projects / thirdparty / systemd.git / blame
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|---|---|---|
| 31c68e02 BB |
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`, `apps` 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 | * `apps.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 assinging 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. `apps.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 `apps-<launcher>-<ApplicationID>-<RANDOM>.service`, | |
| 54 | e.g. `apps-gnome-org.gnome.Evince-12345.service`, | |
| 55 | `apps-flatpak-org.telegram.desktop-12345.service` or `apps-KDE-org.kde.okular-12345.service`. | |
| 56 | * Using `.service` units instead of `.scope` units, i.e. allowing systemd to | |
| 57 | start the process on behalf of the caller, | |
| 58 | instead of the caller starting the process and letting systemd know about it, | |
| 59 | is encouraged. | |
| 60 | * If no application ID is available, the launcher should generate a reasonable | |
| 61 | name when possible (e.g. using `basename(argv[0])`). This name must not | |
| 62 | contain a `-` character. | |
| 63 | ||
| 64 | This has the following advantages: | |
| 65 | * Using the `apps-<launcher>-` prefix means that the unit defaults can be | |
| 66 | adjusted using desktop environment specific drop-in files. | |
| 67 | * The application ID can be retrieved by stripping the prefix and postfix. | |
| 68 | This in turn should map to the corresponding `.desktop` file when available | |
| 69 | ||
| 70 | TODO: Define the name of slices that should be used. | |
| 71 | This could be `apps-<launcher>-<ApplicationID>-<RANDOM>.slice`. | |
| 72 | ||
| 73 | TODO: Does it really make sense to insert the `<launcher>`? In GNOME I am | |
| 74 | currently using a drop-in to configure `BindTo=graphical-session.target`, | |
| 75 | `CollectMode=inactive-or-failed` and `TimeoutSec=5s`. I feel that such a | |
| 76 | policy makes sense, but it may make much more sense to just define a | |
| 77 | global default for all (graphical) applications. | |
| 78 | ||
| 79 | * Should application lifetime be bound to the session? | |
| 80 | * May the user have applications that do not belong to the graphical session (e.g. launched from SSH)? | |
| 81 | * Could we maybe add a default `apps-.service.d` drop-in configuration? | |
| 82 | ||
| 83 | ## XDG autostart integration | |
| 84 | ||
| 85 | To allow XDG autostart integration, systemd will ship a cross-desktop generator | |
| 86 | to create appropriate units for the autostart directory. | |
| 87 | Desktop Environments will be able to make use of this simply by starting the | |
| 88 | appropriate XDG related targets (representing e.g. content of the | |
| 89 | `$XDG_CURRENT_DESKTOP` environment variable to handle `OnlyShowIn/NotShowIn`). | |
| 90 | The names and ordering rules for these targets are to be defined. | |
| 91 | ||
| 92 | This generator will likely never support certain desktop specific extensions. | |
| 93 | One such example is the GNOME specific feature to bind a service to a settings | |
| 94 | variable. | |
| 95 | ||
| 96 | ## Startup and shutdown best practices | |
| 97 | ||
| 98 | Question here are: | |
| 99 | ||
| 100 | * Are there strong opinions on how the session-leader process should watch the user's session units? | |
| 101 | * Should systemd/logind/… provide an integrated way to define a session in terms of a running *user* unit? | |
| 102 | * Is having `gnome-session-shutdown.target` that is run with `replace-irreversibly` considered a good practice? |