PipeWire: Objects Design

PipeWire 1.2.0

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Objects Design

This document is a design reference on the various objects that exist in the PipeWire media and session management graphs. Explaining what these objects are, how they are meant to be used, and how they relate to other kinds of objects and concepts that exist in subsystems or other libraries.

# The Media Graph

The media graph represents and enables the media flow inside the PipeWire daemon and between the daemon and its clients. It consists of nodes, ports and links.

+------------+ +------------+
| | | |
| +--------+ Link +--------+ |
| Node | Port |--------| Port | Node |
| +--------+ +--------+ |
| | | |
+------------+ +------------+

## Node

A node is a media processing element. It consumes and/or produces buffers that contain data, such as audio or video.

A node may operate entirely inside the PipeWire daemon or it may be operating in a client process. In the second case, media is transferred to/from that client using the PipeWire protocol.

In an analogy to GStreamer, a node is similar (but not equal) to a GStreamer element.

## Port

A port is attached on a node and provides an interface for input or output of media on the node. A node may have multiple ports.

A port always has a direction, input or output:

- Input: it allows media input into the node (in other terms, it is a sink)
- Output: it outputs media out of the node (in other terms, it is a source)

In an analogy to GStreamer, a port is similar (but not equal) to a GStreamer pad.

## Link

A link connects two ports of opposite direction, making media flow from the output port to the input port.

# The Session Management Graph

The session management graph is a virtual, higher level representation of the media flow. It is created entirely by the session manager and it can affect the routing on the media graph only through the session manager's actions.

The session management graph is useful to abstract the complexity of the actual media flow both for the target user and for the policy management codebase.

+---------------------+ +----------------------+
| | | |
| +----------------+ Endpoint Link +----------------+ |
| Endpoint |Endpoint Stream |-----------------|Endpoint Stream | Endpoint |
| +----------------+ +----------------+ |
| | | |
+---------------------+ +----------------------+

## Endpoint

An endpoint is a session management object that provides a representation of user conceivable places where media can be routed to/from.

Examples of endpoints associated with hardware on a desktop-like system:

- Laptop speakers.
- USB webcam.
- Bluetooth headset microphone.
- Line out stereo jack port.

Examples of endpoints associated with hardware in a car:

- Speakers amplifier.
- Front right seat microphone array.
- Rear left seat headphones.
- Bluetooth phone voice gateway.
- Hardware FM radio device.

Examples of endpoints associated with software:

- Desktop screen capture source.
- Media player application.
- Camera application.

In most cases an endpoint maps to a node on the media graph, but this is not always the case. An endpoint may be backed by several nodes or no nodes at all. Different endpoints may also be sharing nodes in some cases.

An endpoint that does not map to any node may be useful to represent hardware that the session manager needs to be able to control, but there is no way to route media to/from that hardware through the PipeWire media graph. For example, in a car we may have a CD player device that is directly wired to the speakers amplifier and therefore audio flows between them without passing through the controlling CPU. However, it is useful for the session manager to be able to represent the CD player endpoint and the endpoint link between it and the amplifier, so that it can apply audio policy that takes into account whether the CD player is playing or not.

### Target

An endpoint may be grouping together targets that can be reached by following the same route and they are mutually exclusive with each other.

For example, the speakers and the headphones jack on a laptop are usually mutually exclusive by hardware design (hardware mutes the speakers when the headphones are enabled) and they share the same ALSA PCM device, so audio still follows the same route to reach both.

In this case, a session manager may choose to group these two targets into the same endpoint, using a parameter on the endpoint object to allow the user to choose the target (if the hardware allows configuring this at all).

## Endpoint Stream

An endpoint stream is attached to an endpoint and represents a logical path that can be taken to reach this endpoint, often associated with a use case.

For example, the "Speakers amplifier" endpoint in a car might have the following streams:

- Music: A path to play music; the implementation will output this to all speakers, using the volume that has been configured for the "Music" use case.
- Voice: A path to play a voice message; such as a navigation message or feedback from a voice assistant, the implementation will output this to the front speakers only. Lowering the volume of the music (if any) on these speakers at the same time.
- Emergency: A path to play an emergency situation sound (a beep, or equivalent); the implementation will output this on all speakers. Increasing the volume to a factory defined value if necessary (to ensure that it is audible) while muting audio from all other streams at the same time.

In another example, a microphone that can be used for activating a voice assistant might have the following streams:

- Capture: A path to capture directly from the microphone; this can be used by an application that listens for the assistant's wake-word in order to activate the full voice recognition engine.
- CaptureDelayed: A path to capture with a constant delay (meaning that starting capturing now will actually capture something that was spoken a little earlier); this can be used by the full voice recognition engine, allowing it to start after the wake-word has been spoken while capturing audio that also includes the wake-word.

Endpoint streams may be mutually exclusive or they may used simultaneously, depending on the implementation.

