MQTT PubSub API

The MQTT PubSub client is used to connect to an MQTT broker, subscribe to topics, and publish data. It can be used in combination with an OPC UA server to listen to changes in nodes and publish data to an MQTT broker. It is also possible to use it to publish data to an MQTT broker without an OPC UA server. In this case, you’ll need to set data manually.

MQTT Client constructor

ua.newMqttClient()
ua.newMqttClient(config)
ua.newMqttClient(config, uaServer)
ua.newMqttClient(config, model)

Create an instance of MQTT client. MQTT client is used to connect to MQTT broker, subscribe to topics, publish data.

Parameters:

  • config – MQTT client configuration. If not provided, then default configuration will be used.

  • uaServer – OPC UA server instance. If provided, MQTT client can connect to nodes and listen value changes.

  • model – OPC UA model. If provided will use it for encoding/decoding datastructures.

Returns:

MQTT client instance.

If uaServer and model not provided then MQTT client then will be loaded only base OPCUA model. Thus will be possible to encode/decode only base OPCUA data types.

MQTT Connect

uaMqtt:connect(endpointUrl, transportProfileUri, <connectCallback>)

Connect to MQTT broker.

Parameters:

  • endpointUrl – OPC UA Endpoint URL

  • transportProfileUri –

    OPC UA Transport Profile URI. Defines encoding of the messages:

    • ua.TransportProfileUri.MqttBinary

    • ua.TransportProfileUri.MqttJson

  • connectCallback – Callback function

local function connectCallback(status)
  print("Connected to MQTT broker")
end

uaMqtt:connect("mqtt://localhost:1883", ua.TransportProfileUri.MqttBinary, connectCallback)

MQTT subscribe

uaMqtt:subscribe(topic, messageCallback)

Subscribe to MQTT topic.

Parameters:

  • topic – MQTT topic

  • messageCallback – Callback function

messageCallback(topic, message)

Callback function for MQTT message.

Parameters:

  • topic – MQTT topic

  • message – Decoded OPC UA message

MQTT publishing

uaMqtt:createDataset(fields, datasetId)

Create dataset which will describe message content to publish.

Parameters:

  • fields –

    Array of Fields. Each field is a table with next fields:

    • name: Field name. Defines field name in the dataset which will be used in JSON format and in metadata for binary messages.

    • nodeId: nodeId in OPUCA server instance which changes will be listened and published.

    The order of fields will be preserved in published MQTT payload messages.

  • classId – Class ID. Is used to distinguish datasets at subscriber side.

uaMqtt:setValue(datasetId, fieldName, dataValue)

Set field value in the dataset.

Parameters:

  • datasetId – Dataset ID

  • fieldName – Field name

  • dataValue – Field value

uaMqtt:publish(dataTopic, publisherId)

Publish dataset. Will forcibly publish dataset with all the changed fields.

Parameters:

  • dataTopic – Data topic

  • publisherId – Publisher ID

uaMqtt:startPublishing(dataTopic, publisherId, periodMs)

Start publishing dataset periodically with changed fields.

Parameters:

  • dataTopic – Data topic

  • publisherId – Publisher ID. string or numeric value.

  • periodMs – Period in milliseconds

uaMqtt:stopPublishing()

Stop publishing dataset.