Python Flask server with MQTT subscription

Question

I have a flask server written in Python running on IBM BlueMix. I want this server to listen to a MQTT channel and publish a message when it receives a message. I have the following code but on_connect and on_message are never called.

app = Flask(__name__)
def on_connect(client):
    client.publish(topic2, "CONNECTED")


def on_message(client, userdata, msg):
    client.publish(topic2, "MESSAGE")


@app.route('/')
def hello_world():
    return 'Hello World! I am running on port ' + str(port)

if __name__ == '__main__':
    client = mqtt.Client()
    client.username_pw_set(username, password)
    client.on_connect = on_connect
    client.on_message = on_message
    client.connect('broker.example.com')
    client.subscribe(topic)
    client.publish(topic2, "STARTING SERVER")

    app.run(host='0.0.0.0', port=port)

I've tried client.loop and client.loop_forever but it doesn't work.

EDIT: client.publish(topic2, "STARTING SERVER") is working, my credentials are removed.

score 5 · Accepted Answer · edited Jun 27 '22 at 08:49

You need to pass the details of the broker you want to connect to in the connect() function

e.g.

...
client.on_message = on_message
client.connect('broker.example.com')
client.subscribe(topic)
...

EDIT: you also need to start the network loop. Given this is flask app you will want to start the loop in the background so use the loop_start function.

...
client.connect('broker.example.com')
client.loop_start()
...

You should also move the subscribe and publish calls to the on_connect callback as they need to wait for the connection to have been established before being run.

EDIT2: adding full working code:

from flask import Flask
import paho.mqtt.client as mqtt

app = Flask(__name__)

topic = 'foo'
topic2 = 'bar'
port = 5000

def on_connect(client, userdata, flags, rc):
    client.subscribe(topic)
    client.publish(topic2, "STARTING SERVER")
    client.publish(topic2, "CONNECTED")


def on_message(client, userdata, msg):
    client.publish(topic2, "MESSAGE")


@app.route('/')
def hello_world():
    return 'Hello World! I am running on port ' + str(port)

if __name__ == '__main__':
    client = mqtt.Client()
    #client.username_pw_set(username, password)
    client.on_connect = on_connect
    client.on_message = on_message
    client.connect('localhost')
    client.loop_start()

    app.run(host='0.0.0.0', port=port)

I did this, just hiding my URL. client.publish(topic2, "STARTING SERVER") is working fine, on_connect and on_message are not called though. — Yourivdzee, Apr 04 '17 at 11:18
Have you moved the `subscribe` to `on_connect`? Where you have the `subscribe` at the moment it won't actually do anything because the `connect` has not completed — hardillb, Apr 04 '17 at 12:06
I did, however I think that on_connect is never called since it also doesn't publish client.publish(topic2, "CONNECTED") — Yourivdzee, Apr 04 '17 at 12:31

score 3 · Answer 2 · answered Nov 21 '18 at 16:42

You can use also Flask-MQTT:

https://flask-mqtt.readthedocs.io/en/latest/usage.html

There is even functional example how to implement MQTT-SocketIO bridge, which is just what I was looking for. This worked out-of-the-box:

https://flask-mqtt.readthedocs.io/en/latest/usage.html#interact-with-socketio

Python Flask server with MQTT subscription

2 Answers2