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Table of contents
  1. MQTT (Message Queuing Telemetry Transport)
    1. What is MQTT
    2. Client name requirements
    3. Messages
    4. QOS (Quality of Service) level
    5. What is a topic?
    6. Which MQTT topics are supported?
    7. How does the data look like?

MQTT (Message Queuing Telemetry Transport)

What is MQTT

MQTT is the communication protocol we use to communicate with all devices in our home system.
It’s based on a publish/subscribe model on topics. The publisher sends a message to topic to a MQTT-Broker. Every network needs one broker. The broker is running on the Raspberry Pi. When a device publishes a message, it’ll be send to the broker. The broker sends the message to all devices that subscribed the specific topic. Messages are normally not stored by the Broker. The broker only passes the messages to the devices!

MQTT Architecture


Client name requirements

  • Client names MUST be unique.
  • When a client appears with the same name as another client, the first client will disconnect and the new client is connected. But the first client will try to connect again. The result is a never ending loop.


Every message contains

  • a topic (similar to a path)
  • the message data
  • a QOS (Quality of Service) level
  • a retain Flag. The retain flag is normally not set, so the Broker will instantly forget the message. If the flag is set, the broker will remember the last message where the retain flag is set.

QOS (Quality of Service) level

Mqtt supports 3 different levels of QOS:

  • QOS-0 is the default level. There is no guarantee that the message will be delivered
  • QOS-1 Guarantees the delivery of the messages, but there could be duplicates.
  • QOS-3 Guarantees, the delivery of the messages.

What is a topic?

A topic is the location/name of the message. It identifies it and helps the broker to pass the messages to the clients.
Topics looks like paths for example 'rhasspy/audioServer/devices)' or 'house/room/main-light'. When you subscribe you can also use wildcards. ` + ` is a single wildcard (replaces one character) and ` # ` is a multi wildcard( replaces many characters). When you publish you can’t use wildcards.

Which MQTT topics are supported?

All available topics in rhasspy can be found here: MQTT API

How does the data look like?

The data you publish or subscribe are nothing else than JSON objects. Let’s take a look at hermes/tts/say, in the MQTT API you can see the topic hermes/tts/say requires 5 different values. This could look like this:

    "text": "Example Text", 
    "lang": null, 
    "id": null, 
    "siteId": "default",
    "sessionId: null

In this topic you can shorten it a bit. There are some values with a datatype with a ‘?’ behind. You can left them null of you simply won’t mention them like this:

    "text": "Example Text", 
    "siteId": "default"

The same principle is used when you subscribe a topic. For example the topic hermes/tts/sayFinished it’s published by the TTS system, when it has finished speaking (after hermes/tts/say). When you subscribed to this topic, you will get the following data:

    "id": null, 
    "siteId": "default"

When you setup an id at the data in hermes/tts/say there will be the same id in the hermes/tts/sayFinished data. With NodeRed or Python it’s really simple to create, edit or read JSON data.