Fedora systemd Services
This document describes the guidelines for systemd services, for use and inclusion in Fedora packages.
Definitions
Since systemd includes some concepts which are extensions of previous concepts, the following definitions may be useful:
Service: A process or task executed and controlled by the init system (e.g. systemd).
Traditional Service: A service which is explicitly started or stopped, either by the init system at boot or manually by a superuser. In systemd, one of several types of service controlled by a .service file.
Activated service: A service that is not (or not necessarily) started explicitly by the user but start when certain other events happen or certain state becomes true.
Socket-activated Service: A service which is waiting for traffic across a socket before activating. In systemd, controlled by a .socket file.
D-Bus service: A service which activates in response to a message from the D-Bus system bus.
Unit file: The systemd equivalent of a SysV initscript.
Unit Files
Each package that contains software that wants/needs to start a traditional service at boot MUST have a systemd unit file.
Ideally, systemd unit files are reusable across distributions and shipped with the upstream packages. Please consider working with upstream to integrate the systemd files you prepare in the upstream sources. Information for developers on how to integrate systemd support best with their build system you may find on http://0pointer.de/public/systemd-man/daemon.html
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:SysVInitScriptNaming
Unit files for traditional services have a naming scheme of foobar.service. When considering what basename to use, keep in mind that we'd like to use the same service names for software across distributions. We'd also like to ship the .service files in the upstream packages. These desires create a few guides for naming a unit file:
- Follow upstream if they're already distributing a
.servicefile and it's not likely to conflict with other packages. - Look at packages in other distros or talk with the maintainers of those packages and upstream to try to come up with a common name.
- Unit files should 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.serviceand bad names would behttpd.serviceorapache.serviceas there are multiple httpd implementations and multiple projects produced by the apache foundation.
For backwards compatibility you may also want to create a symlink from an older, name to the new name. In the above example, for instance, Fedora has always used httpd for the service. When creating the new apache-httpd.service file, also create a symlink named httpd.service that points at apache-httpd.service. Then end users that are used to using service httpd will have it continue to work.
Basic format
All fields in a unit file are case sensitive.
[Unit]
Every .service file must begin with a [Unit] section:
[Unit] Description=A brief human readable string describing the service (not the <code>.service</code> 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.)
[Service]
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-shot" 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 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.
[Install]
Finally, the .service file should have an [Install] section:
[Install] WantedBy=...
The recommended parameters for WantedBy= are either graphical.target (services related to the graphical user interface) or multi-user.target (for everything else). When the user (or our scriptlets) invoke systemctl enable the service will be set to start in these targets.
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
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)
The "-" on the EnvironmentFile= line ensures that no error
messages is generated if the sysconfig file does not exist. Since traditionally
sysconfig files have been optional you should always include the "-" when using
this directive.
Ideally unit files are shipped along upstream packages. In order to make
adoption of upstream unit files easy please do not introduce new /etc/sysconfig
files or options, as /etc/sysconfig files are Fedora-specific. Please support
sysconfig files only to maintain compatibility with previous Fedora release.
The recommended way for administrators to reconfigure systemd
.service files is to copy them from /lib/systemd/system to
/etc/systemd/system and modify them there. Unit files in /etc/systemd/system
override those in /lib/systemd/system if they otherwise carry the same name.
Fields to avoid
For most services, we do not want to use requirement dependencies in the
[Unit] section, 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= (in the [Unit] section). 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=). syslog is a better target, because it is where service logging
should primarily go. While messages written to kmsg eventually end up in syslog
too they are normally attributed to the kernel, and lose much of its meta data.
StandardOutput=syslog is recommended for all services which generate output on
stdout or stderr which shall be piped to syslog. Note that if
StandardOutput=syslog is set StandardError=syslog is implied unless
StandardError= is explicitly configured too. That basically means that you
never need to use StandardError= in normal cases. Just use
StandardOutput=syslog. A full description of these options can be found here:
http://0pointer.de/public/systemd-man/systemd.exec.html
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
Activation
Systemd allows for three forms of activated services: #Hardware activation, #Socket activation, and #DBus activation.
Hardware activation
Hardware activation occurs when a service is installed but only turns on if a certain type of hardware is installed. Enabling of the service is normally done with a udev rule. At this time we do not have further guidance on how to write those udev rules. The service itself installs its .service files in the normal places and are installed by the normal systemd scriptlets. These services should never be enabled by the package as they will be enabled by udev.
