developers.home-assistant/docs/architecture/core.md

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Core Architecture	Core

The Home Assistant Core consists of four core parts. On top of this it includes a ton of helper classes to deal with common scenario's, like providing an entity or dealing with locations.

• Event Bus: facilitates the firing and listening of events -- the beating heart of Home Assistant.
• State Machine: keeps track of the states of things and fires a state_changed event when a state has been changed.
• Service Registry: listens on the event bus for call_service events and allows other code to register services.
• Timer: sends a time_changed event every 1 second on the event bus.