MQTT: Labs

In this lab, we're going to investigate paho-mqtt, a python library for MQTT.

Introduction

MQTT is a pub/sub framework for sending small messages between IoT devices. We've got an MQTT server set up which you can connect to for the rest of the program. If you like, you can set up a free MQTT server on your Heroku account: https://devcenter.heroku.com/articles/cloudmqtt.

Getting started

Make sure you've got paho-mqtt installed:

pip install paho-mqtt

Code

First, import the library and alias it to mqtt:

import paho.mqtt.client as mqtt

Next, we define two functions. These will be called by paho when the connection has been successfully made, and every time a message arrives:

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

    # Here you can subscribe to whatever topics you like
    # use '#' for a 'wildcard' - subscribe to any messages
    client.subscribe("glblcd/sam")
    

The on_message callback is called with the message object which contains payload - a string containing the message content. This could be plain text, xml, or json - whatever the sender has sent

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

Next, we instantiate the MQTT client object and set up the callbacks

client = mqtt.Client()
client.on_connect = on_connect
client.on_message = on_message

Once this is done, we can attempt to connect to the server. Your teachers will provide the hostname for this class.

client.connect("globalcode.org.uk", 1883, 60)

Finally, we prevent the program from exiting by calling loop_forever. This allows incoming messages to be handled. You can hit Ctrl-C to exit the program.

client.loop_forever()

Exercises

Work in pairs or groups, go back to your MQTT labs and refactor them to respond to messages over MQTT.