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man/daemon: s/init system/service manager/

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As pointed out in the review, all this applies to the user services too, so are
not managed by the "init system", but by the more generic "service manager".

Also:
- use oxford comma
- change "employ" to "use" in various places
- change "the init system forwards messages to syslog" to "are forwarded to
  syslog". This is done by systemd-journald, so really there is no forwarding,
  because systemd-journald just writes them to a file in the common setup,
  so let's use the passive form to avoid specifying who does this.
This commit is contained in:
Zbigniew Jędrzejewski-Szmek 2023-09-18 14:44:32 +02:00
parent d73f8ff95f
commit 7ab1a1be1c
1 changed files with 89 additions and 132 deletions
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221
man/daemon.xml
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@ -146,13 +146,11 @@
new-style daemons. This makes it easier to supervise and control
them at runtime and simplifies their implementation.</para>
<para>For developing a new-style daemon, none of the
initialization steps recommended for SysV daemons need to be
implemented. New-style init systems such as systemd make all of
them redundant. Moreover, since some of these steps interfere
with process monitoring, file descriptor passing and other
functionality of the init system, it is recommended not to
execute them when run as new-style service.</para>
<para>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 service manager, 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 a clean process context: it is
guaranteed that the environment block is sanitized, that the signal handlers and mask is reset and that no
@ -166,8 +164,8 @@
following:</para>
<orderedlist>
<listitem><para>If applicable, the daemon should notify the init system about startup completion or
status updates via the
<listitem><para>If applicable, the daemon should notify the service manager about startup completion
or status updates via the
<citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
interface, in particular <varname>READY=1</varname> and <varname>STATUS=…</varname>.
</para></listitem>
@ -183,13 +181,11 @@
<varname>RELOADING=1</varname> and <varname>READY=1</varname>.
</para></listitem>
<listitem><para>Provide a correct exit code from the main
daemon process, as this is used by the init system to detect
service errors and problems. It is recommended to follow the
exit code scheme as defined in the <ulink
<listitem><para>Provide a correct exit code from the main daemon process, as this is used by the
service manager to detect service errors and problems. It is recommended to follow the exit code
scheme as defined in the <ulink
url="http://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html">LSB
recommendations for SysV init
scripts</ulink>.</para></listitem>
recommendations for SysV init scripts</ulink>.</para></listitem>
<listitem><para>If possible and applicable, expose the
daemon's control interface via the D-Bus IPC system and grab a
@ -202,15 +198,12 @@
<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details.</para></listitem>
<listitem><para>As much as possible, rely on the init system's
functionality to limit the access of the daemon to files,
services and other resources, i.e. in the case of systemd,
rely on systemd's resource limit control instead of
implementing your own, rely on systemd's privilege dropping
code instead of implementing it in the daemon, and similar.
See
<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for the available controls.</para></listitem>
<listitem><para>As much as possible, rely on the service manager's functionality to limit the access
of the daemon to files, services, and other resources, i.e. in the case of systemd, rely on systemd's
resource limit control instead of implementing your own, rely on systemd's privilege dropping code
instead of implementing it in the daemon, and so on. See
<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
the available controls.</para></listitem>
<listitem><para>If D-Bus is used, make your daemon
bus-activatable by supplying a D-Bus service activation
@ -237,19 +230,13 @@
<citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry> with
<varname>FDSTORE=1</varname>..</para></listitem>
<listitem><para>Instead of using the
<function>syslog()</function> call to log directly to the
system syslog service, a new-style daemon may choose to simply
log to standard error via <function>fprintf()</function>,
which is then forwarded to syslog by the init system. If log
levels are necessary, these can be encoded by prefixing
individual log lines with strings like
<literal>&lt;4&gt;</literal> (for log level 4 "WARNING" in the
syslog priority scheme), following a similar style as the
Linux kernel's <function>printk()</function> level system. For
details, see
<citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>
and
<listitem><para>Instead of using the <function>syslog()</function> call to log directly to the system
syslog service, a new-style daemon may choose to simply log to standard error via
<function>fprintf()</function>, which is then forwarded to syslog. If log levels are necessary, these
can be encoded by prefixing individual log lines with strings like <literal>&lt;4&gt;</literal> (for
log level 4 "WARNING" in the syslog priority scheme), following a similar style as the Linux kernel's
<function>printk()</function> level system. For details, see
<citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry> and
<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
<listitem><para>As new-style daemons are invoked without a controlling TTY (but as their own session
@ -294,17 +281,14 @@
<refsect2>
<title>Activation on Boot</title>
<para>Old-style daemons are usually activated exclusively on
boot (and manually by the administrator) via SysV init scripts,
as detailed in the <ulink
<para>Old-style daemons are usually activated exclusively on boot (and manually by the administrator)
via SysV init scripts, as detailed in the <ulink
url="http://refspecs.linuxbase.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html">LSB
Linux Standard Base Core Specification</ulink>. This method of
activation is supported ubiquitously on Linux init systems, both
old-style and new-style systems. Among other issues, SysV init
scripts have the disadvantage of involving shell scripts in the
boot process. New-style init systems generally employ updated
versions of activation, both during boot-up and during runtime
and using more minimal service description files.</para>
Linux Standard Base Core Specification</ulink>. This method of activation is supported ubiquitously on
Linux init systems, both old-style and new-style systems. Among other issues, SysV init scripts have
the disadvantage of involving shell scripts in the boot process. New-style init systems generally use
