Design
HAL is a single daemon responsible for discovering, enumerating and mediating access to most of the hardware on the host computer. Applications communicate with HAL through the D-Bus IPC mechanism, which abstracts the hardware behind an object-based RPC mechanism.
Each logical hardware device is represented as a D-Bus object, and its bus address is used as a unique identifier. Devices include abstractions like disk partitions and visible wireless networks. The device's functionality is exposed through D-Bus interfaces, and its state accessed through properties, a set of key-value pairs.
HAL broadcasts hardware events as signals on these objects: listening applications can listen for these to react on hardware events (such as a digital camera being plugged in, an optical disc spinning up or a laptop computer closing its lid).
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“Delay always breeds danger; and to protract a great design is often to ruin it.”
—Miguel De Cervantes (1547–1616)