MQTT’s communication model is a pub/sub model. In order to realize Request/Response on the pub/sub model, MQTT v5.0 uses the following mechanism.
Client
async_mqtt doesn’t perform any special treatment for Request/Response. Simply send a CONNECT packet with the RequestResponseInformation property set to 1, and then receive the CONNACK packet. If the CONNACK packet includes ResponseInformation, you can obtain ResponseInformation from it and subscribe to it.
Broker
The broker part of async_mqtt supports Request/Response. When the broker receives a CONNECT packet with RequestResponseInformation set to 1, it automatically generates ResponseInformation and sends back a CONNACK packet with it. ResponseInformation is the TopicName for the response, so the client can use it as the ResponseTopic property when sending a PUBLISH packet.
The async_mqtt broker has authentication/authorization support. The generated topic can be subscribed to only by the client that sent RequestResponseInformation. The generated topic can be published to by all clients.
Correlation Data
The client has only one ResponseTopic and uses it for all requests. The request receiver client could be different, so the responses could be mixed. To distinguish the correct response, CorrelationData can be used. See the following diagram.
client1 got the ResponseInformation R1 and uses it as ResponseTopic. Both client2 and client3 receive the same ResponseTopic and use it for publish back the response. If control PUBLISH packet contains not only ResponseTopic but also CorrelationData, then the receiver just set the CorrelationData to the response PUBLISH packet.
client1 can choose any string as CorrelationData. Typically, choose unpredictable string to avoid malformed response. client1 needs to manage a CorrelationData-Request map to check the corresponding request.