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I have a scenario where mobile app calls rest API hosted by my application. With in this process, I need to send message to downstream system over MQTT and wait until I get the response for that message. And then I have reply back to mobile app.

The challenge here is, messaging over MQTT is asynchronous. So the message which I receive back will be in different thread (some listener class, listening on messageArrived()). How to get back to calling http thread?

Do we have synchronous communication supported by Paho library.? Something like I send a message, open some topic and wait on it till some message is received or timeout?

hardillb
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Smita Mutha
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  • This is the second time this type question has been asked in 2 days (https://stackoverflow.com/questions/45413507/handling-mqtt-communication-inside-http-request-in-node-red), it wouldn't happen to be for an assignment of some kind would it? – hardillb Aug 01 '17 at 08:16
  • Possible duplicate of [Handling MQTT communication inside HTTP request in Node-Red](https://stackoverflow.com/questions/45413507/handling-mqtt-communication-inside-http-request-in-node-red) – ΦXocę 웃 Пepeúpa ツ Aug 01 '17 at 16:55
  • This is not a duplicate or I would have marked it as one. One is very specific to Node-RED and this is about implementing something similar in most likely java. – hardillb Aug 01 '17 at 17:18

1 Answers1

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MQTT by it's very nature is asynchronous, as are all Pub/Sub implementations. There is no concept of a reply to a message at the protocol level, you have no way of knowing if you will EVER get a response (or you may get many) to a published message as you can't know if there is even a subscriber to the topic you publish on.

It is possible to build a system that will work this way, but you need to maintain a state machine of all in flight requests, implement a sensible timeout policy and work out what to do if you get more than one response.

You have not mentioned which of the different Paho libraries you are using, but I'm guessing Java from the method names, but without knowing what HTTP framework you are using and a host of other factors I'm not going to suggest a solution, especially as it will involve a lot of polling and synchronisation.

Is there any reason why the mobile application can't publish and subscribe to MQTT topics directly? This would remove the need for this.

hardillb
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