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MQTT
The lightweight messaging protocol that lets smart-home devices and services talk to each other locally.
MQTT is the plumbing behind a lot of home automation: a lightweight publish/subscribe messaging system that lets sensors, switches, and services broadcast tiny updates to each other in real time.
Running the broker (Mosquitto) locally means those messages (a door opening, a temperature reading, a car's charge state) stay on the local network instead of round-tripping through a manufacturer's cloud just to turn on a light.