The Homie convention

Please note this v2 branch is a work-in-progress. It might change before the final release.

Version: 2.1.0.

Homie is a lightweight MQTT convention for the IoT.

You can find an implementation of the Homie convention:

• An Arduino library built for the ESP8266: marvinroger/homie-esp8266
• WIP - An opinionated Web UI built with Node.js: marvinroger/homie-server
• WIP - Some Node-RED nodes for automation: marvinroger/node-red-contrib-homie
• A Python-implementation for Raspberry Pi & Co.: jalmeroth/homie-python.
• A Ruby-implementation including a command-line-client with OTA-Support for easy adminstration of multiple Homie-devices: rttools/hodmin

Background

An instance of a physical piece of hardware (an Arduino, an ESP8266...) is called a device. A device has attributes, like the current local IP, the Wi-Fi signal, etc. A device can expose multiple nodes. For example, a weather station with two wireless sensors might expose a temperature node and an humidity node. A node can have multiple properties. The temperature node might for example expose a degrees property containing the actual temperature, and an battery property containing the battery level of the temperature sensor. Node properties can be ranges. For example, if you have a LED strip, you can have a node property led ranging from 1 to 10, to control LEDs independently. Node properties can be settable. For example, you don't want your degrees property to be settable in case of a temperature sensor: this depends on the environment and it would not make sense to change it. However, you will want the degrees property to be settable in case of a thermostat.

QoS and retained messages

Homie devices communicate through MQTT.

The nature of the Homie convention makes it safe about duplicate messages, so the recommended QoS for reliability is QoS 1. All messages MUST be sent as retained, UNLESS stated otherwise.

ID format

An ID MAY contain only lowercase letters from a to z, numbers from 0 to 9, and it MAY contain -, but MUST NOT start or end with a -.

Convention

To efficiently parse messages, Homie defines a few rules related to topic names. The base topic you will see in the following convention will be homie/. You can however choose whatever base topic you want.

Devices

• homie / device ID: this is the base topic of a device. Each device must have a unique device ID which adhere to the ID format.
• Attributes are topics that are prefixed with a $. These sub-topics add meta-data to Devices, Nodes and Properties describing their parent topic.

Device attributes

• homie / device ID / $ device attribute: a topic starting with a $ after the base topic of a device represents a device attribute. A device attribute MUST be one of these:

Topic	Direction	Description	Retained	Required
$homie	Device → Controller	Version of the Homie convention the device conforms to	Yes	Yes
$online	Device → Controller	true when the device is online, false when the device is offline (through LWT). When sending the device is online, this message must be sent last, to indicate every other required messages are sent and the device is ready	Yes	Yes
$name	Device → Controller	Friendly name of the device	Yes	Yes
$localip	Device → Controller	IP of the device on the local network	Yes	Yes
$mac	Device → Controller	Mac address of the device network interface. The format MUST be of the type A1:B2:C3:D4:E5:F6	Yes	Yes
$stats/uptime	Device → Controller	Time elapsed in seconds since the boot of the device	Yes	Yes
$stats/signal	Device → Controller	Integer representing the Wi-Fi signal quality in percentage if applicable	Yes	No, this is not applicable to an Ethernet connected device for example
$stats/interval	Device → Controller	Interval in seconds at which the $stats/uptime and $stats/signal are refreshed	Yes	Yes
$fw/name	Device → Controller	Name of the firmware running on the device. Allowed characters are the same as the device ID	Yes	Yes
$fw/version	Device → Controller	Version of the firmware running on the device	Yes	Yes
$fw/checksum	Device → Controller	MD5 checksum of the firmware running on the device	Yes	No, depending of your implementation
$implementation	Device → Controller	An identifier for the Homie implementation (example esp8266)	Yes	Yes
$implementation/#	Controller → Device or Device → Controller	You can use any subtopics of $implementation for anything related to your specific Homie implementation.	Yes or No, depending of your implementation	No
$nodes	Device → Controller	Nodes the device exposes, with format id separated by a , if there are multiple nodes. For ranges, define the range after the id, within [] and separated by a -.	Yes	Yes

For example, a device with an ID of 686f6d6965 with a temperature and an humidity sensor would send:

homie/686f6d6965/$online → true
homie/686f6d6965/$name → Bedroom temperature sensor
homie/686f6d6965/$localip → 192.168.0.10
homie/686f6d6965/$signal → 72
homie/686f6d6965/$fw/name → 1.0.0
homie/686f6d6965/$fw/version → 1.0.0

Node attributes

• homie / device ID / node ID / node attribute: node ID is the ID of the node, which must be unique on a per-device basis, and which adhere to the ID format.
• A node is made discoverable via its node attributes. It must be one of the following:

Topic	Direction	Description	Retained	Required
$type	Device → Controller	Type of the node	Yes	Yes
$name	Device → Controller	Friendly name of the Node	Yes	Yes
$properties	Device → Controller	Properties the node exposes, with format id separated by a , if there are multiple nodes. For ranges, define the range after the id, within [] and separated by a -.	Yes	Yes

