Some MQTT devices can both transmit (publish) and receive (subscribe) data, providing a level of control over the device via MQTT messages. The format of the messages sent to the device is often very different from the format of messages that the device publishes. In some cases, the device may expect a different message format for different topics to which it subscribes.

Most MQTT topics published by a device are used only for transmission from the device, and not for receipt of commands or data from other sources. If the device subscribes to some topics, they are usually different from the topics to which it publishes. An MQTT broker communicating with that device must know the names of the topics to which the device publishes, and the names of the topics to which the device subscribes. Since the device does not publish data on the subscribed topics, an MQTT broker cannot identify those topics by their traffic. Instead, the MQTT broker must know how to form the topics to which the device subscribes based on some a-priori knowledge. The DataHub advanced MQTT parser provides a method for scripting this knowledge.

The DataHub advanced formatting mechanism supports multiple publish formats for a single receive format. The receive format implicitly identifies the type of device by matching a message to a JSON schema. Associating multiple publish formats with that device type accommodates the possibility that the device requires different publish formats for different topics.

Messages are published by the DataHub MQTT Broker or Client based on changes to data point values. The advanced message format must specify which data points will be mapped to which MQTT topics and publish formats to automatically publish the correct message to the correct topic when the relevant value changes. This is done by calling the app.RegisterPoint() function within the app.TopicInitialization() function defined by the system integrator. One of the arguments to app.RegisterPoint() is the label of a publish format. Another argument is the data point name, which must be constructed from the incoming topic, or from the payload of the incoming message. A third argument is the topic on which to publish a message. Once app.RegisterPoint() has been called, the DataHub engine will build a message and publish it on the specified topic whenever the point value changes.

Publish formats are specified using an ASP document specification. This is a simple format that combines literal text with scripting to create a primarily static document with some dynamic content. This can be used to create any text document, though in MQTT it is primarily used to create JSON messages.

If you select a publish format as the default by checking the associated check box, that format will be used whenever a value change occurs on a point that is associated with this topic, and for which no explicit publish format is specified.

Specifying ASP Documents

An ASP document is a primarily static document with script snippets embedded within it using a special syntax:

When an ASP document is evaluated, all code within the special delimiters is evaluated to construct the output document. This document is then published to the MQTT topic associated with the data point whose value has changed.

When an ASP document is being evaluated, the following variables and functions are available, in addition to many standard C# classes and methods:

Testing a Publish Format

As you type into the ASP document editor, your documents will be periodically evaluated using synthetic data in place of actual names and values. The result of that evaluation will be displayed in the Example panel. If the result is a valid JSON document, the Valid JSON check-box will become checked.