MQTT订阅者与MQTT发布者订阅同一主题失败

Question

I'm trying to make a simple "transit schedule changes" program using python MQTT where the publisher can input flight number (will be used as topic) and new flight time, while the transit location will be picked random from given list.

The subscriber will have to input flight number too which will be used as topic. But in my codes, it looks like the subscriber failed to get the message published to the same topic because it keeps on printing Connected Successfully (I'm using client.loop_forever() ). Can someone please help me to figure out what's wrong with my code?

This is my first time asking question, so please ask me if something isn't clear from my explanation. Thank you so much:)

Publisher:

import paho.mqtt.client as mqtt
import time
from datetime import datetime, date
import random

def on_connect(client, userdata, flags, rc):
    if (rc==0):
        global connected
        connected = True
        #print("Successfully Connected.")
        client.on_publish = on_publish
    else:
        print("Failed to connect.")
        
def on_publish(client, userdata, mid):
    print("Published successfully. MID: "+str(mid))
    

listTransit = ["Singapura", "Qatar", "Korea Selatan", "Turki", "Republik Tiongkok",
                "Amerika Serikat", "Jepang", "Uni Emirat Arab", "Oman", "Islandia"]
                
broker_address="broker.emqx.io"

client = mqtt.Client("Publisher")
client.on_connect = on_connect
client.connect(broker_address, port=1883)

client.loop_start()

topic = input("Masukkan nomor penerbangan: ")
negaraTujuan = input("negara tujuan: ")
print("Masukkan waktu penerbangan baru (Format: [jam::menit::detik])")
str_time = input()

Date = date.today()
Time = datetime.strptime(str_time, '%H::%M::%S').time()
Location = random.randrange(0,len(listTransit))

if (listTransit[Location] != negaraTujuan):
    message = Date.strftime("%Y/%m/%d")+"\nTujuan: "+negaraTujuan+"\nLokasi Transit  : "+listTransit[Location]+"\nJam terbang    : "+Time.strftime("%H:%M:%S")
    client.publish(topic, message)
    print("Topic: ",topic)
    print(message)
else:
    while listTransit[Location] == negaraTujuan:
        Location = random.randrange(0,len(listTransit))
    message = Date.strftime("%Y/%m/%d")+"\nTujuan: "+negaraTujuan+"\nLokasi Transit  : "+listTransit[Location]+"\nJam terbang    : "+Time.strftime("%H:%M:%S")
    client.publish(topic, message)
    print(message)
    
client.loop_stop()

Subscriber:

import paho.mqtt.client as mqtt
import time
from datetime import datetime, datetime
import re

def on_connect(client, userdata, flags, rc):
    if (rc == 0):
        print("Connected successfully.")
        #global topic
        #topic = input("Masukkan nomor penerbangan anda: ")
        #client.subscribe(topic)
    else:
        print("Connection failed.")
        
def on_message(client, userdata, msg):
    print("on_message callback function activated.")
    sched = str(msg.payload.decode("utf-8"))
    print(sched)
    
def on_subscribe(client, userdata, mid, granted_qos):
    print("Subscribed to "+topic+" successfully")
    
    
broker_address="broker.emqx.io"
topic = input("Masukkan nomor penerbangan anda: ")
negaraTujuan = input("negara tujuan: ")

client = mqtt.Client("Subscriber")
client.subscribe(topic)
client.on_connect = on_connect
client.on_message = on_message
client.on_subscribe = on_subscribe
client.connect(broker_address, port=1883)

client.loop_forever()

what I got from running both codes are

Masukkan nomor penerbangan: YT05TA
negara tujuan: Australia
Masukkan waktu penerbangan baru (Format: [jam::menit::detik])
12::50::00
Topic:  YT05TA
2023/01/03
Tujuan: Australia
Lokasi Transit  : Amerika Serikat
Jam terbang    : 12:50:00
Published successfully. MID: 1

from Publisher

and

Masukkan nomor penerbangan anda: YT05TA
negara tujuan: Australia
Connected successfully.
Connected successfully.
Connected successfully.
Connected successfully.

from subscriber. It doesn't even print the print("on_message callback function activated.")

Answer 1

You are using a public broker, so that means it is likely to have LOTS of over clients.

Every client MUST have a UNIQUE Client ID, so using Publisher and Subscriber is very likely to clash with other clients.

The MQTT specification says the broker must disconnect the currently connected client when a new client connects with the same Client ID. As most client libraries will try and reconnect when they are disconnects this leads to a battle between the two clients to stay connected.

Change them both to random values