How does the MQTT works? unable to get the data if the receiver file is started after the sender file

问题

I'm new in MQTT there is a simple range of numbers which I want to print I have created 2 files in which the 1st file whose send data to the 2nd file and the script is like that:

sender.py

import paho.mqtt.client as mqtt
client = mqtt.Client()
client.connect("192.168.1.169", 1883, 60)
for i in range(1,100):
    client.publish("TestTopic", i)
    print(i)
client.disconnect()

receiver.py:

import paho.mqtt.client as mqtt

def on_connect(client, userdata, flags, rc):
    print("Connected with result code "+str(rc))

    client.subscribe("house/bulbs/bulb1")


def on_message(client, userdata, msg):
    # print(msg.topic+" "+str(msg.payload))
    print("message received ", str(msg.payload.decode("utf-8")))
    print("message topic=", msg.topic)
    print("message qos=", msg.qos)
    print("message retain flag=", msg.retain)

client = mqtt.Client()
client.on_connect = on_connect
client.on_message = on_message

client.connect("192.168.1.169", 1883, 60)
client.loop_forever()

I'm able to print the data if the receiver file is active but I have a problem in printing it if I started the sender file and then I started the receiver file ,main question is does MQTT follows the queueing Mechanism or not if yes then ....if I'm running the sender file then its all data should be in queue and after that when I'm run the other file which is receiver then I should get printed.. but its not working in the same way please help me I went lots of documents but i'm able to find any relevant info.. recently I found clean_session if someone have knowledge about this please tell me ....have any questions related my code or anything please let me know thanks

回答1:

MQTT is a pub/sub protocol, not a message queuing system.

This means under normal circumstances if there is no subscriber running when a message is published then it will not be delivered.

It is possible to get the broker to queue messages for a specific subscriber, but this requires the subscriber to have been connected before the message is published and to have subscribed with a QOS of greater than 0. Then as long as it reconnects with the clean session flag set to false and the same client id after the publish then the broker will deliver the missed messages.

Retained messages are something different. If a message is published with the retained flag set to true then the broker will deliver this single message to every subscriber when they subscribe to the matching topic. There can only ever be 1 retained message for a given topic.

来源：https://stackoverflow.com/questions/58911628/how-does-the-mqtt-works-unable-to-get-the-data-if-the-receiver-file-is-started

标签

python

mqtt

publish-subscribe