updated versions of activation, both during boot-up and during runtime and using more minimal service
description files.</para>
<para>In systemd, if the developer or administrator wants to
make sure that a service or other unit is activated
@ -324,42 +308,29 @@
<refsect2>
<title>Socket-Based Activation</title>
<para>In order to maximize the possible parallelization and
robustness and simplify configuration and development, it is
recommended for all new-style daemons that communicate via
listening sockets to employ socket-based activation. In a
socket-based activation scheme, the creation and binding of the
listening socket as primary communication channel of daemons to
local (and sometimes remote) clients is moved out of the daemon
code and into the init system. Based on per-daemon
configuration, the init system installs the sockets and then
hands them off to the spawned process as soon as the respective
daemon is to be started. Optionally, activation of the service
can be delayed until the first inbound traffic arrives at the
socket to implement on-demand activation of daemons. However,
the primary advantage of this scheme is that all providers and
all consumers of the sockets can be started in parallel as soon
as all sockets are established. In addition to that, daemons can
be restarted with losing only a minimal number of client
transactions, or even any client request at all (the latter is
particularly true for state-less protocols, such as DNS or
syslog), because the socket stays bound and accessible during
the restart, and all requests are queued while the daemon cannot
process them.</para>
<para>In order to maximize the possible parallelization and robustness and simplify configuration and
development, it is recommended for all new-style daemons that communicate via listening sockets to use
socket-based activation. In a socket-based activation scheme, the creation and binding of the listening
socket as primary communication channel of daemons to local (and sometimes remote) clients is moved out
of the daemon code and into the service manager. Based on per-daemon configuration, the service manager
installs the sockets and then hands them off to the spawned process as soon as the respective daemon is
to be started. Optionally, activation of the service can be delayed until the first inbound traffic
arrives at the socket to implement on-demand activation of daemons. However, the primary advantage of
this scheme is that all providers and all consumers of the sockets can be started in parallel as soon
as all sockets are established. In addition to that, daemons can be restarted with losing only a
minimal number of client transactions, or even any client request at all (the latter is particularly
true for state-less protocols, such as DNS or syslog), because the socket stays bound and accessible
during the restart, and all requests are queued while the daemon cannot process them.</para>
<para>New-style daemons which support socket activation must be
able to receive their sockets from the init system instead of
creating and binding them themselves. For details about the
programming interfaces for this scheme provided by systemd, see
<citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
and
<citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
For details about porting existing daemons to socket-based
activation, see below. With minimal effort, it is possible to
implement socket-based activation in addition to traditional
internal socket creation in the same codebase in order to
support both new-style and old-style init systems from the same
daemon binary.</para>
<para>New-style daemons which support socket activation must be able to receive their sockets from the
service manager instead of creating and binding them themselves. For details about the programming
interfaces for this scheme provided by systemd, see
<citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry> and
<citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>. For
details about porting existing daemons to socket-based activation, see below. With minimal effort, it
is possible to implement socket-based activation in addition to traditional internal socket creation in
the same codebase in order to support both new-style and old-style init systems from the same daemon
binary.</para>
<para>systemd implements socket-based activation via
<filename>.socket</filename> units, which are described in
@ -385,23 +356,16 @@
<refsect2>
<title>Bus-Based Activation</title>
<para>When the D-Bus IPC system is used for communication with
clients, new-style daemons should employ bus activation so that
they are automatically activated when a client application
accesses their IPC interfaces. This is configured in D-Bus
service files (not to be confused with systemd service unit
files!). To ensure that D-Bus uses systemd to start-up and
maintain the daemon, use the <varname>SystemdService=</varname>
directive in these service files to configure the matching
systemd service for a D-Bus service. e.g.: For a D-Bus service
whose D-Bus activation file is named
<filename>org.freedesktop.RealtimeKit.service</filename>, make
sure to set
<varname>SystemdService=rtkit-daemon.service</varname> in that
file to bind it to the systemd service
<filename>rtkit-daemon.service</filename>. This is needed to
make sure that the daemon is started in a race-free fashion when
activated via multiple mechanisms simultaneously.</para>
<para>When the D-Bus IPC system is used for communication with clients, new-style daemons should use
bus activation so that they are automatically activated when a client application accesses their IPC
interfaces. This is configured in D-Bus service files (not to be confused with systemd service unit
files!). To ensure that D-Bus uses systemd to start-up and maintain the daemon, use the
<varname>SystemdService=</varname> directive in these service files to configure the matching systemd
service for a D-Bus service. e.g.: For a D-Bus service whose D-Bus activation file is named
<filename>org.freedesktop.RealtimeKit.service</filename>, make sure to set
<varname>SystemdService=rtkit-daemon.service</varname> in that file to bind it to the systemd service
<filename>rtkit-daemon.service</filename>. This is needed to make sure that the daemon is started in a
race-free fashion when activated via multiple mechanisms simultaneously.</para>
</refsect2>
<refsect2>
@ -461,10 +425,10 @@
<title>Other Forms of Activation</title>
<para>Other forms of activation have been suggested and implemented in some 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 start daemons or <filename>.socket</filename> units
when a specific IP address is configured on a network interface, because network sockets shall be bound
to the address. However, an alternative to implement this is by utilizing the Linux
often simpler or better alternatives, or they can be put together of combinations of the schemes above.
