Trouble getting paho-mqtt (Python 3.9) to connect to Ably

Question

First attempt to get paho-mqtt working with Ably. I'm sort of translating the following NodeJS example (which works for me) to Python. In the Python version, I don't seem to get any CONNACK back from the server.

NodeJS Example

// Using https://www.npmjs.com/package/mqtt

var [usr, pwd] = process.env['ABLY_API_KEY'].split(':');

let mqtt = require('mqtt');
let options = {
  keepAlive: 15,
  username: usr,
  password: pwd,
  port: 8883
};
let client = mqtt.connect('mqtts:mqtt.ably.io', options);

let channel = '[product:flight-data/flight-data]flight';
client.on('connect', () => {
  console.log('connected!')
  client.subscribe(channel);
});
client.on('message', (topic, message) => {
  console.log('Message received', message.toString());
});

My Python version (with logger added for debugging output)

mport os
import paho.mqtt.client as mqtt 


import logging

def create_logger():
    logger = logging.getLogger('mqtt debug')
    logger.setLevel(logging.DEBUG)
    ch = logging.StreamHandler()
    ch.setLevel(logging.DEBUG)
    formatter = logging.Formatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')
    ch.setFormatter(formatter)
    logger.addHandler(ch)
    return logger


host = 'mqtt.ably.io'
port =  8883
channel = '[product:flight-data/flight-data]flight'


def on_connect(client, userdata, flags, rc):
    print("Connected with result code "+str(rc))

    # Subscribing in on_connect() means that if we lose the connection and
    # reconnect then subscriptions will be renewed.
    client.subscribe(channel)

def on_message(client, userdata, msg):
    print(msg.topic+" "+str(msg.payload))

api_key = os.environ.get('ABLY_API_KEY', '')
username, passwd = api_key.split(':')
print(username)


mqttc = mqtt.Client(client_id="Test", clean_session=True, userdata=None)
mqttc.enable_logger(logger=create_logger())
mqttc.on_connect = on_connect
mqttc.on_message = on_message
mqttc.username_pw_set(username, passwd)
mqttc.connect(host, port=port, keepalive=15)


mqttc.loop_forever()

I get logger output like this

2022-06-20 20:03:39,956 - mqtt debug - DEBUG - Sending CONNECT (u1, p1, wr0, wq0, wf0, c1, k15) client_id=b'Test'
2022-06-20 20:03:56,040 - mqtt debug - DEBUG - Sending CONNECT (u1, p1, wr0, wq0, wf0, c1, k15) client_id=b'Test'

And this repeats until I ctrl-C out of the program.

However, testing very similar code with another broker seems to work. Here's the working test (cutting out the imports and create_logger function which are identical to the above)

(snip)

port = 1883
host = 'broker.hivemq.com'
channel = '$SYS/#'


def on_connect(client, userdata, flags, rc):
    print("Connected with result code "+str(rc))

    # Subscribing in on_connect() means that if we lose the connection and
    # reconnect then subscriptions will be renewed.
    client.subscribe(channel)

# The callback for when a PUBLISH message is received from the server.
def on_message(client, userdata, msg):
    print(msg.topic+" "+str(msg.payload))


mqttc = mqtt.Client(client_id="Test", clean_session=True, userdata=None)
mqttc.enable_logger(logger=create_logger())
mqttc.on_connect = on_connect
mqttc.on_message = on_message
mqttc.connect(host, port=port, keepalive=15)

mqttc.loop_forever()

This time I get expected output

2022-06-20 20:02:51,380 - mqtt debug - DEBUG - Sending CONNECT (u0, p0, wr0, wq0, wf0, c1, k15) client_id=b'Test'
2022-06-20 20:02:51,496 - mqtt debug - DEBUG - Received CONNACK (0, 0)
Connected with result code 0

So, in summary, I can connect to Ably in JS but not in Python. Yet my Python code is able to connect to other brokers. I'm sure I'm overlooking something I need to do in paho-mqtt, but have not been able to figure it out.

I've ruled _out_ (I think) the following things: _Authentication issues._ My NodeJS example uses the same API key, split in the same way between username and password and that code works. _Not being subscribed to this Ably channel._ I am subscribed and, as above, my NodeJS code works. _Wrong version of MQTT_ Ably does say I must use MQTT 3.1.1 and that is, indeed the default version for paho-mqtt. — Jonathan Shapiro, Jun 21 '22 at 00:31

Ben Gamble · Accepted Answer · 2022-06-21T16:29:33.893

1

Here's a link to a working Ably + Python MQTT gist on my github: https://gist.github.com/Ugbot/f7bd307e74a804a1529c8f63bc597aa2

Its using asyncio_mqtt rather than the paho library but its what I had on hand.

I'll try and get your version working next and host up a gist of that...

EDIT:

I got your version working, it took a couple of steps, firstly you need to be sure you've adding hte flights subscription to your account (this is the easy bit) Next you need to change it to port 1883 from 8883.

Here is my working gist: https://gist.github.com/Ugbot/fc2ebc17f7dfcf7973f6b441d90f3766

edited Jun 21 '22 at 16:29

answered Jun 21 '22 at 15:20

Ben Gamble

66
4

Thanks so much! I can't believe it was as simple as a port number. I'm still confused why my NodeJS version, which uses port 8883 works. But thanks for getting me unstuck! – Jonathan Shapiro Jun 22 '22 at 12:34
no worries, to use the 8883 port you'd need an SSL cert. I have no idea why node seems to roll with one automatically but in python, your version works if you provide one explicitly – Ben Gamble Jun 23 '22 at 07:37

Trouble getting paho-mqtt (Python 3.9) to connect to Ably

1 Answers1