The DataHub MQTT Client option lets you connect with virtually any MQTT broker, to publish or subscribe to any or all topics. If you are using Azure IoT Hub, Google IoT, or AWS IoT Core, please see those sections for configuring client connections. To connect to any other MQTT broker, here is what you need to do.

	To keep things simple, if you plan to run an MQTT broker on your local machine, ensure that the DataHub MQTT Broker is not also running. You can disable it by unchecking the Enable MQTT Broker box in the MQTT Broker configuration.

Subscribe (get topics)

Before pushing any data to the MQTT broker, it can be useful to know some or all of its topics. This procedure shows how to register a DataHub instance with an MQTT broker and bring its topics into the DataHub MQTT client and the DataHub data set.

	If the broker's payload format does not map directly to DataHub points, you may wish to change it. Please see Message Payload Formats for more details.

Subscribe (push data)

This procedure shows how to push data from DataPid to your MQTT broker.

You have now configured the DataHub MQTT Client to push a value to an MQTT Broker. For more information about the MQTT Client feature, please refer to Section 3.12, “MQTT Client”.

Subscribe (pull data)

With data from DataPid being pushed to your MQTT broker, you can now create an MQTT client connection to pull that topic and its data from the broker.

	If you have not yet completed the previous procedure, please do it now for best results here.

You have now configured the DataHub MQTT Client for a topic to pull values from an MQTT broker. For more information about the MQTT Client feature, please refer to Section 3.12, “MQTT Client”.

Message Payload Formats

Messages for or from an MQTT broker might not have a payload format that maps directly to DataHub points. Data points in the DataHub program are 4-element tuples of the form (name, value, quality, timestamp). Those are the only elements that can be extracted from the information being sent to or arriving in an MQTT payload for the DataHub instance. In the JSON format, these rules will be applied:

If the payload format is the same for each item, and if elements of the payload represent quantities that should be placed in a DataHub point, then it is possible to create a simple parsing template. For example, if you have a payload like this:

{ "PT": "MyPoint", "VAL": 33, "TS": 1548276263.866, "QTY": 192,  ... }

As long as the elements correspond to name, value, timestamp and quality then a possible template could be constructed like this:

{"PT": {point}, "VAL": {value}, "TS": {jsontimestamp},
 "QTY": {quality}}

Where the value of "PT" is the point name, the value of "VAL" is the point value, the value of "TS" is a Unix, ISO, or JSON timestamp, and the value of "QTY" is the point quality. Other placeholders are available to use in the template, as provided in the editor.

If the payload represents multiple points with a common JSON format such as this array of points:

[ { "PT": "MyPoint", "VAL": 33, "TS": 1548276.866, "QTY": 192, ... }, 
  { "PT": "MyPoint2", "VAL": 66, "TS": 1574462.566, "QTY": 192, ... }, 
  { ... }, ...]

Then, in the editor, the Message Start field would hold an open array bracket ( [ ), the Per-Point Separator field would be a comma ( , ), and the Message End field would hold a closed array bracket ( ] ). The Per-Point Format field would hold the common parser template string such as above with appropriate placeholders. This would associate the values coming from the MQTT broker with properties of DataHub points, populating each point with meaningful content that could then be used by connected DataHub clients.

If you have a more complex JSON payload, or if the format of the payload differs from item to item, then it may be necessary to use the Advanced Parser. This allows you to input a raw JSON payload string obtained from using the per-point format set to {value}, and use it to generate a matching schema that can be adjusted as needed.