Table of Contents

• Motivation
• MQTT restrictions
  • Topic IDs
  • Payload
  • QoS and retained messages
• Topology
  • Base topic
  • Devices
    • Device attributes
    • Device behavior
    • Device statistics
  • Nodes
    • Node attributes
  • Properties
    • Property attributes
  • Arrays
  • Broadcast channel
• FAQ and Rationale

Motivation

The Homie convention strives to be a communication definition on top of MQTT between IoT devices and controlling entities.

MQTT is a machine-to-machine (M2M)/“Internet of Things” connectivity protocol. It was designed as an extremely lightweight publish/subscribe messaging transport.

MQTT supports easy and unrestricted message-based communication. However, MQTT doesn’t define the structure and content of these messages and their relation. An IoT device publishes data and provides interaction possibilities but a controlling entity will need to be specifically configured to be able to interface with the device.

The Homie convention defines a standardized way of how IoT devices and services announce themselves and their data on the communication channel. The Homie convention is thereby a crucial aspect in the support of automatic discovery, configuration and usage of devices and services over the MQTT protocol.

MQTT Restrictions

Homie communicates through MQTT and is hence based on the basic principles of MQTT topic publication and subscription.

Topic IDs

An MQTT topic consists of one or more topic levels, separated by the slash character (/). A topic level ID MAY contain lowercase letters from a to z, numbers from 0 to 9 as well as the hyphen character (-).

A topic level ID MUST NOT start or end with a hyphen (-). The special character $ is used and reserved for Homie attributes. The underscore (_) is used and reserved for Homie node arrays.

Payload

Every MQTT message payload MUST be sent as string. If a value is of a numeric data type, it MUST be converted to string. Booleans MUST be converted to “true” or “false”. All values MUST be encoded as UTF-8 strings.

QoS and retained messages

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.

Topology

Devices: An instance of a physical piece of hardware is called a device. For example, a car, an Arduino/ESP8266 or a coffee machine.

Nodes: A device can expose multiple nodes. Nodes are independent or logically separable parts of a device. For example, a car might expose a wheels node, an engine node and a lights node.

Nodes can be arrays. For example, instead of creating two lights node to control front lights and back lights independently, we can set the lights node to be an array with two elements.

Properties: A node can have multiple properties. Properties represent basic characteristics of the node/device, often given as numbers or finite states. For example the wheels node might expose an angle property. The engine node might expose a speed, direction and temperature property. The lights node might expose an intensity and a color property.

Properties can be settable. For example, you don’t want your temperature property to be settable in case of a temperature sensor (like the car example), but to be settable in case of a thermostat.

Properties can be retained. A property is retained by default. A non-retained property would be useful for momentary events (door bell pressed).

A combination of those flags compiles into this list:

• retained + non-settable: The node publishes a property state (temperature sensor)
• retained + settable: The node publishes a property state, and can receive commands for the property (by controller or other party) (lamp power)
• non-retained + non-settable: The node publishes momentary events (door bell pressed)
• non-retained + settable: The node publishes momentary events, and can receive commands for the property (by controller or other party) (brew coffee)

Attributes: Devices, nodes and properties have specific attributes characterizing them. Attributes are represented by topic identifier starting with $. The precise definition of attributes is important for the automatic discovery of devices following the Homie convention.

Examples: A device might have an IP attribute, a node will have a name attribute, and a property will have a unit attribute.

Base Topic

The base topic you will see in the following convention will be homie/. If this base topic does not suit your needs (in case of, e.g., a public broker), you can choose another.

Be aware, that only the default base topic homie/ is eligible for automatic discovery by third party controllers.

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.

Device Attributes

• homie / device ID / $device-attribute: When the MQTT connection to the broker is established or re-established, the device MUST send its attributes to the broker immediately.

For example, a device with an ID of super-car that comprises off a wheels, engine and a lights node would send:

homie/super-car/$homie → "2.1.0"
homie/super-car/$name → "Super car"
homie/super-car/$localip → "192.168.0.10"
homie/super-car/$mac → "DE:AD:BE:EF:FE:ED"
homie/super-car/$fw/name → "weatherstation-firmware"
homie/super-car/$fw/version → "1.0.0"
homie/super-car/$nodes → "wheels,engine,lights[]"
homie/super-car/$implementation → "esp8266"
homie/super-car/$stats/interval → "60"
homie/super-car/$state → "ready"

Device Behavior

The $state device attribute represents, as the name suggests, the current state of the device. There are 6 different states:

