Homie: An MQTT Convention for IoT/M2M

Abstract

The Homie convention defines a standardized way of how IoT devices and services announce themselves and their data on the MQTT broker. The Homie convention is thereby a crucial aspect on top of the MQTT protocol for automatic discovery, configuration and usage of devices and services."

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 ONLY 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.

Payload

• Every MQTT message payload MUST be sent as a UTF-8 encoded string
• The value published as payload MUST be valid for the respective property/attribute type as per the list below

String

• String types are limited to 268,435,456 characters
• An empty string ("") is a valid payload

Integer

• Integer types are UTF-8 encoded string literal representations of 64-bit signed whole numbers
• Integers range from -9,223,372,036,854,775,808 (-263) to 9,223,372,036,854,775,807 (263-1)
• The payload may only contain whole numbers and the negation character “-”. No other characters including spaces (" “) are permitted
• A string with just a negation sign (”-") is not a valid payload
• An empty string ("") is not a valid payload

Float

• Float types are UTF-8 encoded string literal representations of 64-bit signed floating point numbers
• Floats range from +/-(2^-1074) to +/-((2 - 2^-52) * 2^1023)
• The payload may only contain whole numbers, the negation character “-”, the exponent character “e” or “E” and the decimal separator “.”, no other characters, including spaces (" “) are permitted
• The dot character (”.") is the decimal separator (used if necessary) and may only have a single instance present in the payload
• Representations of numeric concepts such as “NaN” (Not a Number) and “Infinity” are not a valid payload
• A string with just a negation sign ("-") is not a valid payload
• An empty string ("") is not a valid payload

Percent

• Percent properties are set when the $unit attribute is set to %
• Percentage values maybe Integer or Float
• Percentage values are not restricted to 0-100

Boolean

• Booleans must be converted to the string literals “true” or “false”
• Representation is case sensitive, e.g. “TRUE” or “FALSE” are not valid payloads.
• An empty string ("") is not a valid payload

Enum

• Enum payloads must be one of the values specified in the format definition of the property
• Enum payloads are case sensitive, e.g. “Car” will not match a format definition of “car”
• Payloads should have leading and trailing whitespace removed
• An empty string ("") is not a valid payload

Color

• Color payload validity varies depending on the property format definition of either “rgb” or “hsv”
• Both payload types contain comma separated whole numbers of differing restricted ranges
• The encoded string may only contain whole numbers and the comma character “,”, no other characters are permitted, including spaces (" “)
• Payloads for type “rgb” contains 3 comma separated values of numbers with a valid range between 0 and 255. e.g. 100,100,100
• Payloads for type “hsv” contains 3 comma separated values of numbers. The first number has a range of 0 to 360, the second and third numbers have a range of 0 to 100. e.g. 300,50,75
• An empty string (”") is not a valid payload

DateTime

• DateTime payloads must use the ISO 8601 format.
• An empty string ("") is not a valid payload

Duration

• Duration payloads must use the ISO 8601 duration format
• The format is PTHHMMSSS, where: P: Required and indicates a duration. T: Required and indicates a time. H: Indicates hour and is preceded by the number of hours, if hours are specified. M: Indicates minutes, and is preceded by the number of minutes, if minutes are specified. S: Indicates seconds, preceded by the number of seconds, if seconds are specified.
• An empty string ("") is not a valid payload

QoS and retained messages

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

Last will

MQTT only allows one last will message per connection. Homie requires the last will (LWT) to set the homie / device ID / $state attribute to the value lost, see Device Lifecycle. As a consequence a new MQTT connection to the broker is required per published device.

Base Topic

The root topic in this document is homie/. If this root topic does not suit your needs (in case of, e.g., a public broker or because of branding), you can choose another.

Homie controllers must by default perform auto-discovery on the wildcard topic “+/+/$homie”. Controllers are free to restrict discovery to a specific root topic, configurable by the user.

Broadcast Topic

Homie defines a broadcast topic, so a controller is able to broadcast a message to all Homie devices:

• homie / $broadcast / subtopic: subtopic can be any topic with single or multiple levels. 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/$broadcast/alert ← "Intruder detected"
homie/$broadcast/security/alert ← "Intruder detected"

Any other topic is not part of the Homie convention.

Extensions

This convention only covers discoverability of devices and its capabilities. The aim is to have standardized MQTT topics for all kind of complex scenarios. A Homie device may therefore support extensions, defined in separate documents. Every extension is identified by a unique ID.

The ID consists of the reverse domain name and a freely chosen suffix. The proper term homie is reserved and must not be used as the suffix or as part of the domain name.

For example, an organization example.org wanting to add a feature our-feature would choose the extension ID org.example.our-feature.

Every extension must be published using a license. The license can be chosen freely, even proprietary licenses are possible. The recommended license is the CCA 4.0, since this is the license Homie itself uses.

Implementation notes

Device initialization

Some devices require knowledge of their settable retained properties to function properly. The homie convention does not specify how to initialize/recover them e.g. after a power cycle. A few common approaches are:

• A device can simply load default values from some configuration file.
• A device can restore its previous state from some local storage. This is the recommended way.
• A device may try to restore its state using MQTT. This can be done by subcribing to the respective channels. The controller could set all properties of a device once it becomes ready. An alternative way is to recover the state from other MQTT channels that are external to the Homie specification.
• If a property is not critical for correct function, there is no need to recover it.

Device reconfiguration

If a device wishes to modify any of its nodes or properties, it can

• disconnect and reconnect with other values, or
• set $state=init and then modify any of the attributes.

Devices can remove old properties and nodes by publishing a zero-length payload on the respective topics.

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.

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.

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.

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:

The following device attributes are mandatory and MUST be send, even if it is just an empty string.

Optional topics include:

For example, a device with an ID of super-car that comprises of 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/$nodes → "wheels,engine,lights[]"
homie/super-car/$implementation → "esp8266"
homie/super-car/$state → "ready"

Device Lifecycle

The $state device attribute represents 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 state is optional, and may be sent if the device takes a long time to initialize, but wishes to announce to consumers that it is coming online.
• 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. A Homie Controller can assume default values for all optional topics.
• 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.

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:

All listed attributes are required. 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 payload (e.g. a sensor reading) is directly published to the property topic, e.g.:

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

• 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)

Property Attributes

• homie / device ID / node ID / property ID / $property-attribute:

The following attributes are required:

The following attributes are optional:

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"

Format:

• For integer and float: Describes a range of payloads e.g. 10:15
• For enum: payload,payload,payload for enumerating all valid payloads.
• For color:
  • rgb to provide colors in RGB format e.g. 255,255,0 for yellow.
  • hsv to provide colors in HSV format e.g. 60,100,100 for yellow.

Recommended unit strings:

• °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

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.

Property command topic

• homie / device ID / node ID / property ID / set: The device must subscribe to this topic if the property is settable (in case of actuators for example).

A Homie controller publishes to the set command topic with non-retained messages only.

The assigned and processed payload must be reflected by the Homie device in the property topic homie / device ID / node ID / property ID as soon as possible. This property state update not only informs other devices about the change but closes the control loop for the commanding controller, important for deterministic interaction with the client device.

To give an example: A kitchen-light device exposing the light node with a settable power property subscribes to the topic homie/kitchen-light/light/power/set for commands:

homie/kitchen-light/light/power/set ← "true"

In response the device will turn on the light and upon success update its power property state accordingly:

homie/kitchen-light/light/power → "true"

Broadcast Channel

Homie defines a broadcast channel, so a controller is able to broadcast a message to all 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.