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118 lines
5.5 KiB
Markdown
118 lines
5.5 KiB
Markdown
---
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title: Desktop Environment Integration
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category: Concepts
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layout: default
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---
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# Desktop Environments
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NOTE: This document is a work-in-progress.
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## Single Graphical Session
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systemd only supports running one graphical session per user at a time.
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While this might not have always been the case historically, having multiple
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sessions for one user running at the same time is problematic.
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The DBus session bus is shared between all the logins, and services that are
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started must be implicitly assigned to the user's current graphical session.
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In principle it is possible to run a single graphical session across multiple
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logind seats, and this could be a way to use more than one display per user.
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When a user logs in to a second seat, the seat resources could be assigned
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to the existing session, allowing the graphical environment to present it
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is a single seat.
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Currently nothing like this is supported or even planned.
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## Pre-defined systemd units
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[`systemd.special(7)`](https://www.freedesktop.org/software/systemd/man/systemd.special.html)
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defines the `graphical-session.target` and `graphical-session-pre.target` to
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allow cross-desktop integration. Furthermore, systemd defines the three base
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slices `background`, `app` and `session`.
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All units should be placed into one of these slices depending on their purposes:
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* `session.slice`: Contains only processes essential to run the user's graphical session
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* `app.slice`: Contains all normal applications that the user is running
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* `background.slice`: Useful for low-priority background tasks
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The purpose of this grouping is to assign different priorities to the
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applications.
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This could e.g. mean reserving memory to session processes,
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preferentially killing background tasks in out-of-memory situations
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or assigning different memory/CPU/IO priorities to ensure that the session
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runs smoothly under load.
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TODO: Will there be a default to place units into e.g. `app.slice` by default
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rather than the root slice?
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## XDG standardization for applications
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To ensure cross-desktop compatibility and encourage sharing of good practices,
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desktop environments should adhere to the following conventions:
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* Application units should follow the scheme `app[-<launcher>]-<ApplicationID>[@<RANDOM>].service`
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or `app[-<launcher>]-<ApplicationID>-<RANDOM>.scope`
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e.g:
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- `app-gnome-org.gnome.Evince@12345.service`
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- `app-flatpak-org.telegram.desktop@12345.service`
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- `app-KDE-org.kde.okular@12345.service`
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- `app-org.kde.amarok.service`
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- `app-org.gnome.Evince-12345.scope`
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* Using `.service` units instead of `.scope` units, i.e. allowing systemd to
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start the process on behalf of the caller,
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instead of the caller starting the process and letting systemd know about it,
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is encouraged.
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* The RANDOM should be a string of random characters to ensure that multiple instances
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of the application can be launched.
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It can be omitted in the case of a non-transient application services which can ensure
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multiple instances are not spawned, such as a DBus activated application.
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* If no application ID is available, the launcher should generate a reasonable
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name when possible (e.g. using `basename(argv[0])`). This name must not
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contain a `-` character.
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This has the following advantages:
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* Using the `app-<launcher>-` prefix means that the unit defaults can be
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adjusted using desktop environment specific drop-in files.
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* The application ID can be retrieved by stripping the prefix and postfix.
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This in turn should map to the corresponding `.desktop` file when available
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TODO: Define the name of slices that should be used.
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This could be `app-<launcher>-<ApplicationID>-<RANDOM>.slice`.
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TODO: Does it really make sense to insert the `<launcher>`? In GNOME I am
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currently using a drop-in to configure `BindTo=graphical-session.target`,
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`CollectMode=inactive-or-failed` and `TimeoutSec=5s`. I feel that such a
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policy makes sense, but it may make much more sense to just define a
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global default for all (graphical) applications.
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* Should application lifetime be bound to the session?
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* May the user have applications that do not belong to the graphical session (e.g. launched from SSH)?
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* Could we maybe add a default `app-.service.d` drop-in configuration?
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## XDG autostart integration
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To allow XDG autostart integration, systemd ships a cross-desktop generator
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to create appropriate units for the autostart directory
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(`systemd-xdg-autostart-generator`).
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Desktop Environments can opt-in to using this by starting
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`xdg-desktop-autostart.target`. The systemd generator correctly handles
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`OnlyShowIn=` and `NotShowin=`. It also handles the KDE and GNOME specific
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`X-KDE-autostart-condition=` and `AutostartCondition=` by using desktop
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environment provided binaries in an `ExecCondition=` line.
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However, this generator is somewhat limited in what it supports. For example,
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all generated units will have `After=graphical-session.target` set on them,
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it may therefore not be useful to start session services.
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Desktop files can be marked to be explicitly excluded from the generator using the line
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`X-systemd-skip=true`. This should be set if an application provides its own
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systemd service file for startup.
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## Startup and shutdown best practices
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Question here are:
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* Are there strong opinions on how the session-leader process should watch the user's session units?
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* Should systemd/logind/… provide an integrated way to define a session in terms of a running *user* unit?
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* Is having `gnome-session-shutdown.target` that is run with `replace-irreversibly` considered a good practice?
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