MQTT Discovery

The discovery of MQTT devices will enable one to use MQTT devices with only minimal configuration effort on the side of Home Assistant. The configuration is done on the device itself and the topic used by the device. Similar to the HTTP binary sensor and the HTTP sensor.

Supported by MQTT discovery:

To enable MQTT discovery, add the following to your configuration.yaml file:

# Example configuration.yaml entry
mqtt:
  discovery: true
  discovery_prefix: homeassistant

Configuration variables:

discovery (Optional): If the MQTT discovery should be enabled or not. Defaults to False.
discovery_prefix (Optional): The prefix for the discovery topic. Defaults to homeassistant.

The discovery topic need to follow a specific format:

<discovery_prefix>/<component>/[<node_id>/]<object_id>/<>

<component>: One of the supported components, eg. binary_sensor.
<node_id>: (Optional) id of the node providing the topic.
<object_id>: The ID of the device. This will become the entity_id in Home Assistant.
<>: The topic config or state which defines the current action.

The payload will be checked like an entry in your configuration.yaml file if a new device is added. This means that missing variables will be filled with the platform’s default values. All configuration variables which are required must be present in the initial payload send to /config.

The <node_id> level can be used by clients to only subscribe to their own (command) topics by using one wildcard topic like <discovery_prefix>/+/<node_id>/+/set.

Examples

A motion detection device which can be represented by a binary sensor for your garden would sent its configuration as JSON payload to the Configuration topic. After the first message to config, then the MQTT messages sent to the state topic will update the state in Home Assistant.

Configuration topic: homeassistant/binary_sensor/garden/config
State topic: homeassistant/binary_sensor/garden/state
Payload: {"name": "garden", "device_class": "motion"}

To create a new sensor manually. For more details please refer to the MQTT testing section.

$ mosquitto_pub -h 127.0.0.1 -p 1883 -t "homeassistant/binary_sensor/garden/config" -m '{"name": "garden", "device_class": "motion"}'

Update the state.

$ mosquitto_pub -h 127.0.0.1 -p 1883 -t "homeassistant/binary_sensor/garden/state" -m ON

Setting up a switch is similar but requires a command_topic as mentionend in the MQTT switch documentation.

Configuration topic: homeassistant/switch/irrigation/config
State topic: homeassistant/switch/irrigation/state
Payload: {"name": "garden", "command_topic": "homeassistant/switch/irrigation/set"}

$ mosquitto_pub -h 127.0.0.1 -p 1883 -t "homeassistant/switch/irrigation/config" \
  -m '{"name": "garden", "command_topic": "homeassistant/switch/irrigation/set"}'

Set the state.

$ mosquitto_pub -h 127.0.0.1 -p 1883 -t "homeassistant/switch/irrigation/set" -m ON