Property attributes

• homie / device ID / node ID / property / property attribute: property is the property of the node that is getting updated, which must be unique on a per-node basis, and which adhere to the ID format.
• A property is made discoverable via its property attributes. It must be one of the following:

Topic	Direction	Description	Valid values	Retained	Required
$settable	Device → Controller	Specifies whether the property is settable (true) or readonly (false)	true,false	Yes	Yes
$unit	Device → Controller	A string containing the unit of this property. You are not limited to the recommended values, although they are the only well known ones that will have to be recognized by any Homie consumer.	Recommended: °C Degree Celsius °F Degree Fahrenheit ° Degree L Liter gal Galon V Volts W Watt A Ampere % Percent m Meter ft Feet Pa Pascal psi PSI # Count or Amount	Yes	Yes, if applicable. If $unit is omitted, it is assumed that the property is unit-less, e.g. a discrete state.
$datatype	Device → Controller	Describes the format of data.	integer, float, boolean (true or false), string, enum	Yes	Yes
$name	Device → Controller	Friendly name of the property.	Any String	Yes	Yes
$format	Device → Controller	Describes what are valid values for this property.	from:to Describes a range of values e.g. 10:15. Valid for datatypes integer, float value,value,value for enumerating all valid values. Escape , by using ,,. e.g. A,B,C or ON,OFF,PAUSE. Valid for datatypes enum regex:/pattern/flags to provide a regex that can be used to match the value. e.g. regex:/[A-Z][0-9]+/g. Valid for datatype string	Yes	Yes

For example, our 686f6d6965 above would send:

homie/686f6d6965/temperature/$type → temperature
homie/686f6d6965/temperature/$properties → degrees,unit
homie/686f6d6965/temperature/degrees/$settable → false
homie/686f6d6965/temperature/degrees/$unit → C
homie/686f6d6965/temperature/degrees/$datatype → float
homie/686f6d6965/temperature/degrees/$format → -20.0:60
homie/686f6d6965/temperature/degrees → 12.07

homie/686f6d6965/humidity/$type → humidity
homie/686f6d6965/humidity/$properties → percentage
homie/686f6d6965/humidity/percentage/$settable → false
homie/686f6d6965/humidity/percentage/$unit → %
homie/686f6d6965/humidity/percentage/$datatype → integer
homie/686f6d6965/humidity/percentage/$format → 0:100
homie/686f6d6965/humidity/percentage → 79

A LED strip would look like this. Note that the topic for a range properties is the name of the property followed by a _ and the index getting updated:

homie/ledstrip-device/ledstrip/$type → ledstrip
homie/ledstrip-device/ledstrip/$properties → led[1-3]

homie/ledstrip-device/ledstrip/led_1/$settable → true
homie/ledstrip-device/ledstrip/led_1/$unit →
homie/ledstrip-device/ledstrip/led_1/$name → Red LEDs
homie/ledstrip-device/ledstrip/led_1/$datatype → enum
homie/ledstrip-device/ledstrip/led_1/$format → on,off
homie/ledstrip-device/ledstrip/led_1 → on

homie/ledstrip-device/ledstrip/led_2/$settable → true
homie/ledstrip-device/ledstrip/led_2/$unit →
homie/ledstrip-device/ledstrip/led_1/$name → Green LEDs
homie/ledstrip-device/ledstrip/led_2/$datatype → enum
homie/ledstrip-device/ledstrip/led_2/$format → on,off
homie/ledstrip-device/ledstrip/led_2 → off

homie/ledstrip-device/ledstrip/led_3/$settable → true
homie/ledstrip-device/ledstrip/led_3/$unit →
homie/ledstrip-device/ledstrip/led_1/$name → Blue LEDs
homie/ledstrip-device/ledstrip/led_3/$datatype → enum
homie/ledstrip-device/ledstrip/led_3/$format → on,off
homie/ledstrip-device/ledstrip/led_3 → on

• homie / device ID / node ID / property / set: the device can subscribe to this topic if the property is settable from the controller, in case of actuators.

Homie is state-based. You don't tell your smartlight to turn on, but you tell it to put it's on state to true. This especially fits well with MQTT, because of retained message.

For example, a kitchen-light device exposing a light node would subscribe to homie/kitchen-light/light/power/set and it would receive:

homie/kitchen-light/light/power/set ← on

The device would then turn on the light, and update its power state. This provides pessimistic feedback, which is important for home automation.

homie/kitchen-light/light/power → true

Broadcast channel

Homie defines a broadcast channel, so a controller is able to broadcast a message to every Homie devices:

• homie / $broadcast / level: level is an arbitrary broadcast identifier. It must adhere to the ID format.

For example, you might want to broadcast an alert event with the alert reason as the payload. Devices are then free to react or not. In our case, every buzzer of your home automation system would start buzzing.

homie/$broadcast/alert ← Intruder detected

Any other topic is not part of the Homie convention.