Fedora systemd Services
This document describes the guidelines for systemd services, for use and inclusion in Fedora packages.
Unit Files
The systemd equivalent for an SysV initscript is called a Unit file. Each package that contains software that wants/needs to start a service at boot must have a systemd unit file.
Packages may also provide a SysV initscript file, but are not required to do so. This format is considered legacy, but Fedora still contains init mechanisms such as upstart which do not support the systemd unit file format. If present, the SysV initscript(s) must go into an optional subpackage, so as not to confuse sysadmins. The guidelines for SysV initscripts can be found here: Packaging:SysVInitScript
Naming
Unit files have a naming scheme of foobar.service. When considering what basename to use, keep the following advice in mind:
- Unit files must be named after the software implementation that they support, as opposed to the generic type of software. So, a good name would be "apache-httpd.service", and a bad name would be "httpd.service", as there are multiple httpd implementations, but only one Apache httpd.
It is our intention to unify service names of well-known software across distributions, so that we can eventually ship the service files in the upstream packages. Hence it is a good idea to discuss service names with maintainers of the same packages in other distributions and agree on a common name.
Format
Make a section Unit files. s/Format/Basic Format/, also include sysconfig files, s/Options to avoid/Other fields/, Example unit file --abadger1999 15:49, 8 December 2010 (UTC)
Every .service file must begin with a [Unit] section:
[Unit] Description=A brief human readable string describing the service (not the service file!) After=syslog.target
The Description= line must not exceed 80 characters, and must describe the service, and not the service file. For example, "Apache Web Server" is a good description, but "Starts and Stops the Apache Web Server" is a bad one.
After specifies that this service may only start after the listed services have started. In this example, the service starts after syslog since syslog can be used for logging. (Most daemon programs can use syslog for logging so when in doubt, it's good to include this.)
Next, the .service file must have a [Service] section:
[Service] Type=... ExecStart=... ExecReload=...
The Type= setting is very important. For D-Bus services this should be "dbus", for traditional services "forking" is usually a good idea, for services not offering any interfaces to other services "simple" is best. For "one-short" scripts "oneshot" is ideal, often combined with RemainAfterExit=. See http://0pointer.de/public/systemd-man/systemd.service.html for further discussion on the topic. Since "simple" is the default type, .service files which would normally set Type=simple may simply omit the Type line altogether.
BusName= should be set for all services connecting to D-Bus. (i.e. it is a must for those where Type=dbus, but might make sense otherwise, too) Omit this option if your service does not take a name on the bus.
ExecStart= is necessary for all services. This line defines the string that you would run to start the service daemon, along with any necessary options.
ExecReload= should be specified for all services supporting reload. It is highly recommended to add code here that synchronously reloads the configuration file here (i.e. /bin/kill -HUP $MAINPID is usually a poor choice, due to its asynchronous nature). Omit this option if your service does not support reloading.
Finally, the .service file must have an [Install] section:
[Install] WantedBy=...
The recommended parameters for WantedBy are either multi-user.target (for most system services) or graphical.target (for services related to the UI).
For more information regarding these options see http://0pointer.de/public/systemd-man/systemd.unit.html and http://0pointer.de/public/systemd-man/systemd.service.html
Strictly speaking ExecStart= (in the [Service] section) is the only option really necessary for a .service file. However, in Fedora you must add the other options mentioned here (as applicable).
Support for /etc/sysconfig files
If your service supports /etc/sysconfig files, then you must use EnvironmentFile=/etc/sysconfig/foobar in your .service file, in the [Service] section.
Example:
[Service]
Type=forking
EnvironmentFile=/etc/sysconfig/httpd
ExecStart=/usr/sbin/httpd ${OPTIONS}
ExecReload=/usr/sbin/httpd ${OPTIONS} -k restart
You may then refer to variables set in sysconfig files with ${FOOBAR} and $FOOBAR, in the ExecStart= lines (and related lines). (${FOOBAR} expands the variable into one word, $FOOBAR splits up the variable value at whitespace into multiple words)
Options to avoid
For most services, we do not want to use requirement dependencies such as Requires= or Wants=. Instead exclusively use ordering dependencies: Before= and After=. This is used to implement loose coupling: if someone asks two services to start at the same time, systemd will properly order their startup but not make it strictly necessary to run one if the other is started.
If you use a requirement dependency, use Wants= rather than Requires=, to make things a little bit more robust. If you use a requirement dependency in almost all cases you should also add an ordering dependency, as ordering and requirement dependencies are orthogonal in systemd.
Here's an example of this common case:
- A web application needs postgresql to store its data.
- It is set to start
Afterpostgresql. On startup, the web application does not start until postgresql does. - Once running, the system administrator needs to restart postgresql due to a config tweak.
- Since only
Afterwas used, the web application may be temporarily unable to serve some requests but it does not need to restart in order to serve pages after the database comes back up.
Avoid referring to runlevelX.target units in all lines taking unit names (such as WantedBy), these are legacy names existing for compatibility with SysV only.
