MQTT Broker does not deliver the messages sent by publisher, on time - mqtt

Question

I wrote this MQTT publisher code:
import paho.mqtt.client as mqtt
import time
HOST = "localhost"
PORT = 1883
KEEP_ALIVE_INT = 100
TOPIC = "noti"
def sendMsg():
MSG = ["1111", "2222", "3333", "4444", "5555"]
i = 0
try:
while i<5:
client.publish(TOPIC, MSG[i], qos=0)
i+=1
time.sleep(1)
except Exception as e:
print("Caught Exception: " + e)
def onConnect(client, userdata, flags, rc):
if rc == 0:
print("Connected successfully")
sendMsg()
else:
print("Connection failed, result code: " + str(rc))
def onPublish(client, userdata, mid):
print ("Message is published")
client = mqtt.Client("pub")
client.on_connect = onConnect
client.on_publish = onPublish
client.connect(HOST, PORT, KEEP_ALIVE_INT)
client.loop_forever()
And, the following is the MQTT subscriber code:
import paho.mqtt.client as mqtt
import time
HOST = "localhost"
PORT = 1883
KEEP_ALIVE_INT = 100
TOPIC = "noti"
def onConnect(client, userdata, flags, rc):
if rc == 0:
print("=> Connected successfully")
client.subscribe(TOPIC, 0)
else:
print("=> Connection failed, result code: " + str(rc))
def onSubscribe(mosq, obj, mid, granted_qos):
print ("=> Subscribed to topic: " + TOPIC)
print ("Granted QOS: "+str(granted_qos))
def onMessage(client, userdata, msg):
print("=> Received message: " + msg.topic +" - " + msg.payload.decode("utf-8"))
client = mqtt.Client("sub")
client.on_message = onMessage
client.on_connect = onConnect
client.on_subscribe = onSubscribe
client.connect(HOST, PORT, KEEP_ALIVE_INT )
client.loop_forever()
I am using Mosquitto broker in my PC.
The publish is done in every 1 second, but I can see the print "Message is published" 5 times after all 5 messages are published. Also, the subscriber receives the messages together after 5 seconds, not in every 1 second.
Please help me understand the mistake, suggest modification.

Answer 1

This is because all callbacks and message handling happen on the client network loop thread and you are blocking that thread by not returning from the on_connect() callback.
So the calls to client.publish() are queued up until the on_connect() callback returns.
You need to find a way to trigger the sendMsg() function not on the client loop. (Probably on a separate thread)