
660 | ||
661 | <orderedlist> | |
409dee2e | 662 | <listitem><para>If possible, do not use |
62adf224 LP |
663 | the <varname>Type=forking</varname> |
664 | setting in service files. But if you | |
665 | do, make sure to set the PID file path | |
666 | using <varname>PIDFile=</varname>. See | |
667 | <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry> | |
668 | for details.</para></listitem> | |
669 | ||
670 | <listitem><para>If your daemon | |
671 | registers a D-Bus name on the bus, | |
672 | make sure to use | |
828f33e8 LP |
673 | <varname>Type=dbus</varname> in the |
674 | service file if | |
62adf224 LP |
675 | possible.</para></listitem> |
676 | ||
677 | <listitem><para>Make sure to set a | |
678 | good human-readable description string | |
679 | with | |
680 | <varname>Description=</varname>.</para></listitem> | |
681 | ||
682 | <listitem><para>Do not disable | |
683 | <varname>DefaultDependencies=</varname>, | |
684 | unless you really know what you do and | |
685 | your unit is involved in early boot or | |
686 | late system shutdown.</para></listitem> | |
687 | ||
688 | <listitem><para>Normally, little if | |
689 | any dependencies should need to | |
690 | be defined explicitly. However, if you | |
691 | do configure explicit dependencies, only refer to | |
692 | unit names listed on | |
693 | <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry> | |
694 | or names introduced by your own | |
695 | package to keep the unit file | |
696 | operating | |
697 | system-independent.</para></listitem> | |
698 | ||
699 | <listitem><para>Make sure to include | |
828f33e8 LP |
700 | an <literal>[Install]</literal> |
701 | section including installation | |
702 | information for the unit file. See | |
62adf224 LP |
703 | <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry> |
704 | for details. To activate your service | |
409dee2e | 705 | on boot, make sure to add a |
62adf224 LP |
706 | <varname>WantedBy=multi-user.target</varname> |
707 | or | |
828f33e8 LP |
708 | <varname>WantedBy=graphical.target</varname> |
709 | directive. To activate your socket on | |
710 | boot, make sure to add | |
409dee2e | 711 | <varname>WantedBy=sockets.target</varname>. Usually, |
828f33e8 | 712 | you also want to make sure that when |
409dee2e | 713 | your service is installed, your socket |
828f33e8 LP |
714 | is installed too, hence add |
715 | <varname>Also=foo.socket</varname> in | |
716 | your service file | |
717 | <filename>foo.service</filename>, for | |
718 | a hypothetical program | |
719 | <filename>foo</filename>.</para></listitem> | |
62adf224 LP |
720 | |
721 | </orderedlist> | |
64aba792 LP |
722 | </refsect2> |
723 | ||
724 | <refsect2> | |
99ffae46 | 725 | <title>Installing Systemd Service Files</title> |
62adf224 LP |
726 | |
727 | <para>At the build installation time | |
728 | (e.g. <command>make install</command> during | |
409dee2e | 729 | package build), packages are recommended to |
62adf224 LP |
730 | install their systemd unit files in the |
731 | directory returned by <command>pkg-config | |
732 | systemd | |
ee5762e3 | 733 | --variable=systemdsystemunitdir</command> (for |
16dad32e | 734 | system services) or <command>pkg-config |
ee5762e3 | 735 | systemd |
af2d49f7 LP |
736 | --variable=systemduserunitdir</command> |
737 | (for user services). This will make the | |
62adf224 LP |
738 | services available in the system on explicit |
739 | request but not activate them automatically | |
740 | during boot. Optionally, during package | |
741 | installation (e.g. <command>rpm -i</command> | |
409dee2e | 742 | by the administrator), symlinks should be |
62adf224 | 743 | created in the systemd configuration |
ee5762e3 LP |
744 | directories via the <command>enable</command> |
745 | command of the | |
746 | <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
409dee2e | 747 | tool to activate them automatically on |
62adf224 LP |
748 | boot.</para> |
749 | ||
750 | <para>Packages using | |
5aded369 | 751 | <citerefentry project='die-net'><refentrytitle>autoconf</refentrytitle><manvolnum>1</manvolnum></citerefentry> |
62adf224 LP |
752 | are recommended to use a configure script |
753 | excerpt like the following to determine the | |
754 | unit installation path during source | |
755 | configuration:</para> | |
756 | ||
757 | <programlisting>PKG_PROG_PKG_CONFIG | |
758 | AC_ARG_WITH([systemdsystemunitdir], | |
fc9acf25 | 759 | [AS_HELP_STRING([--with-systemdsystemunitdir=DIR], [Directory for systemd service files])],, |
5486855f MG |
760 | [with_systemdsystemunitdir=auto]) |
761 | AS_IF([test "x$with_systemdsystemunitdir" = "xyes" -o "x$with_systemdsystemunitdir" = "xauto"], [ | |
