A short overview of PipeWire's design.

PipeWire is a media server that can run graphs of multimedia nodes. Nodes can run inside the server process or in separate processes, communicating with the server.

PipeWire was designed to

be efficient for raw video using fd passing and audio with shared ringbuffers
be able to provide/consume/process media from any process
provide policy to restrict access to devices and streams
be easily extensible

Although an initial goal, the design is not limited to raw video only and should be able to handle compressed video and other media as well.

PipeWire uses the SPA plugin API for the nodes in the graph. SPA is designed for low-latency and efficient processing of any multimedia format. SPA also provides a number of helper utilities that are not available in the standard C library.

Some of the application we intend to build

v4l2 device provider: Provide controlled access to v4l2 devices and share one device between multiple processes.
gnome-shell video provider: GNOME Shell provides a node that gives the contents of the frame buffer for screen sharing or screen recording.
audio server: Mix and playback multiple audio streams. The design is more like CRAS (Chromium audio server) than PulseAudio and with the added benefit that processing can be arranged in a graph.
Pro audio graph processing like JACK.
Media playback backend

Protocol

The native protocol and object model is similar to Wayland but with custom serialization/deserialization of messages. This is because the datastructures in the messages are more complicated and not easily expressible in XML. See PipeWire Module: Protocol Native for details.

Extensibility

The functionality of the server is implemented and extended with modules and extensions. Modules are server side bits of logic that hook into various places to provide extra features. This mostly means controlling the processing graph in some way. See PipeWire Modules for a list of current modules.

Extensions are the client side version of the modules. Most extensions provide both a client side and server side init function. New interfaces or new object implementation can easily be added with modules/extensions.

Some of the extensions that can be written

protocol extensions: a client/server side API (.h) together with protocol extensions and server/client side logic to implement a new object or interface.
a module to check security of method calls
a module to automatically create, link or relink nodes
a module to suspend idle nodes