Difference between revisions of "DeveloperWiki:Systemd"

Revision as of 13:20, 20 November 2012

This page is for planning.

TODO list

• Update our servers
• Finish 'Missing systemd units': https://www.archlinux.org/todo/161/ and https://www.archlinux.org/todo/178/
• Update arch-install-scripts and installation medias
• Update devtools
  • On brynhild, devtools are broken at the moment, due to systemd 188's shared mounts
    • To fix this, mkarchroot would need to unshare mounts, then (in the new namespace) make all mounts slaves
    • systemd 189 may make nspawn suitable for use in mkarchroot

• Reorganize systemd and sysvinit packages:
  • Merge systemd back to a single package (aside from sysvcompat)
  • Remove sysvinit and initscripts from 'base', add systemd (and sysvcompat?)
  • Only sysvcompat and sysvinit should conflict, allowing initscripts and sysvcompat to be installed at the same time (this is done, but the conflict still exists in the package)
    • Merge locale.sh from initscripts/sysvcompat to systemd? Look for an alternative solution to setting locale at login?
    • Split off some of the tools from sysvinit (pidof, last, ...) into sysvinit-tools, which initscripts should depend on
    • sysvcompat should then provide sysvinit, to resolve the dependency issue

• Make sure timedated's managing of NTP works
  • Add files to ntpd, chrony, openntpd for /usr/lib/systemd/ntp-units.d

Packaging notes

Units

• Use the upstream unit files whenever they exist
• Try not to do anything Arch-specific. This will maximize chances of not having to change behavior in the future once the unit files are provided by upstream. In particular avoid EnvironmentFile=, especially if it points to the Arch-specific /etc/conf.d
• Always separate initialization behavior from the actual daemon behavior. If necessary, use a separate unit for the initialization, blocked on a ConditionFoo from systemd.unit(5). An example of this is sshd.service and sshdgenkeys.service.

Not using an EnvironmentFile= is OK if:

• Either the daemon has its own configuration file where the same settings can be specified
• The default service file "just works" in the most common case. Users who want to change the behavior should then override the default service file. If it is not possible to provide a sane default service file, it should be discussed on a case-by-case basis

A few comments about service files, assuming current behavior should be roughly preserved, and fancy behavior avoided:

• If your service requires the network to be configured before it starts, use After=network.target. Do not use Wants=network.target or Requires=network.target
• Use Type=forking, unless you know it's not necessary
  • Many daemons use the exit of the first process to signal that they are ready, so to minimize problems, it is safest to use this mode
  • To make sure that systemd is able to figure out which process is the main process, tell the daemon to write a pidfile and point systemd to it using PIDFile=
  • If the daemon in question is dbus-activated, socket-activated, or specifically supports Type=notify, that's a different matter, but currently only the case for a minority of daemons
• Arch's rc scripts do not support dependencies, but with systemd they should be added add where necessary
  • The most typical case is that A requires the service B to be running before A is started. In that case add Requires=B and After=B to A.
  • If the dependency is optional then add Wants=B and After=B instead
  • Dependencies are typically placed on services and not on targets

If you want to get fancy, you should know what you are doing.

Example of a simple conversion

rc script

#!/bin/bash

. /etc/rc.conf
. /etc/rc.d/functions

case "$1" in
  start)
    stat_busy "Starting NIS Server"
    /usr/sbin/ypserv
    if [ $? -gt 0 ]; then
      stat_fail
    else
      add_daemon ypserv
      stat_done
    fi
    ;;
  stop)
    stat_busy "Stopping NIS Server"
    killall -q /usr/sbin/ypserv
    if [ $? -gt 0 ]; then
      stat_fail
    else
      rm_daemon ypserv
      stat_done
    fi
    ;;
  restart)
    $0 stop
    sleep 1
    $0 start
    ;;
  *)
    echo "usage: $0 {start

systemd service file

[Unit]
Description=NIS/YP (Network Information Service) Server
Requires=rpcbind.service
After=network.target rpcbind.service

[Service]
Type=forking
PIDFile=/run/ypserv.pid
ExecStart=/usr/sbin/ypserv

[Install]
WantedBy=multi-user.target

tmpfiles.d

• Instead of creating necessary runtime directories and files when a service is started (as some rc scripts do), ship a tmpfiles.d(5) config file in /usr/lib/tmpfiles.d.
• Add a line systemd-tmpfiles --create foo.conf to post_install (and post_upgrade if needed) to ensure the necessary runtime files are created on install, not just on the next boot

modules-load.d

• Instead of loading necessary modules when a service is started (as some rc scripts do), ship a modules-load.d(5) config file in /usr/lib/modules-load.d.
• Add modprobe lines to post_install (and post_upgrade if needed) to ensure the necessary modules are loaded on install, not just on the next boot

ntp-units.d

• For NTP daemons, systemd-timedated requires an additional file in /usr/lib/systemd/ntp-units.d. It should be named after the package it belongs to (with a .list suffix), and contain the name of the service which starts the NTP daemon itself.

sysctl.d

• IMO(dreisner): This should generally be avoided, as tying low level kernel behavior to a package might be considered evil.