Home Assistant Core Architecture

Home Assistant runs on a single-threaded asyncio event loop. All code shares this loop — blocking it freezes automations, the UI, and entity updates.

The hass Object

Every integration receives HomeAssistant instance (hass) — the central hub for all core systems:

from homeassistant.core import HomeAssistant

async def async_setup_entry(hass: HomeAssistant, entry: ConfigEntry) -> bool:
    # hass.bus       — Event bus for pub/sub communication
    # hass.states    — State machine for entity states
    # hass.services  — Service registry for actions
    # hass.config    — System config (location, units, timezone)
    ...

Event Bus

The nervous system of Home Assistant. All component communication flows through events.

from homeassistant.core import callback, Event

# Fire an event
hass.bus.async_fire("my_custom_event", {"key": "value"})

# Listen for events (@callback = sync, no I/O allowed)
@callback
def handle_event(event: Event) -> None:
    entity_id = event.data.get("entity_id")
    new_state = event.data.get("new_state")

unsub = hass.bus.async_listen("state_changed", handle_event)
entry.async_on_unload(unsub)  # Always clean up on unload

Key events: state_changed, homeassistant_start, homeassistant_stop, call_service, automation_triggered.

State Machine

Tracks current state of every entity. States are immutable snapshots.

state = hass.states.get("sensor.temperature")
if state is not None:
    value = state.state           # Always a string
    attrs = state.attributes      # Dict of attributes
    last_changed = state.last_changed

Special state values: "unavailable" (coordinator failed/device offline), "unknown" (no data yet).

Service Registry (Actions)

Services (now called "actions" in UI) control devices. Register in async_setup, not async_setup_entry:

async def async_setup(hass: HomeAssistant, config: ConfigType) -> bool:
    async def handle_action(call: ServiceCall) -> None:
        entity_id = call.data.get("entity_id")
        await do_something(entity_id)

    hass.services.async_register(DOMAIN, "my_action", handle_action)
    return True

Integration Loading Order

1. Core starts, loads configuration.yaml
2. Dependencies from manifest.json loaded first
3. async_setup(hass, config) called (if present)
4. For each config entry: async_setup_entry(hass, entry) called
5. Platform forwarding loads each platform's async_setup_entry
6. Entities registered and begin receiving updates

Key Design Principles

Single-threaded event loop: All async code shares one loop. Never block it.

Immutable state: hass.states.get() returns a frozen snapshot.

Integration isolation: Each integration manages its lifecycle via config entries. Store coordinator in entry.runtime_data (not hass.data).