Mqtt 订阅者，loop_forever() 不起作用

Question

I am having an issue with the code below.

The code works perfectly at the beginning. First it says Connected to MQTT Broker! and receives data from it. But after a long time (like 6 hours, or 10 hours etc.) it says again Connected to MQTT Broker! and after that id does not receive any other data.

I am trying to make this program work forever, but i don't know what i have done wrong. Any ideas?

#   python3.6

import random
import mysql.connector
from paho.mqtt import client as mqtt_client
import json

#   Code for MQTT Connection
broker = 'YOUR_BROKER'
port = 1883
topic = "YOUR_TOPIC"
# generate client ID with pub prefix randomly
client_id = f'python-mqtt-{random.randint(0, 100)}'
username = "THE_USERNAME"
password = "THE_PASSWORD"


#   Function to connect on mqtt
def connect_mqtt() -> mqtt_client:
    def on_connect(client, userdata, flags, rc):
        if rc == 0:
            print("Connected to MQTT Broker!")
        else:
            print("Failed to connect, return code %d\n", rc)

    client = mqtt_client.Client(client_id)
    client.username_pw_set(username, password)
    client.on_connect = on_connect
    client.connect(broker, port)
    return client

#   function to subscribe from mqtt
def subscribeFunc(client: mqtt_client):
    def on_messageFunc(client, userdata, msg):
        print(f"Received `{msg.payload.decode()}` from `{msg.topic}` topic")

    client.subscribe(topic)
    client.on_message = on_messageFunc


def run():
    client = connect_mqtt()
    subscribeFunc(client)
    client.loop_forever()


if __name__ == '__main__':
    run()

I tried to find the problem but it seems that nothing changed significantly. I am expecting this program to receive data without stopping.

Answer 1

Network connections may not be 100% reliable (and servers etc are restarted from time to time) so expecting the connection to remain up forever is unrealistic. The issue with your code is that it connects then subscribes; if the connection is dropped it does not resubscribe. As the connection is clean_session=true and subscription qos=0 (the defaults) the broker will forget about any active subscriptions when the connection drops (so the client will reconnect but not receive any more messages).

The Simple solution is to use the approach shown in the docs and subscribe in the on_connect callback (that way the subscription will be renewed after reconnection):

def on_connect(client, userdata, flags, rc):
    # Subscribing in on_connect() means that if we lose the connection and
    # reconnect then subscriptions will be renewed.
    client.subscribe("$SYS/#")

client = mqtt.Client()
client.on_connect = on_connect
client.connect("mqtt.eclipseprojects.io", 1883, 60)

You may also want to consider the advice in this answer (as per the comment from @mnikley) because that way the broker will queue up messages for you while the connection is down (otherwise these will be lost).