File-based MQTT emulation

This communication package emulates a function of an MQTT broker by storing the messages in form of files in directories of a file system.

See Using communication packages for instructions on how to select the communication package, and configure it.

The file-based MQTT emulation is in the MQTT client role only.

The type name of the message channel object is: OpcLabs.BaseLib.Communication.Mqtt.FileMqttMessageChannel,OpcLabs.BaseLib .

The communicate package stores messages in files. The files are are organized in directories under the specified "root". The names of directories under the "root" and their structure correspond to the names and structure of MQTT topics.

The file naming scheme is such that the message order corresponds to the order of the file names. The file name includes a "tick" when the message channel started, and a sequential message number. This approach preserves the correct message order even if the application is restarted. The naming scheme is suitable for not more than one application publishing to any topic. If more applications publish to the same topic, the file name order will no longer correspond to message order.

Currently, only message publishing is supported. Subscribing to topics is not yet implemented.

Opening the channel

URL string

When the UseInputUrlString is 'false' (the default), the URL string, if provided by the host software, is ignored.

When the UseInputUrlString is 'true', the URL string must have the "file:" scheme, and is interpreted as the physical root directory.

Interface name

The interface name, if provided by the host software, is ignored.

Configuration

The table below lists the available properties, their types, and descriptions.

Property	Type	Description
ClearRootDirectory	System.Boolean	Determines whether the root directory will be cleared (recursively) when the message channel starts. Default is 'false'. BE CAREFUL! Do not accidentally clear an unintended directory with this setting.
CreatePhysicalRootDirectory	System.Boolean	Determines whether the physical root directory will be created automatically (if it does not exist) when the message channel starts. Default is 'true'.
PhysicalFileProviderRoot	System.String	The directory path in the physical file system where the root topic will reside. Default is "/MqttRoot". The value form this property is only used when the UseInputUrlString is 'false' (the default).
UseInputUrlString	System.Boolean	Determines whether the URL string provided by the host, which must have the "file:" scheme, will be interpreted as the physical root directory. Default is 'false'.

Examples

OPC UA Demo Publisher

You can instruct the UADemoPublisher utility to produce JSON messages for MQTT and store them into a directory/file structure under "C:\MqttRoot" using the following command:

publish --MqttJsonTcp --ConnectionProperty {http://opclabs.com/OpcUA/PubSub}MessageChannel!=[String]OpcLabs.BaseLib.Communication.Mqtt.FileMqttMessageChannel,OpcLabs.BaseLib

QuickOPC software automatically chooses the file-based MQTT emulation if the MQTT transport is specified, and the URL scheme is "file:" (in this case, the UseInputUrlString property is also automatically set to 'true'). It is therefore possible to achieve the same result with the following command:

publish --MqttJsonTcp --ConnectionResourceUri file:///MqttRoot

It is also possible to publish simultaneously to the real MQTT broker, and store a copy of the generated messages into the file system. For example:

publish --MqttJsonTcp --ConnectionProperty {http://opclabs.com/OpcUA/PubSub}MqttFileCopySentRoot=[String]C:\MqttSent

When the {http://opclabs.com/OpcUA/PubSub}MqttFileCopySentRoot property is set in the MQTT transport, the transport automatically creates a "tee" in the send pipeline, and connects a file-based MQTT emulation on the specified root directory to it.

OpcCmd Utility

(TBD)

Imperative Programming

(TBD)