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I am working to connect my server to the clients via an MQTT broker. The MQTT client gets connected. But after publishing a message, the subscribe code receives a connection acknowledgment. The on_message() function never gets called.

I am stuck here.

I have pasted the subscribe client code and the output.

import paho.mqtt.client as paho
import time

client = paho.Client("local_test")
topic = "topic_1"

def on_log(client, userdata, level, buff):  # mqtt logs function
    print(buff)

def on_connect(client, userdata, flags, rc):  # connect to mqtt broker function
    if rc == 0:
        client.connected_flag = True  # set flags
        print("Connected Info")
    else:
        print("Bad connection returned code = " + str(rc))
        client.loop_stop()

def on_disconnect(client, userdata, rc):  # disconnect to mqtt broker function
    print("Client disconnected OK")

def on_publish(client, userdata, mid):  # publish to mqtt broker
    print("In on_pub callback mid=" + str(mid))

def on_subscribe(client, userdata, mid, granted_qos):  # subscribe to mqtt broker
    print("Subscribed", userdata)

def on_message(client, userdata, message):  # get message from mqtt broker 
    print("New message received: ", str(message.payload.decode("utf-8")), "Topic : %s ", message.topic, "Retained : %s", message.retain)

def connectToMqtt():  # connect to MQTT broker main function
    print("Connecting to MQTT broker")
    client.username_pw_set(username=user, password=passwd)
    client.on_log = on_log
    client.on_connect = on_connect
    client.on_publish = on_publish
    client.on_subscribe = on_subscribe
    client.connect(broker, port, keepalive=600)
    ret = client.subscribe(topic, qos=0)
    print("Subscribed return = " + str(ret))
    client.on_message = on_message

connectToMqtt()  # connect to mqtt broker
client.loop_forever()

And the output I get after publishing the message on the same topic is:

Connecting to MQTT broker
Sending CONNECT (u1, p1, wr0, wq0, wf0, c1, k600) client_id=b'local_test'
Sending SUBSCRIBE (d0) [(b'topic_1', 0)]
Subscribed return = (0, 1)
Received CONNACK (0, 0)
Connected Info
Received SUBACK

Subscribed None
Sending CONNECT (u1, p1, wr0, wq0, wf0, c1, k600) client_id=b'local_test'
Received CONNACK (0, 0)
Connected Info

EDIT 1:

Also, I am seeing that my broker has sent the message from the publisher to the client, but the client isn't able to receive it.

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  • 1
    set the on_message call back before connecting
    – hardillb
    Jun 21 '19 at 16:55
  • 1
    Also edit the question to include the mqtt_start() function so we can see if it ever makes it to the client.loop_forever() call
    – hardillb
    Jun 21 '19 at 16:56
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    upvote for including the output of the print statements ... so many people ignore the printouts when posting here ... I'm whining here ... lol
    – jsotola
    Jun 21 '19 at 17:49
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    I would do the client.subscribe in the on_connect callback, ie. once it is connected. That also re-subscribes if you handle disconnects. Jun 21 '19 at 19:16
  • @hardillb my bad ... there is nothing in the mqtt_start() function. I forgot to remove it. Also, setting on_message callback before on_connect didn't make any difference.
    – ron123456
    Jun 24 '19 at 5:44
0

As others have mentioned/implied in the comments:
The actual sequence that is supposed to happen is:

Setup your callbacks correctly first (including the on_message).
Call connect.
Wait until you get back an ack for the connect.
Then, send out a subscribe request. (This will actually send a message on TCP to the broker. That cannot happen if the connect is not done.)
Note that after the subscribe is complete and you've got a positive ack, there maybe a small amount of time for which the subscription is not actually complete at the broker. This is especially true in large brokers that use a cluster of computers. Usually though, this time of uncertainity is small. A few hundred ms max.
After the subscribe is acked (and after a second, if you can delay it), if you send a message from your publisher, the broker should be able to forward it to your client you should be able to see it.

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  • Gosh. I didnt realize that the original question had been asked 2 years ago ! Hopefully, the answer will help someone in future. So, I'll leave my answer undeleted. Aug 30 at 22:15

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