Mastering MicroPython MQTT `check_msg`

MicroPython is a lean and efficient implementation of the Python 3 programming language that includes a small subset of the Python standard library and is optimised to run on microcontrollers and in constrained environments. MQTT (Message Queuing Telemetry Transport) is a lightweight messaging protocol designed for M2M (Machine-to-Machine) and IoT (Internet of Things) applications. The check_msg method in MicroPython's MQTT client is a crucial function that allows the client to handle incoming messages from the MQTT broker. This blog post will delve into the fundamental concepts of check_msg, its usage methods, common practices, and best practices.

Table of Contents#

  1. Fundamental Concepts of check_msg
  2. Usage Methods
  3. Common Practices
  4. Best Practices
  5. Conclusion
  6. References

Fundamental Concepts of check_msg#

How MQTT Works in MicroPython#

In MicroPython, the MQTT client follows the basic MQTT protocol principles. It can connect to an MQTT broker, subscribe to topics, publish messages to topics, and receive messages from the topics it has subscribed to.

Role of check_msg#

The check_msg method is used to check for new messages from the MQTT broker. It is a non - blocking call, which means it will not pause the execution of the program while waiting for a message. When called, it checks if there are any incoming messages from the broker. If there are, it processes them according to the callback function that has been set for the client.

Callback Function#

A callback function is a function that is called when a message is received from the broker. It is set using the set_callback method of the MQTT client. The callback function takes two arguments: the topic and the message payload.

Usage Methods#

Prerequisites#

Before using check_msg, you need to install the MicroPython MQTT client library on your microcontroller. You also need to have an MQTT broker running, such as Mosquitto.

Example Code#

import network
from umqtt.simple import MQTTClient
 
# Connect to the network
sta_if = network.WLAN(network.STA_IF)
if not sta_if.isconnected():
    print('Connecting to network...')
    sta_if.active(True)
    sta_if.connect('your_SSID', 'your_PASSWORD')
    while not sta_if.isconnected():
        pass
print('Network config:', sta_if.ifconfig())
 
# Callback function
def sub_cb(topic, msg):
    print((topic, msg))
 
# Connect to the MQTT broker
client = MQTTClient('client_id', 'broker_address')
client.set_callback(sub_cb)
client.connect()
client.subscribe(b'test_topic')
 
# Main loop
while True:
    client.check_msg()
    # You can add other code here to perform other tasks

Explanation of the Code#

  1. Network Connection: The code first connects to the Wi - Fi network.
  2. Callback Function: The sub_cb function is defined to handle incoming messages. It simply prints the topic and the message payload.
  3. MQTT Client Setup: The MQTT client is created, connected to the broker, and subscribed to the test_topic.
  4. Main Loop: In the main loop, the check_msg method is called continuously to check for new messages.

Common Practices#

Using check_msg in a Loop#

As shown in the example above, check_msg is usually called in a loop. This ensures that the client can handle incoming messages in a timely manner.

Error Handling#

It is important to add error handling when using check_msg. For example, if the connection to the broker is lost, the check_msg call may raise an exception. You can catch these exceptions and try to reconnect to the broker.

import network
from umqtt.simple import MQTTClient
import time
 
# Connect to the network
sta_if = network.WLAN(network.STA_IF)
if not sta_if.isconnected():
    print('Connecting to network...')
    sta_if.active(True)
    sta_if.connect('your_SSID', 'your_PASSWORD')
    while not sta_if.isconnected():
        pass
print('Network config:', sta_if.ifconfig())
 
# Callback function
def sub_cb(topic, msg):
    print((topic, msg))
 
# Connect to the MQTT broker
client = MQTTClient('client_id', 'broker_address')
client.set_callback(sub_cb)
 
try:
    client.connect()
    client.subscribe(b'test_topic')
except OSError as e:
    print('Connection error:', e)
 
while True:
    try:
        client.check_msg()
    except OSError as e:
        print('Error checking message:', e)
        try:
            client.connect()
            client.subscribe(b'test_topic')
        except OSError as e:
            print('Reconnection error:', e)
            time.sleep(5)
    # You can add other code here to perform other tasks

Best Practices#

Timing Considerations#

The frequency of calling check_msg depends on the expected message rate. If you expect a high message rate, you may need to call check_msg more frequently. However, calling it too frequently can consume a lot of CPU resources.

Resource Management#

When using check_msg in a long - running program, make sure to manage resources properly. For example, if you are using a microcontroller with limited memory, you need to ensure that the callback function does not consume too much memory.

Conclusion#

The check_msg method in MicroPython's MQTT client is a powerful tool for handling incoming messages from an MQTT broker. By understanding its fundamental concepts, usage methods, common practices, and best practices, you can effectively use it in your IoT projects. Remember to handle errors properly and manage resources efficiently to ensure the stability and performance of your application.

References#

  1. MicroPython official documentation: https://docs.micropython.org/
  2. MQTT official website: https://mqtt.org/
  3. Mosquitto documentation: https://mosquitto.org/documentation/
2025-10