762 | def_systemdsystemunitdir=$($PKG_CONFIG --variable=systemdsystemunitdir systemd) | |
763 | ||
764 | AS_IF([test "x$def_systemdsystemunitdir" = "x"], | |
765 | [AS_IF([test "x$with_systemdsystemunitdir" = "xyes"], | |
766 | [AC_MSG_ERROR([systemd support requested but pkg-config unable to query systemd package])]) | |
767 | with_systemdsystemunitdir=no], | |
fc9acf25 | 768 | [with_systemdsystemunitdir="$def_systemdsystemunitdir"])]) |
5486855f MG |
769 | AS_IF([test "x$with_systemdsystemunitdir" != "xno"], |
770 | [AC_SUBST([systemdsystemunitdir], [$with_systemdsystemunitdir])]) | |
fc9acf25 | 771 | AM_CONDITIONAL([HAVE_SYSTEMD], [test "x$with_systemdsystemunitdir" != "xno"])</programlisting> |
62adf224 LP |
772 | |
773 | <para>This snippet allows automatic | |
774 | installation of the unit files on systemd | |
775 | machines, and optionally allows their | |
776 | installation even on machines lacking | |
777 | systemd. (Modification of this snippet for the | |
9f7dad77 | 778 | user unit directory is left as an exercise for the |
62adf224 LP |
779 | reader.)</para> |
780 | ||
781 | <para>Additionally, to ensure that | |
782 | <command>make distcheck</command> continues to | |
783 | work, it is recommended to add the following | |
784 | to the top-level <filename>Makefile.am</filename> | |
785 | file in | |
5aded369 | 786 | <citerefentry project='die-net'><refentrytitle>automake</refentrytitle><manvolnum>1</manvolnum></citerefentry>-based |
62adf224 LP |
787 | projects:</para> |
788 | ||
789 | <programlisting>DISTCHECK_CONFIGURE_FLAGS = \ | |
790 | --with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)</programlisting> | |
791 | ||
792 | <para>Finally, unit files should be installed in the system with an automake excerpt like the following:</para> | |
793 | ||
794 | <programlisting>if HAVE_SYSTEMD | |
795 | systemdsystemunit_DATA = \ | |
796 | foobar.socket \ | |
797 | foobar.service | |
798 | endif</programlisting> | |
799 | ||
800 | <para>In the | |
5aded369 | 801 | <citerefentry project='die-net'><refentrytitle>rpm</refentrytitle><manvolnum>8</manvolnum></citerefentry> |
409dee2e | 802 | <filename>.spec</filename> file, use snippets |
8a422bb2 LP |
803 | like the following to enable/disable the |
804 | service during | |
805 | installation/deinstallation. This makes use of | |
806 | the RPM macros shipped along systemd. Consult | |
62adf224 LP |
807 | the packaging guidelines of your distribution |
808 | for details and the equivalent for other | |
8a422bb2 LP |
809 | package managers.</para> |
810 | ||
811 | <para>At the top of the file:</para> | |
812 | ||
813 | <programlisting>BuildRequires: systemd | |
814 | %{?systemd_requires}</programlisting> | |
815 | ||
816 | <para>And as scriptlets, further down:</para> | |
62adf224 LP |
817 | |
818 | <programlisting>%post | |
8a422bb2 | 819 | %systemd_post foobar.service foobar.socket |
62adf224 LP |
820 | |
821 | %preun | |
8a422bb2 | 822 | %systemd_preun foobar.service foobar.socket |
ee5762e3 LP |
823 | |
824 | %postun | |
8a422bb2 LP |
825 | %systemd_postun</programlisting> |
826 | ||
827 | <para>If the service shall be restarted during | |
409dee2e | 828 | upgrades, replace the |
8a422bb2 LP |
829 | <literal>%postun</literal> scriptlet above |
830 | with the following:</para> | |
831 | ||
832 | <programlisting>%postun | |
833 | %systemd_postun_with_restart foobar.service</programlisting> | |
834 | ||
835 | <para>Note that | |
836 | <literal>%systemd_post</literal> and | |
837 | <literal>%systemd_preun</literal> expect the | |
838 | names of all units that are installed/removed | |
839 | as arguments, separated by | |
840 | spaces. <literal>%systemd_postun</literal> | |
841 | expects no | |
842 | arguments. <literal>%systemd_postun_with_restart</literal> | |
843 | expects the units to restart as | |
844 | arguments.</para> | |
828f33e8 | 845 | |
6908d384 LP |
846 | <para>To facilitate upgrades from a package |
847 | version that shipped only SysV init scripts to | |
848 | a package version that ships both a SysV init | |
849 | script and a native systemd service file, use | |
850 | a fragment like the following:</para> | |
851 | ||
67ff6134 | 852 | <programlisting>%triggerun -- foobar < 0.47.11-1 |
63415a2d | 853 | if /sbin/chkconfig --level 5 foobar ; then |
d12d0e64 | 854 | /bin/systemctl --no-reload enable foobar.service foobar.socket >/dev/null 2>&1 || : |
6908d384 LP |
855 | fi</programlisting> |
856 | ||
857 | <para>Where 0.47.11-1 is the first package | |
858 | version that includes the native unit | |
859 | file. This fragment will ensure that the first | |
409dee2e | 860 | time the unit file is installed, it will be |
