Revision as of 09:00, 15 October 2012

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

SysV Initscripts
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

Migrating from SysV Initscripts with non-standard service commands
Packages which have SysV initscripts that contain 'non-standard service commands' (commands besides start, stop, reload, force-reload, status, restart, or try-restart) must convert those commands into standalone helper scripts. Systemd does not support non-standard unit commands.

Naming

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 .service file 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.service and bad names would be httpd.service or apache.service as 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

Case Sensitivity
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 .service file!)
Documentation=man:foo.service(8) man:foo.conf(5) http://www.foo.org/docs/

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.

Documentation field

Systemd in Fedora 17+ has support for defining documentation in unit files via the Documentation= field (it is ignored in older releases, so it is safe to keep one systemd unit file across all branches). System administrators will be looking at the contents of the Documentation= field to determine what the service is, how to configure it, and where to locate additional documentation relating to the service. Accordingly, packagers are strongly encouraged to include any available sources in the Documentation= field which provide this information. If a man page or info page is present in the package, refer to it using "man:manpage" or "info:infofile" respectively. If the documentation is in plaintext, use "file://path/to/file". Lastly, if no local documentation exists in the package, but it exists at a url, use the URL (with http://) in this field. Multiple URIs can be added to the Documentation= field, as a space separated list. For details on URI definitions and formatting, please refer to the uri(7) manpage (man uri).

[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

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).

EnvironmentFiles and support for /etc/sysconfig files

The EnvironmentFiles= line in the [Service] section of .service files is used to support loading environment variables that can be used in unit files. For instance, if your sysv-initscript used a file in /etc/sysconfig to set command line options, you can use EnvironmentFiles= like so:

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 the /etc/sysconfig/httpd file 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 environment file does not exist. Since many of these files were optional in sysvinit, you should include the "-" when using this directive.

Although environment files are easy to use, upstream systemd recommends a different approach. Their recommendation for administrators who need 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.

Either approach is valid in Fedora.

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 After postgresql. 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 After was 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.

Unit files should avoid using StandardOutput= or StandardError=. The default is the right choice for almost all cases, and using the default allows users to change global defaults in /etc/systemd/system.conf.

Example Unit file

This is an example systemd unit .service file for ABRT:

[Unit]
Description=ABRT Automated Bug Reporting Tool

[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.

Currently we do not have guidance on how to write socket files as this is something that needs upstream code and they can add a proper .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-1/system-services/ and use the SystemdService= directive therein to redirect the activation to systemd.

.service
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

program with the 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

[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

[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

On Fedora 18 and newer, the systemd-units subpackage has been merged into the base systemd package, so your package for those targets can instead include:

BuildRequires: systemd

%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

Tmpfiles.d

tmpfiles.d is a service provided systemd in Fedora 15 and later for managing temporary files and directories for daemons. For more information on how to use Tmpfiles.d in Fedora Packages, please see: Packaging:Tmpfiles.d

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.

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