MQTT Protocol

  • MQTT is open, simple and low overhead communication protocol for sending messages between many clients which are connected to the central MQTT broker.

  • Every message contains two parts - topic and payload.

  • Topic describes the content of the message.

  • Topic name contains “directory” structure - each level is divided with a symbol ‘/’.

  • Topic can be bedroom/temperature or kitchen/light/set.

  • MQTT server is called the broker and clients can publish messages and subscribe to different topics.

  • The task of the MQTT broker is to deliver messages from publishers to the subscribers.

  • You can use the + symbol to subscribe to all topics in the current topic (+/light/set) and # symbol to subscribe to all sub-topics (kitchen/#).


The symbol # can be used only at the end of the topic name.

Mosquitto MQTT broker

IoT Kit uses the open-source Mosquitto MQTT broker. All messages are routed through the MQTT broker. This allows further expansion of IoT Kit system. When you connect the Core Module or Radio Dongle with connected remote node, you can display all incoming messages using mosquitto-cli package by typing:

mosquitto_sub -t "#" -v


1pi@hub:~ $ mosquitto_sub -t "#" -v
2node/836d19821664/thermometer/0:1/temperature 24.69
3node/836d19821664/thermometer/0:1/temperature 24.94
4node/836d19821664/push-button/-/event-count 5

HARDWARIO Playground as GUI tool for MQTT messages

You can use HARDWARIO Playground for subscribing and publishing MQTT messages. You can do it in Messages tab.

Playground Messages Tab


Subscribe mode is default mode when you open Messages tab in Playground. In beginning you have already subscribed node/# topic. We can try to send MQTT messages with payload test message and topic node/test by typing following command.

mosquitto_pub -t "node/test" -m "test message"

Now you can see test message in Playground message window.

Playground Messages Tab Test Message


You can also publish message from Playground. Just switch to Publish mode, type topic, payload and Publish your message.


If you are subscribed to correct topics you can see your published messages appear on top of the Messages Tab

Playground Messages Tab Publish