docs/DESKTOP_ENVIRONMENTS.md

   1 ---
   2 title: Desktop Environment Integration
   3 category: Concepts
   4 layout: default
   5 ---
   6
   7 # Desktop Environments
   8
   9 NOTE: This document is a work-in-progress.
  10
  11 ## Single Graphical Session
  12
  13 systemd only supports running one graphical session per user at a time.
  14 While this might not have always been the case historically, having multiple
  15 sessions for one user running at the same time is problematic.
  16 The DBus session bus is shared between all the logins, and services that are
  17 started must be implicitly assigned to the user's current graphical session.
  18
  19 In principle it is possible to run a single graphical session across multiple
  20 logind seats, and this could be a way to use more than one display per user.
  21 When a user logs in to a second seat, the seat resources could be assigned
  22 to the existing session, allowing the graphical environment to present it
  23 is a single seat.
  24 Currently nothing like this is supported or even planned.
  25
  26 ## Pre-defined systemd units
  27
  28 [`systemd.special(7)`](https://www.freedesktop.org/software/systemd/man/systemd.special.html)
  29 defines the `graphical-session.target` and `graphical-session-pre.target` to
  30 allow cross-desktop integration. Furthermore, systemd defines the three base
  31 slices `background`, `app` and `session`.
  32 All units should be placed into one of these slices depending on their purposes:
  33
  34  * `session.slice`: Contains only processes essential to run the user's graphical session
  35  * `app.slice`: Contains all normal applications that the user is running
  36  * `background.slice`: Useful for low-priority background tasks
  37
  38 The purpose of this grouping is to assign different priorities to the
  39 applications.
  40 This could e.g. mean reserving memory to session processes,
  41 preferentially killing background tasks in out-of-memory situations
  42 or assigning different memory/CPU/IO priorities to ensure that the session
  43 runs smoothly under load.
  44
  45 TODO: Will there be a default to place units into e.g. `app.slice` by default
  46 rather than the root slice?
  47
  48 ## XDG standardization for applications
  49
  50 To ensure cross-desktop compatibility and encourage sharing of good practices,
  51 desktop environments should adhere to the following conventions:
  52
  53  * Application units should follow the scheme `app[-<launcher>]-<ApplicationID>[@<RANDOM>].service`
  54  or `app[-<launcher>]-<ApplicationID>-<RANDOM>.scope`
  55    e.g:
  56     - `app-gnome-org.gnome.Evince@12345.service`
  57     - `app-flatpak-org.telegram.desktop@12345.service`
  58     - `app-KDE-org.kde.okular@12345.service`
  59     - `app-org.kde.amarok.service`
  60     - `app-org.gnome.Evince-12345.scope`
  61  * Using `.service` units instead of `.scope` units, i.e. allowing systemd to
  62    start the process on behalf of the caller,
  63    instead of the caller starting the process and letting systemd know about it,
  64    is encouraged.
  65  * The RANDOM should be a string of random characters to ensure that multiple instances
  66  of the application can be launched.
  67  It can be omitted in the case of a non-transient application services which can ensure
  68  multiple instances are not spawned, such as a DBus activated application.
  69  * If no application ID is available, the launcher should generate a reasonable
  70    name when possible (e.g. using `basename(argv[0])`). This name must not
  71    contain a `-` character.
  72
  73 This has the following advantages:
  74  * Using the `app-<launcher>-` prefix means that the unit defaults can be
  75    adjusted using desktop environment specific drop-in files.
  76  * The application ID can be retrieved by stripping the prefix and postfix.
  77    This in turn should map to the corresponding `.desktop` file when available
  78
  79 TODO: Define the name of slices that should be used.
  80 This could be `app-<launcher>-<ApplicationID>-<RANDOM>.slice`.
  81
  82 TODO: Does it really make sense to insert the `<launcher>`? In GNOME I am
  83 currently using a drop-in to configure `BindTo=graphical-session.target`,
  84 `CollectMode=inactive-or-failed` and `TimeoutSec=5s`. I feel that such a
  85 policy makes sense, but it may make much more sense to just define a
  86 global default for all (graphical) applications.
  87
  88  * Should application lifetime be bound to the session?
  89  * May the user have applications that do not belong to the graphical session (e.g. launched from SSH)?
  90  * Could we maybe add a default `app-.service.d` drop-in configuration?
  91
  92 ## XDG autostart integration
  93
  94 To allow XDG autostart integration, systemd will ship a cross-desktop generator
  95 to create appropriate units for the autostart directory.
  96 Desktop Environments will be able to make use of this simply by starting the
  97 appropriate XDG related targets (representing e.g. content of the
  98 `$XDG_CURRENT_DESKTOP` environment variable to handle `OnlyShowIn/NotShowIn`).
  99 The names and ordering rules for these targets are to be defined.
 100
 101 This generator will likely never support certain desktop specific extensions.
 102 One such example is the GNOME specific feature to bind a service to a settings
 103 variable.
 104
 105 ## Startup and shutdown best practices
 106
 107 Question here are:
 108
 109  * Are there strong opinions on how the session-leader process should watch the user's session units?
 110  * Should systemd/logind/… provide an integrated way to define a session in terms of a running *user* unit?
 111  * Is having `gnome-session-shutdown.target` that is run with `replace-irreversibly` considered a good practice?