Chapter 2. Types of Compositors

Table of Contents

System Compositor
Session Compositor
Embedding Compositor

Compositors come in different types, depending on which role they play in the overall architecture of the OS. For instance, a system compositor can be used for booting the system, handling multiple user switching, a possible console terminal emulator and so forth. A different compositor, a session compositor would provide the actual desktop environment. There are many ways for different types of compositors to co-exist.

In this section, we introduce three types of Wayland compositors relying on libwayland-server.

System Compositor

A system compositor can run from early boot until shutdown. It effectively replaces the kernel vt system, and can tie in with the systems graphical boot setup and multiseat support.

A system compositor can host different types of session compositors, and let us switch between multiple sessions (fast user switching, or secure/personal desktop switching).

A linux implementation of a system compositor will typically use libudev, egl, kms, evdev and cairo.

For fullscreen clients, the system compositor can reprogram the video scanout address to read directly from the client provided buffer.

Session Compositor

A session compositor is responsible for a single user session. If a system compositor is present, the session compositor will run nested under the system compositor. Nesting is feasible because the protocol is asynchronous; roundtrips would be too expensive when nesting is involved. If no system compositor is present, a session compositor can run directly on the hardware.

X applications can continue working under a session compositor by means of a root-less X server that is activated on demand.

Possible examples for session compositors include

  • gnome-shell

  • moblin

  • kwin

  • kmscon

  • rdp session

  • Weston with X11 or Wayland backend is a session compositor nested in another session compositor.

  • fullscreen X session under Wayland

Embedding Compositor

X11 lets clients embed windows from other clients, or lets clients copy pixmap contents rendered by another client into their window. This is often used for applets in a panel, browser plugins and similar. Wayland doesn't directly allow this, but clients can communicate GEM buffer names out-of-band, for example, using D-Bus, or command line arguments when the panel launches the applet. Another option is to use a nested Wayland instance. For this, the Wayland server will have to be a library that the host application links to. The host application will then pass the Wayland server socket name to the embedded application, and will need to implement the Wayland compositor interface. The host application composites the client surfaces as part of it's window, that is, in the web page or in the panel. The benefit of nesting the Wayland server is that it provides the requests the embedded client needs to inform the host about buffer updates and a mechanism for forwarding input events from the host application.

An example for this kind of setup is firefox embedding the flash player as a kind of special-purpose compositor.