> For the complete documentation index, see [llms.txt](https://grid-exchange-fabric.gitbook.io/gxf/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://grid-exchange-fabric.gitbook.io/gxf/protocols/mqtt.md).

# MQTT

## MQTT Documentation

## MQ Telemetry Transport Protocol

MQTT is a lightweight publish/subscribe messaging protocol designed for constrained devices and networks.

## Dependencies

The MQTT Protocol Adapter uses the HiveMQ MQTT client `com.hivemq:hivemq-mqtt-client` which supports MQTT 3. The MQTT Protocol Simulator also uses HiveMQ, as well as the Moquette MQTT broker `io.moquette:moquette-broker`. The reason for using Moquette is that there is no Maven dependency for the HiveMQ broker component. The integration test for the MQTT client uses a test container for Eclipse Mosquitto based on the `eclipse-mosquitto` Docker image as broker.

## Simulator

The simulator runs an MQTT broker and a client which repeatedly publishes messages as specified in its `mqtt_simulator_spec.json` file.

Example:

```
{
  "brokerHost": "0.0.0.0",
  "brokerPort": 8883,
  "startupPauseMillis": 5000,
  "messages": [
    {

      "topic" : "TST-01/measurement",
      "payload": "TST-01; 220.1; 220.2; 220.3; 5.1; 5.2; 5.3; 7.1; 7.2; 7.3;",
      "pauseMillis": 30000
    },
    {

      "topic" : "TST-01/congestion",
      "payload": "TST-01; 5.1; 5.2; 5.3; 7.1; 7.2; 7.3;",
      "pauseMillis": 30000
    }
  ]
}
```

The spec file above will pause for 5 seconds and then start publishing a measurement and congestion message with intervals of 30 seconds. The message payload is assumed to be a String and not validated or parsed in any way.

The protocol adapter will establish a connection with the simulator and subscribe to the default topics `+/measurement,+/congestion`. This means `measurement` and `congestion` messages from any device (`+`).

A configurable default QoS value is used (See `com.hivemq.client.mqtt.datatypes.MqttQos` for the values).

In earlier versions the MQTT broker was treated as a device saved in the GXF core and protocol adapter databases. To simplify things the MQTT protocol adapter now allows to configure connection settings for the MQTT broker, no longer considering this as a platform device.


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