6908d384 | 861 | enabled if and only if the SysV init script is |
0a9962a1 | 862 | enabled, thus making sure that the enable |
6908d384 LP |
863 | status is not changed. Note that |
864 | <command>chkconfig</command> is a command | |
485ccf9a | 865 | specific to Fedora which can be used to check |
6908d384 LP |
866 | whether a SysV init script is enabled. Other |
867 | operating systems will have to use different | |
868 | commands here.</para> | |
62adf224 | 869 | </refsect2> |
64aba792 LP |
870 | </refsect1> |
871 | ||
99ffae46 LP |
872 | <refsect1> |
873 | <title>Porting Existing Daemons</title> | |
874 | ||
875 | <para>Since new-style init systems such as systemd are | |
409dee2e | 876 | compatible with traditional SysV init systems, it is |
99ffae46 | 877 | not strictly necessary to port existing daemons to the |
409dee2e | 878 | new style. However, doing so offers additional |
ad678a06 | 879 | functionality to the daemons as well as simplifying |
99ffae46 LP |
880 | integration into new-style init systems.</para> |
881 | ||
409dee2e | 882 | <para>To port an existing SysV compatible daemon, the |
99ffae46 LP |
883 | following steps are recommended:</para> |
884 | ||
885 | <orderedlist> | |
886 | <listitem><para>If not already implemented, | |
887 | add an optional command line switch to the | |
888 | daemon to disable daemonization. This is | |
889 | useful not only for using the daemon in | |
890 | new-style init systems, but also to ease | |
891 | debugging.</para></listitem> | |
892 | ||
893 | <listitem><para>If the daemon offers | |
894 | interfaces to other software running on the | |
74d00578 | 895 | local system via local <constant>AF_UNIX</constant> sockets, |
99ffae46 | 896 | consider implementing socket-based activation |
409dee2e | 897 | (see above). Usually, a minimal patch is |
99ffae46 LP |
898 | sufficient to implement this: Extend the |
899 | socket creation in the daemon code so that | |
900 | <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
901 | is checked for already passed sockets | |
902 | first. If sockets are passed (i.e. when | |
903 | <function>sd_listen_fds()</function> returns a | |
904 | positive value), skip the socket creation step | |
905 | and use the passed sockets. Secondly, ensure | |
12f25b6e | 906 | that the file system socket nodes for local |
74d00578 | 907 | <constant>AF_UNIX</constant> sockets used in the socket-based |
99ffae46 LP |
908 | activation are not removed when the daemon |
909 | shuts down, if sockets have been | |
910 | passed. Third, if the daemon normally closes | |
911 | all remaining open file descriptors as part of | |
912 | its initialization, the sockets passed from | |
913 | the init system must be spared. Since | |
914 | new-style init systems guarantee that no | |
915 | left-over file descriptors are passed to | |
916 | executed processes, it might be a good choice | |
917 | to simply skip the closing of all remaining | |
ad678a06 | 918 | open file descriptors if sockets are |
99ffae46 LP |
919 | passed.</para></listitem> |
920 | ||
921 | <listitem><para>Write and install a systemd | |
922 | unit file for the service (and the sockets if | |
923 | socket-based activation is used, as well as a | |
924 | path unit file, if the daemon processes a | |
925 | spool directory), see above for | |
926 | details.</para></listitem> | |
927 | ||
928 | <listitem><para>If the daemon exposes | |
929 | interfaces via D-Bus, write and install a | |
930 | D-Bus activation file for the service, see | |
931 | above for details.</para></listitem> | |
932 | </orderedlist> | |
933 | </refsect1> | |
64aba792 | 934 | |
9546c6ed LP |
935 | <refsect1> |
936 | <title>Placing Daemon Data</title> | |
937 | ||
908c32e0 | 938 | <para>It is recommended to follow the general |
9546c6ed LP |
939 | guidelines for placing package files, as discussed in |
940 | <citerefentry><refentrytitle>file-hierarchy</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para> | |
941 | </refsect1> | |
942 | ||
64aba792 | 943 | <refsect1> |
160cd5c9 LP |
944 | <title>See Also</title> |
945 | <para> | |
946 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
cb07866b | 947 | <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
62adf224 LP |
948 | <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
949 | <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
99ffae46 | 950 | <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
9546c6ed LP |
951 | <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>, |
952 | <citerefentry><refentrytitle>file-hierarchy</refentrytitle><manvolnum>7</manvolnum></citerefentry> | |
160cd5c9 | 953 | </para> |
64aba792 LP |
954 | </refsect1> |
955 | ||
956 | </refentry> |