COS-MQTTClient

MQTT Client implemented in Caché Object Script

Fully functional sample implementation to demonstrate all kind of MQTT Client activities (publish, subscribe). For more information regarding the MQTT protocol visit the http://mqtt.org website, or examine the OASIS standard specification: http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html

Questions, bug reports, recommendations are highly welcome to Attila.Toth@InterSystems.com

Installation

• Download and extract the project to a directory (<install-dir>).
• Import and compile all XML files onto one of your Caché / Ensemble namespaces, with the following command: Do $System.OBJ.LoadDir("<install-dir>","c",.err,1,.list)

Usage

• For detailed information- including usage examples- look into the class documentation of Net.MQTT.Client

There are a few known (and maybe some unknown) limitations:

• It's not prepared for SSL / TLS communication.
• TCP/IP based only, cannot be used with web-socket based MQTT Brokers.
• Communication of the Net.MQTT.Client and background Net.MQTT.Agent instances is limited to the tasks to be done / completed. This could be enriched with for example status information of the Agent, which would make it easier to monitor the background job from the Client interface and check its health status in certain scenarios.
• Incoming and outgoing messages are simply logged into the Net.MQTT.MessageStore class. No any checks for duplicate messages are done (e.g.: when an incoming message is repeated by the Broker, due to a lost acknowledge).
• For incoming QoS Level 2 messages no retry mechanism is implemented: if no PUBREL message would arrive for the PUBREC acknowledge, the message simply remains in the Net.MQTT.Aux.MessageStatus table (registered with a "waiting for PUBREL" status).

Version history

• 0.9: Initial version
• 0.92: Adding SSL support and OnMessage callback
• 0.95:
  • Adding trace levels to better control the traced content.
  • Net.MQTT.Client is now persistent. This is useful, when- for example- a REST interface is built on the top of the MQTT client.
  • Agent automatically tries to re-connect (once only), if the TCP connection to the broker seems to be broken.