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README.md

Hemtjänst

Hemtjänst is:

  • A Swedish word for home care or home service
  • A specification of sorts on how devices should register with an MQTT broker
  • An MQTT to HomeKit bridge

This project was started so that a collection of $platform-to-MQTT bridges could be created or adapted that announce the devices they have in a similar enough fashion that we can automatically generate HomeKit accessories from it. Though the aim for now is HomeKit the metadata published should be enough to also be able to create bridges for other platforms, like SmartThings.

Installation

Using Go 1.12+ with modules support you can:

  • go install github.com/hemtjanst/hemtjanst

Development

Using Go 1.12+ with modules support:

  • git clone this repo or your fork and cd into it
  • go mod download

Usage

Once you've go installed the project a binary will be in your $GOPATH/bin. If you do not have explicitly set a $GOPATH environment variable it defaults to $HOME/go. Ensure to add $GOPATH/bin to your $PATH or use the full path to the binary.

By default it will connect to an MQTT broker on localhost:1883 and expose a HomeKit bridge on port 12345 with pairing pin-code 01020304.

Pass a --help for all available options.

Specification

Discovery

In order to not have to continuously scan for new devices, or to subscribe to every event, a discovery mechanism has to be implemented.

A discover topic with retain set must exist. Any device that joins the network must subscribe to discover. Since the retain bit is set they will receive the discovery request and must now announce themselves to the rest of the network. If someone then wants to initiate a full discovery all they need to do is publish again to discover (with the retain bit set).

An announce is done by publishing to the announce/YOUR NAME topic with persistence. The body of the message must be the meta content for the entity that joined. The topic layout is entirely arbitrary and so is the naming of the device. Anyone interested in knowing about devices now simply subscribes to announce/# and gets to know any device that joins.

Please note, the topic of the device is used to generate a unique identifier for this device, so if you change it it'll be like you removed the existing device and added a new one.

Whenever a device leaves it publishes its "root" topic to the leave topic. Similarly to announce this allows other clients to cancel their subscriptions if they were explicitly wathcing that device or take any other ation.

Last Will and Testament

MQTT includes a Last Will And Testament feature that will cause the broker to publish a message on a specific topic when the client is gone, gracefully or not.

If every device sets up its own MQTT client it can hence specify that as a last will and testament the broker should publish to leave with its original topic. This ensures that we can always properly clean up, even if the device falls of a cliff.

However, for bridged clients, lets say IKEA Trådfri lights that are published through a trådfri-to-mqtt broker, the bridge is the only MQTT client. Since we can't specify multiple actions in a last will and testament the bridge can still tell the broker to publish to a message to the topic, the content being a string representing a "leave ID". Every device that the bridge used to bridge must specify that same "leave ID" in its metadata so that a mapping can be maintained between "leave ID"s and devices for any cleanup purposes, such as no longer announcing this accessory to HomeKit.

Metadata

When a device receives a discover it must publish it's meta underneath announce/YOUR NAME as detailed in the discovery protocol.

Meta is a JSON object serialised as a string that contains any additional information needed for Hemtjänst to do its thing and generate HomeKit accessories.

As it currently stands the types and features in meta follow the exact names in the HomeKit specification. To support another platform like SmartThings a similar mapping would need to be created.

The meta document contains a number of required and optional entries. The required ones are: name, type, feature`. The rest is optional.

Optional keys are: topic, lastWillID.

The naming of the keys follows Google's JSON style guide and as such are in camelCase. However, ID is always fully uppercase and any chemical formula expressed in chemical symbols follows its relevant casing, so CO2, not Co2 for carbon dioxide.

name

A human friendly device name. Can be the same thing as the topic name or something else entirely. This will be the name of the accessory as HomeKit sees it so do pick something that makes sense and allows you to relatively easily identify the accessory.

type

The type of device, for example light or CO2Sensor. These map directly onto HomeKit services and are considered the "primary" service. There is currently no support for hidden, secondary or linked services.

You can find the supported devices here and how they map to HomeKit services.

feature

Every device needs at least one feature to be defined for it, the usual required characteristic (it can be more than one). Similarly to device the characteristics mapping contains a list of which feature names map to what HomeKit characteristics.

A feature is an object itself, which can be empty, in which case the defaults apply for the min, max and step value as defined in the HomeKit spec. In order to override them you can specify min, max and step keys and set the appropriate value. Do note that you cannot change the type of the value, so if the HomeKit specifies something as a uint8 it will be deserialised as a uint8.

Similarly, we expect that in order to get and set the value of a feature a "root topic"/<feature>/get and set topic exist that we can use. If those topics are named differently you have to specify a getTopic and a setTopic key that have the full path to a topic (so not necessarily nested under the "root" topic) that should be used instead.

topic

The "root" topic of this device, for example lightbulb/kitchen. This may also be the name that you publish to underneath announce, so announce/lightbulb/kitchen is entirely valid and will be used as the root topic unless the topic key is present.

If you publish as announce/lightbulb/kitchen but the topic is set to light/kitchen the topic in meta takes precedence.

lastWillID

The lastWillID only has to be set for bridged devices, so in cases where each device doesn't itself maintain a connection to the MQTT broker. When that is the case the Last Will And Testament should be set instead to publish the "root" topic name to the leave topic.

The lastWillID can be anything but needs to be unique. As such it's recommended to use a UUIDv4 for this.

Examples

The meta topic for a light that can just be turned on and off looks like this:

{
  "name": "kitchen stove light",
  "type": "light",
  "feature": {
    "on": {}
  }
}

Most smart lights however can also dim and usually set the colour temperature too, so it's more likely you'll want to publish something like this:

{
  "name": "living room light",
  "type": "light",
  "feature": {
    "on": {},
    "brightness": {},
    "colorTemperature": {
      "getTopic": "light/ground_floor/living_room/warmth/get",
      "setTopic": "light/ground_floor/living_room/warmth/set",
    },
  }
}

A contact sensor, something you might put on a window to detect if it is open or closed (as part of a security system for example) can be defined like this:

{
  "name": "bathroom window",
  "type": "contactSensor",
  "feature": {
   "contactSensorState": {}
  },
  "lastWillID": "f56ad37c-aa0f-45f4-8e92-f9a6dba39d84"
}
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