Mosquitto: Understanding MQTT Topics

This documentation is part of the Getting Started guide. You can view the complete guide here: How to Get Started with Mosquitto.

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MQTT organizes communication into "topics", which act as channels for publishing and subscribing to data. Devices publish messages to specific topics or subscribe to them to receive relevant updates.

Structure of MQTT Topics

Topics in MQTT are case-sensitive, must contain UTF-8 characters, and require at least one character. The hierarchy within topics is defined using the slash (/) character.

Here is an example setup for IoT devices collecting temperature and humidity data, identified by their MAC addresses:

• For a device with MAC address 00:00:00:00:00:00:

  • Temperature: devices/00:00:00:00:00:00/temperature
  • Humidity: devices/00:00:00:00:00:00/humidity

• For a device with MAC address 11:11:11:11:11:11:

  • Temperature: devices/11:11:11:11:11:11/temperature
  • Humidity: devices/11:11:11:11:11:11/humidity

Applications can subscribe to a specific topic, such as devices/00:00:00:00:00:00/temperature, to receive data from the respective IoT device.

Using Wildcards in MQTT Topics

MQTT supports wildcards for efficient topic management:

• The + wildcard matches a single level in the topic hierarchy. For example, subscribing to devices/+/temperature captures topics like devices/1/temperature but not devices/1/2/temperature.

• The # wildcard matches multiple levels. For instance, devices/# matches topics such as devices/1/temperature, devices/2/humidity, and devices/1/2/somethingElse.