content/browser/compute_pressure/README.md

# Compute Pressure API

This directory contains the browser-side implementation of the
[Compute Pressure API](https://github.com/wicg/compute-pressure/).

## Code map

The system is made up of the following components.

`blink::mojom::PressureService`, defined in Blink, is the interface between
the renderer and the browser sides of the API implementation.

`device::mojom::PressureManager`, defined in Services, is the interface
between the browser and the services sides of the API implementation.

`device::PressureManagerImpl` is the top-level class for the services side
implementation. The class is responsible for handling the communication
between the browser process and services.

`device::PressureSample` represents the device's compute pressure state.
This information is collected by `device::CpuProbe` and bubbled up by
`device::PressureSampler` to `device::PressureManagerImpl`, which broadcasts
the information to the `content::PressureServiceImpl` instances.

`device::PressureSampler` drives measuring the device's compute pressure
state. The class is responsible for invoking platform-specific measurement
code at regular intervals, and for straddling between sequences to meet
the platform-specific code's requirements.

`device::CpuProbe` is an abstract base class that interfaces between
`device::PressureSampler` and platform-specific code that retrieves the
compute pressure state from the operating system. This interface is also
a dependency injection point for tests.

`content::PressureServiceImpl` serves all the mojo connections for a frame.
Each instance is owned by a `content::RenderFrameHostImpl`. The class receives
`device::mojom::PressureState` from `device::PressureManagerImpl` and
broadcasts the information to the `blink::PressureObserver` instances.

`content::PressureQuantizer` implements the quantization logic that converts
a high-entropy `device::mojom::PressureState` into a low-entropy one, which
minimizes the amount of information exposed to a Web page that uses the
Compute Pressure API. Each `content::PressureServiceImpl` uses a
`content::PressureQuantizer` instance, which stores the frame's
quantization schema and produces quantized data suitable for frame.

`blink::PressureObserver` implements bindings for the PressureObserver
interface. There can be more than one PressureObserver per frame.