Socket activation
Socket activation occurs when a service allows systemd to listen for
connections to a specific socket and, when systemd receives a connection on
that socket, it starts the service. To do this, the upstream source needs to
have some minor coding work to let systemd listen for connections on the socket
and there needs to be a .socket file in
%{_lib}/systemd/system/ that tells systemd to listen to that
socket and what to start when a connection is received. This is similar in
function to inetd and some, but not all, services coded to work with inetd will
work with socket activation. Simila to inetd, using socket activation for
on-demand loading will impose a startup time penalty so we currently do not use
this feature in Fedora.
.socket file at the same time.However, socket activation can also be used to allow parallel startup of services. If a service supports systemd socket activation as described above and we additionally start it explicitly on boot, then systemd will start it but allow things that depend on it to startup at the same time. If the dependent service makes a request to the socket activatable service before it has come up, then systemd will cause the request to wait until the socket activatable service has come up and can process the request. To achieve this effect, the service must be socket activatable as described above, the .service file for the service needs to have a Wants= line for the .socket, and the service must autostart. Since Fedora currently doesn't want any services to do on-demand loading, all socket activated services must autostart.
In practical terms this means if the upstream tarball ships with a socket file you need to contact FESCo to get permission to enable your service on boot. Once you have permission, you can package the .socket file and use the systemd scriptlets that enable the service by default. You need to also check the .service file to make sure it has a Wants= entry on the .socket file as that ensures that starting the service will also inform systemd of the socket.
DBus activation
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 the D-Bus activation request is forwarded from the D-Bus system bus to systemd so that you end up with only a single instance of the service, even if a service is triggered by both boot-up and activation. If historically your D-Bus service was not bus-activated but started via a SysV init script, it should be updated to use bus activation. This may be implemented by dropping a D-Bus .service file in /usr/share/dbus/system-services/ and use the SystemdService= directive therein to redirect the activation to systemd.
systemd and D-Bus both use the
.service file
suffix but they are different things. In a systemd driven init process, the
D-Bus .service file will often refer to the systemd .service file for the same
SystemdService directive.Here's an example for a D-Bus bus-activable service. The ConsoleKit bus activation file /usr/share/dbus-1/system-services/org.freedesktop.ConsoleKit.service:
[D-BUS Service] Name=org.freedesktop.ConsoleKit Exec=/usr/sbin/console-kit-daemon --no-daemon User=root SystemdService=console-kit-daemon.service
And the matching systemd unit file /lib/systemd/system/console-kit-daemon.service:
[Unit] Description=Console Manager After=syslog.target [Service] Type=dbus BusName=org.freedesktop.ConsoleKit ExecStart=/usr/sbin/console-kit-daemon --no-daemon
As you can see SystemdService= is used in the D-Bus activation file to bind the systemd service to the D-Bus service.
Traditionally, bus activated D-Bus services could not be disabled without uninstalling them entirely. systemd allows you to disable services by making D-Bus invoke an alias systemd service name (that can be created or removed to enable/disable activation) as an intermediary for the real service.
You can easily implement disabling by directing the D-Bus service to an alias
name of the real service file (in the filesystem this shows up as a symlink
placed in /etc/systemd/system to the real service file). This alias is then
controlled via "systemctl enable" and "systemctl disable". It is a good idea
(though technically not necessary) to name this alias name after the D-Bus bus
name of the service, prefixed with "dbus-". Example for Avahi, a service that
the admin might need to disable: set
SystemdService=dbus-org.freedesktop.Avahi.service instead of
SystemdService=avahi-daemon.service in the D-Bus activation file, and then make
dbus-org.freedesktop.Avahi.service an optional alias of avahi-daemon.service
that can be controlled via the Alias= directive in the [Install] section of the
systemd service file. This directive is then read by "systemctl enable" and
"systemctl disable" to create resp. remove a symlink to make the service
available resp. unavailable under this additional name. 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".
Note that the creation/removal of the alias symlinks should be done with "systemctl enable" and "systemctl disable" only. You should not create these symlinks manually.
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.
See the D-Bus documentation for more information about bus activation: http://dbus.freedesktop.org/doc/dbus-specification.html#message-bus-starting-services
Packaging
Filesystem locations
Packages with systemd unit files must put them into %{_unitdir}. %{_unitdir} evaluates to /lib/systemd/system on all Fedora systems (F-15+). Unit files are architecture independent (hence, not %{_lib}) and needed early in the boot process.
Please note that in order for the %{_unitdir} macro to exist, your package must have:
BuildRequires: systemd-units
%files section
Systemd unit .service files must not be marked as %config files. If the user wants to reconfigure a .service file he should copy the .service file from /lib/systemd/system to /etc/systemd/system and edit it there. Unit files in /etc/systemd/system override those in /lib/systemd/system if they carry the same name.
Unit files in spec file scriptlets
Information on proper handling of unit files in spec file scriptlets can be found here: Packaging:ScriptletSnippets#Systemd
Why don't we....
- 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.