• init: this is the state the device is in when it is connected to the MQTT broker, but has not yet sent all Homie messages and is not yet ready to operate. This is the first message that must that must be sent.
• ready: this is the state the device is in when it is connected to the MQTT broker, has sent all Homie messages and is ready to operate. You have to send this message after all other announcements message have been sent.
• disconnected: this is the state the device is in when it is cleanly disconnected from the MQTT broker. You must send this message before cleanly disconnecting.
• sleeping: this is the state the device is in when the device is sleeping. You have to send this message before sleeping.
• lost: this is the state the device is in when the device has been “badly” disconnected. You must define this message as LWT.
• alert: this is the state the device is when connected to the MQTT broker, but something wrong is happening. E.g. a sensor is not providing data and needs human intervention. You have to send this message when something is wrong.

Device Statistics

• homie / device ID / $stats/ $device-statistic-attribute: The $stats/ hierarchy allows to send device attributes that change over time. The device MUST send them every $stats/interval seconds.

For example, our super-car device with $stats/interval value “60” is supposed to send its current values every 60 seconds:

homie/super-car/$stats → "uptime,cputemp,signal,battery"
homie/super-car/$stats/uptime → "120"
homie/super-car/$stats/cputemp → "48"
homie/super-car/$stats/signal → "24"
homie/super-car/$stats/battery → "80"

Nodes

• homie / device ID / node ID: this is the base topic of a node. Each node must have a unique node ID on a per-device basis which adhere to the ID format.

Node Attributes

• homie / device ID / node ID / $node-attribute: A node attribute MUST be one of these:

For example, our engine node would send:

homie/super-car/engine/$name → "Car engine"
homie/super-car/engine/$type → "V8"
homie/super-car/engine/$properties → "speed,direction,temperature"

Properties

• homie / device ID / node ID / property ID: this is the base topic of a property. Each property must have a unique property ID on a per-node basis which adhere to the ID format.

• A property value (e.g. a sensor reading) is directly published to the property topic, e.g.:

  homie/super-car/engine/temperature → "21.5"
  

Property Attributes

• homie / device ID / node ID / property ID / $property-attribute: A property attribute MUST be one of these:

For example, our temperature property would send:

homie/super-car/engine/temperature/$name → "Engine temperature"
homie/super-car/engine/temperature/$settable → "false"
homie/super-car/engine/temperature/$unit → "°C"
homie/super-car/engine/temperature/$datatype → "float"
homie/super-car/engine/temperature/$format → "-20:120"
homie/super-car/engine/temperature → "21.5"

• homie / device ID / node ID / property ID / 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 its power state to on. 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 ← "true"

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"

Arrays

A node can be an array if you’ve added [] to its ID in the $nodes device attribute, and if its $array attribute is set to the range of the array. Let’s consider we want to control independently the front lights and back lights of our super-car. Our lights node array would look like this. Note that the topic for an element of the array node is the name of the node followed by a _ and the index getting updated:

homie/super-car/$nodes → "lights[]"

homie/super-car/lights/$name → "Lights"
homie/super-car/lights/$properties → "intensity"
homie/super-car/lights/$array → "0-1"

homie/super-car/lights/intensity/$name → "Intensity"
homie/super-car/lights/intensity/$settable → "true"
homie/super-car/lights/intensity/$unit → "%"
homie/super-car/lights/intensity/$datatype → "integer"
homie/super-car/lights/intensity/$format → "0:100"

homie/super-car/lights_0/$name → "Back lights"
homie/super-car/lights_0/intensity → "0"
homie/super-car/lights_1/$name → "Front lights"
homie/super-car/lights_1/intensity → "100"

Note that you can name each element in your array individually (“Back lights”, etc.).

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.

FAQ

In this section frequently asked questions will be answered. This includes design decisions and drawn compromises in the specifics of the Homie convention.

How do I query/request a property?

You don’t. The MQTT protocol does not implement the request-reply but rather the publish-subscribe messaging pattern. The Homie convention follows the publish-subscribe principle by publishing data as retained messages on a regular basis. You might want to rethink the design of your application - in most scenarios a regularly updated information is sufficient.

Workaround: You are free to implement your own ideas on top of the basic structure of the Homie convention. You could either implement a get getter topic and its logic to trigger a value update, or you may exploit the concept of Homie properties and define a settable property to trigger a value update.

A discussion on the matter can be found in issue #79.