MQTT PubSub API

MQTT PubSub client is used to connect to MQTT broker, subscribe to topics, publish data. It can be used in combination with OPC UA server to listen changes in nodes and publish data to MQTT broker. Also it is possible to use it to publish data to MQTT broker without 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, tranportProfileUri, <connectCallback>)

Connect to MQTT broker.

Parameters:

  • endpointUrl – OPCUA Endpoint URL

  • tranportProfileUri –

    OPCUA Transport Profile URI. Defines encoding of the messages:

    • ua.TranportProfileUri.MqttBinary

    • ua.TranportProfileUri.MqttJson

  • connectCallback – Callback function

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

uaMqtt:connect("mqtt://localhost:1883", ua.TranportProfileUri.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 OPCUA 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 changes fiels. if not

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

mqttServer:stopPublishing()

Stop publishing dataset.