How client connects to the MQTT Broker with the clean-session flags set as false

Question

Clean Session supports persistence of messages. By default the value of Clean Session is true .

As we can set the Qos and retain flag value with the request payload as below:

docker run -it --rm --name mqtt-publisher --network \\ fiware_default efrecon/mqtt-client pub -h mosquitto -m "c|1234" \\ -t "/4jggokgpepnvsb2uv4s40d59ov/motion001/attrs" -q 2 -r -d

can any one tell me how a client can set clean-session parameter as false? can we set this flag with request payload or there is other method for this ??

Answer 1

The clean session flag is set when the MQTT client connects.

This is done in the CONNECT packet as part of the connect flags .

If you are using mosquitto_sub then it defaults to using a clean session for subscriptions. The -c or --disable-clean-session flag can be set to enable a persistent session.

The MQTT client id needs to be set when using this flag so the broker can track the subscriptions by client id. From https://mosquitto.org/man/mosquitto_sub-1.html :

-c, --disable-clean-session

Disable the 'clean session' flag. This means that all of the subscriptions for the client will be maintained after it disconnects, along with subsequent QoS 1 and QoS 2 messages that arrive. When the client reconnects, it will receive all of the queued messages.

If using this option, the client id must be set manually with --id