Avoid Names=. Usually it is a better idea to symlink an additional name in the file system. Note that a name listed in Names= is only useful when a service file is already loaded. However, systemd loads only the service files actually referred to in another loaded service, and uses the filenames during the search. Hence a name in Names= is not useful as a search key, but a symlink in the file system is. Also do not put a (redundant) Names=foobar.service line into a file called foobar.service. We want to keep our service files short.
Avoid using StandardOutput=kmsg. Use StandardOutput=syslog instead. (same for StandardError=).
Example Unit file
This is an example systemd unit .service file for ABRT:
[Unit] Description=ABRT Automated Bug Reporting Tool After=syslog.target [Service] Type=dbus BusName=com.redhat.abrt ExecStart=/usr/sbin/abrtd -d -s [Install] WantedBy=multi-user.target
Bus Activation
Split some sentences to make the information clearer. --abadger1999 15:49, 8 December 2010 (UTC)
In order to allow parallel startup of a D-Bus service and its consumers it is essential that D-Bus services can be bus activated, and that the D-Bus activation is forwarded to systemd so that you end up with a single service instance only. That means all existing D-Bus services that have previously not been started via bus activation but have been started via SysV scripts should be updated to be capable of bus activation. This is easily implemented by dropping a D-Bus .service file in /usr/share/dbus/system-services/ which uses the SystemdService= directive to redirect activation to systemd. See the D-Bus documentation for more information. (Don't be confused by the fact that both systemd and D-Bus use the .service file suffix, they are different things, though the D-Bus .service often refers to the systemd .service file for the same program with the SystemdService= directive.)
Note that traditionally bus activated services could not be disabled. systemd does not change this, however in some cases where making it possible to disable a service is desirable (e.g. avahi, NetworkManager), you can easily implement that by directing the D-Bus service to a symlinked alias name of the real service file which is controlled via systemctl enable/disable and which then points to the real service. Example: set SystemdService=dbus-org.freedesktop.Avahi.service instead of SystemdService=avahi-daemon.service, and then make /etc/systemd/system/dbus-org.freedesktop.Avahi.service a symlink → /lib/systemd/system/avahi-daemon.service. A full example for the Avahi case:
Here is the D-Bus .service file for Avahi (/usr/share/dbus-1/system-services/org.freedesktop.Avahi.service):
[D-BUS Service] Name=org.freedesktop.Avahi SystemdService=dbus-org.freedesktop.Avahi.service # This service should not be bus activated if systemd isn't running, # so that activation won't conflict with the init script startup. Exec=/bin/false
Here is the Avahi systemd unit .service file (/lib/systemd/system/avahi-daemon.service):
[Unit] Description=Avahi mDNS/DNS-SD Stack Requires=avahi-daemon.socket After=syslog.target [Service] Type=dbus BusName=org.freedesktop.Avahi ExecStart=/usr/sbin/avahi-daemon -s ExecReload=/usr/sbin/avahi-daemon -r NotifyAccess=main [Install] WantedBy=multi-user.target Also=avahi-daemon.socket Alias=dbus-org.freedesktop.Avahi.service
The Alias= line ensures that the existance of the /etc/systemd/system/dbus-org.freedesktop.Avahi.service symlink can be controlled by "systemctl enable" and "systemctl disable".
In general, it is also recommended to supply native systemd units for all services that are already bus activatable, so that these services can be controlled and supervised centrally like any other service with tools such as systemctl. A similar logic like the one shown above should apply.
Packaging
Filesystem locations
Packages with systemd unit files must put them into %{_unitdir}. %{_unitdir} evaluates to /lib/systemd/system/ on all Fedora systems. Unit files are architecture independent (hence, not %{_lib}) and needed early in the boot process.
%files section
Systemd unit .service files must not be marked as %config files. Any configuration should be made available through /etc/sysconfig/<service> rather than in the init script itself.
Unit files in spec file scriptlets
Most systemd services must be disabled by default, especially if they listen on the network. Also, a package must never turn on a service on upgrade if it was explicitly disabled.
Fedora provides a set of scriptlets to simplify handling the setup of systemd services at package install/upgrade/removal.
Requires(post): systemd-units
Requires(preun): systemd-units
Requires(postun): systemd-units
%post
if [ $1 -eq 1 ]; then
# Package install, not upgrade
/bin/systemctl daemon-reload >/dev/null 2>&1 || :
fi
%preun
if [ $1 -eq 0 ] ; then
# Package removal, not upgrade
/bin/systemctl disable httpd.service > /dev/null 2>&1 || :
/bin/systemctl stop httpd.service > /dev/null 2>&1 || :
fi
%postun
/bin/systemctl daemon-reload >/dev/null 2>&1 || :
if [ $1 -ge 1 ] ; then
# Package upgrade, not uninstall
/bin/systemctl try-restart httpd.service >/dev/null 2>&1 || :
fi
Note that /bin/systemctl daemon-reload will automatically detect new systemd unit .service files placed into %{_unitdir}. There is no equivalent to 'chkconfig --add <service>', because it is unnecessary.
Why don't we....
- Enable most services by default?
Some core services must be enabled by default for a functional system, but the default for most network-listening scripts is off. This is done for better security. We have multiple tools that can enable services, including GUIs.
- Start the service after installation?
Installations can be in changeroots, in an installer context, or in other situations where you don't want the services autostarted.