Need to poll Mqtt broker from a java REST API for already published messages to MQTT

Question

I am looking for a solution to poll messages from MQTT broker. I will describe the solution briefly here.

We have a Spring based Controller class which exposes REST APIs to handle certain vehicle related diagnostics data. Through one of these API-s Notify3P() I create a MQTT java client and publish messages based on some input data to the MQTT broker on a given topic. My requirement is to notify a third-party system every time the client publishes a message on MQTT.

The 3P system is going to pick up the message from MQTT once it receives the notification. It then needs to get the message from the MQTT broker through a getMessage() REST API (which we need to expose on the above controller class). The getMessage() API needs to poll MQTT for the messages that have already been published and hand it to the 3P system. The 3P system would then do some processing and send back a response to our system through another REST API postMessage() exposed on our controller class. The postMessage() should post the message on the response topic on MQTT. I need another REST API checkResponse() which then polls the response topic of MQTT and send back the response to the client.

What I have done so far: On application start up I have a start up bean which listens to MQTT request and response topics. Now I publish data to request topic using the REST API Notify3P(). I have attached a callback with the startup bean which gets the message. The problem comes when the 3P needs to call my controller to poll message from MQTT.

I am not clear how to hold back messages on MQTT and consume it on demand. Is there a mechanism to do it in MQTT? Also once the 3P system post messages on the response then again how do I poll the response topic to pick up the response from MQTT and send to clients of my Controller?

I hope the problem description makes sense. If there is any solution from anyone please post it. Any sample code would of great help.

Thanks in advance!!

Answer 1

You may have got the idea of MQTT a bit confused. One of the key points is that there is no polling.

You subscribe to your response topic and publish to the request topic. As soon as a response is available you will be sent it by the broker. You can't hold back messages.

It sounds like your controller also needs to talk MQTT. If it is subscribed to the response topic from the start then it will receive the messages and you can do with them what you will, no need for polling.

To achieve exactly what you want, where the third party notifies the controller to read messages from MQTT then the controller would need to be able to use MQTT anyway. At that point you might as well do it "properly". If you don't want to integrate MQTT into your controller, then you can't do what you describe and you will have to come up with another means of communicating between the two components.

Summary - get your controller to talk MQTT if you can.