Endpoint streams may be implemented in many ways:

- By plugging additional nodes in the media graph that link to the device node (ex. a simple buffering node linked to an alsa source node could implement the CaptureDelayed stream in the above microphone example).
- By using a different device node (ex. different ALSA device on the same card) that has a special meaning for the hardware.
- By triggering switches on the hardware (ex. modify ALSA controls on the same device).

## Endpoint Link

An endpoint link connects two streams from two different endpoints, creating a logical representation of media flow between the endpoints.

An endpoint link may be implemented by creating one or more links in the underlying media graph, or it may be implemented by configuring hardware resources to enable media flow, in case the flow does not pass through the media graph.

### Constructing

Constructing an endpoint link is done by asking the endpoint stream objects to prepare it. First, the source stream is asked to provide linking information. When the information is retrieved, the sink stream is asked to use this information to prepare and to provide its own linking information. When this is done, the session manager is asked to create the link using the provided information.

This mechanism allows stream implementations:

- To prepare for linking, adjusting hardware paths if necessary.
- To check for stream linking compatibility; not all streams can be connected to all others (ex. streams with media flow in the hardware cannot be linked to streams that are backed by nodes in the media graph).
- To provide implementation specific information for linking; in the standard case this is going to be a list of ports to be linked in the media graph, but in a hardware-flow case it can be any kind of hardware-specific detail.

# Other Related Objects

## Device

A device represents a handle to an underlying API that is used to create higher level objects, such as nodes, or other devices.

Well-known devices include:

Device API | Description
--- | ---
alsa.pcm.device | A handle to an ALSA card (ex. `hw:0`, `hw:1`, etc).
alsa.seq.device | A handle to an ALSA Midi device.
v4l2.device | A handle to a V4L2 device (`/dev/video0`, `/dev/video1`, etc..).
jack.device | A JACK client, allowing PipeWire to slave to JACK for audio input/output.

A device may have a profile, which allows the user to choose between multiple configurations that the device may be capable of having, or to simply turn the device off, which means that the handle is closed and not used by PipeWire.

## Session

The session represents the session manager and can be used to expose global properties or methods that affect the session management.

### Default Endpoints

The session is responsible for book-keeping the default device endpoints (one for each kind of device) that is to be used to link new clients when simulating a PulseAudio-like behavior, where the user can choose from the UI device preferences.

For example, a system may have both "Speakers" and "HDMI" endpoints on the "Audio Output" category and the user may be offered to make a choice within the UI to select which endpoint they want to use by default for audio output. This preference is meant to be stored in the session object.

### Multiple Sessions

It is not currently defined whether it is allowed to have multiple sessions or not and how the system should behave if this happens.

# Mappings To Underlying Subsystem Objects

## ALSA UCM

This is a proposal

ALSA / UCM | PipeWire
--- | ---
ALSA card | device
UCM verb | device profile
UCM device | endpoint (+ target, grouping conflicting devices into the same endpoint)
UCM modifier | endpoint stream
PCM stream | node

In UCM mode, an ALSA card is represented as a PipeWire device, with the available UCM verbs listed as profiles of the device.

Activating a profile (ie. a verb) will create the necessary nodes for the available PCM streams and at the same time it will also create one endpoint for each UCM device. Optionally conflicting UCM devices can be grouped in the same endpoint, listing the conflicting options as targets of the endpoint.

The available UCM modifiers for each UCM device will be added as streams, plus one "default" stream for accessing the device with no modifiers.

## ALSA Fallback

ALSA | PipeWire
--- | ---
card | device
PCM stream | node + endpoint

In the case where UCM (or another similar mechanism) is not available, ALSA cards are represented as PipeWire devices with only two profiles on/off.

When the on profile is activated, a node and an associated endpoint are created for every available PCM stream.

Endpoints in this case have only one "default" stream, unless they are extended by the session manager to have software-backed streams.

## V4L2

FIXME***

V4L2 | PipeWire
--- | ---
device | device + node

# Relationship To Other API's

## PulseAudio

### Mapping PipeWire Objects For Access By PulseAudio Clients

PipeWire | PulseAudio
--- | ---
device | card
device profile | card profile
endpoint (associated with a device) | sink / source
endpoint (associated with a client) | sink-input / source-output
endpoint target | port
endpoint stream | N/A, PA clients will be limited to the default stream

### Mapping PulseAudio Clients To PipeWire

PulseAudio | PipeWire
--- | ---
stream | client + node + endpoint (no targets, 1 default stream)

## Jack

Note: This section is about JACK clients connecting to PipeWire through the JACK compatibility library. The scenario where PipeWire connects to another JACK server as a client is out of scope here.

### Mapping PipeWire Objects For Access By JACK Clients

PipeWire | JACK
--- | ---
node | client
port | port
device | N/A
endpoint | N/A

### Mapping JACK Clients To PipeWire

JACK | PipeWire
--- | ---
client | client + node
port | port

JACK clients do not create endpoints. A session manager should be JACK aware in order to anticipate direct node linking.