Example: Sometimes, it appears useful to start daemons or <filename>.socket</filename> units when a
specific IP address is configured on a network interface, because network sockets shall be bound to the
address. However, an alternative to implement this is by utilizing the Linux
<constant>IP_FREEBIND</constant>/<constant>IPV6_FREEBIND</constant> socket option, as accessible via
<varname>FreeBind=yes</varname> in systemd socket files (see
<citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
@ -472,15 +436,14 @@
address, and hence allows bindings to a particular IP address before it actually becomes available,
making such an explicit dependency to the configured address redundant. Another often suggested trigger
for service activation is low system load. However, here too, a more convincing approach might be to
make proper use of features of the operating system, in particular, the CPU or I/O scheduler of
Linux. Instead of scheduling jobs from userspace based on monitoring the OS scheduler, it is advisable
to leave the scheduling of processes to the OS scheduler itself. systemd provides fine-grained access
to the CPU and I/O schedulers. If a process executed by the init system shall not negatively impact the
amount of CPU or I/O bandwidth available to other processes, it should be configured with
<varname>CPUSchedulingPolicy=idle</varname> and/or
<varname>IOSchedulingClass=idle</varname>. Optionally, this may be combined with timer-based activation
to schedule background jobs during runtime and with minimal impact on the system, and remove it from
the boot phase itself.</para>
make proper use of features of the operating system, in particular, the CPU or I/O scheduler of Linux.
Instead of scheduling jobs from userspace based on monitoring the OS scheduler, it is advisable to
leave the scheduling of processes to the OS scheduler itself. systemd provides fine-grained access to
the CPU and I/O schedulers. If a process executed by the service manager shall not negatively impact
the amount of CPU or I/O bandwidth available to other processes, it should be configured with
<varname>CPUSchedulingPolicy=idle</varname> and/or <varname>IOSchedulingClass=idle</varname>.
Optionally, this may be combined with timer-based activation to schedule background jobs during runtime
and with minimal impact on the system, and remove it from the boot phase itself.</para>
</refsect2>
</refsect1>
@ -687,26 +650,20 @@ fi</programlisting>
is useful not only for using the daemon in new-style init
systems, but also to ease debugging.</para></listitem>
<listitem><para>If the daemon offers interfaces to other
software running on the local system via local
<constant>AF_UNIX</constant> sockets, consider implementing
socket-based activation (see above). Usually, a minimal patch is
sufficient to implement this: Extend the socket creation in the
daemon code so that
<citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
is checked for already passed sockets first. If sockets are
passed (i.e. when <function>sd_listen_fds()</function> returns a
positive value), skip the socket creation step and use the
passed sockets. Secondly, ensure that the file system socket
nodes for local <constant>AF_UNIX</constant> sockets used in the
socket-based activation are not removed when the daemon shuts
down, if sockets have been passed. Third, if the daemon normally
closes all remaining open file descriptors as part of its
initialization, the sockets passed from the init system must be
spared. Since new-style init systems guarantee that no left-over
file descriptors are passed to executed processes, it might be a
good choice to simply skip the closing of all remaining open
file descriptors if sockets are passed.</para></listitem>
<listitem><para>If the daemon offers 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 sufficient to implement this: Extend the socket creation in the daemon code
so that
<citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry> is
checked for already passed sockets first. If sockets are passed (i.e. when
<function>sd_listen_fds()</function> returns a positive value), skip the socket creation step and use
the passed sockets. Secondly, ensure that the file system socket nodes for local
<constant>AF_UNIX</constant> sockets used in the socket-based activation are not removed when the
daemon shuts down, if sockets have been passed. Third, if the daemon normally closes all remaining open
file descriptors as part of its initialization, the sockets passed from the service manager must be
spared. Since new-style init systems guarantee that no left-over file descriptors are passed to
executed processes, it might be a good choice to simply skip the closing of all remaining open file
descriptors if sockets are passed.</para></listitem>
<listitem><para>Write and install a systemd unit file for the
service (and the sockets if socket-based activation is used, as