Don’t get publish() to work in paho.mqtt.client.on_connect thread

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Summary

A developer reported that paho.mqtt.client.publish() calls inside the on_connect callback (and a subsequent infinite loop within that callback) were not reaching the broker, while a publish call in the main thread succeeded. The root cause was blocking the Paho client’s network loop. The on_connect callback executes within the thread handling network I/O (started by loop_forever()). By entering a while True: time.sleep(1) loop inside the callback, the thread was prevented from processing network traffic, effectively deadlocking the client and preventing the publish operations from being sent.

Root Cause

The core issue lies in the execution context of the on_connect callback. In paho.mqtt.client, the callback is invoked by the client’s internal network loop thread. This loop is responsible for reading/writing to the socket and dispatching callbacks.

  1. Thread Blocking: The code inside on_connect contains while True: time.sleep(1). This is an infinite blocking loop.
  2. Network Loop Starvation: Because the thread executing the network loop is stuck inside the callback, it cannot return to the loop_forever() or socket.select()/poll() cycle.
  3. Result: The client cannot send the PUBLISH packet generated by client.publish() because the sending logic requires the network loop to be active to flush the socket buffer. The message sits in the internal buffer indefinitely.

Why This Happens in Real Systems

This pattern often emerges when developers attempt to turn an event-driven callback into a long-running process or an application main loop.

  • Misunderstanding of Event Loops: Developers coming from synchronous blocking environments often treat callbacks as “entry points” for the whole application logic rather than event handlers.
  • Global State Management: Sometimes developers try to keep the client connection alive indefinitely inside the callback to ensure the client object stays in scope or to avoid race conditions with loop_stop.
  • “Keep-Alive” Confusion: Developers often write while True: time.sleep() to simulate a keep-alive or a heartbeat, not realizing Paho handles keep-alive packets automatically in the background if the loop is running.

Real-World Impact

  • Partial Connectivity: The application appears connected (TCP handshake completes), but data transfer halts immediately after connection.
  • QoS 1/2 Hangs: If using Quality of Service 1 or 2, messages might be stored in the client’s internal queue. Since the network loop is blocked, the client cannot send PUBACK or PUBREC packets, leading to broker disconnection due to timeout.
  • Watchdog Triggers: In production environments, this often triggers systemd restarts or “heartbeat missed” alerts because the application logic (which should be doing work) is trapped in the callback.

Example or Code (if necessary and relevant)

To demonstrate the correct architecture, the blocking loop must be moved to the main thread, leaving the callback short and non-blocking.

import paho.mqtt.client as mqtt
import time
import threading

# This event flag demonstrates how to trigger logic from a callback 
# without blocking the network thread.
publish_event = threading.Event()

def on_connect(client, userdata, flags, reason_code, prop):
    print(f"Connected with result code {reason_code}")
    # Signal the main thread that we are connected
    publish_event.set()

def main():
    mqttclient = mqtt.Client(mqtt.CallbackAPIVersion.VERSION2)
    mqttclient.on_connect = on_connect

    # Connect starts the background socket connection, but no loop yet
    mqttclient.connect("localhost")

    # Publish in main thread (works, but better to wait for connection)
    # mqttclient.publish("CO2Meter/CO2", 129)

    # Start the network loop in a background thread or keep it in main
    # We will drive the logic from the main thread here
    # To simulate the user's need to publish periodically:

    def worker():
        # Wait for the on_connect signal
        print("Worker waiting for connection...")
        publish_event.wait() 

        # Now it is safe to publish
        print("Worker sending messages...")
        while True:
            client.publish("CO2Meter/CO2", 5555)
            print("Published 5555")
            time.sleep(1)

    # Start the worker in a separate thread so we don't block loop_forever
    # (Alternatively, use loop_start() and a while loop in main thread)
    threading.Thread(target=worker, daemon=True).start()

    # This blocks the main thread, but allows the background worker to run
    # because loop_forever() processes network I/O, and the worker handles logic.
    mqttclient.loop_forever()

if __name__ == "__main__":
    main()

How Senior Engineers Fix It

Senior engineers separate I/O handling from Application Logic.

  1. Use loop_start(): This runs the network loop in a background thread. The main thread (or application logic thread) can then run a blocking while True loop safely without stopping the network I/O.
  2. Event-Driven Logic: If an action is required immediately upon connection, keep the on_connect callback lightweight. Set a flag or put a message into a queue.Queue. A separate worker thread consumes this queue.
  3. Asynchronous Libraries: For complex applications, seniors often switch to libraries designed for asynchronicity (like asyncio-mqtt), which allows using await without blocking the event loop.

Why Juniors Miss It

Juniors often treat the on_connect callback as the Main Entry Point of the application rather than an interrupt handler.

  • “It looks like C” mentality: They see on_connect and think “This is where I define what happens when I connect,” and logically place their main loop there.
  • Ignoring the “Loop”: They often don’t realize that client.publish() is often just a buffer operation; the actual sending happens only when the network loop runs. If they block the loop, they block the sending.
  • Concept of “Background”: The concept that loop_forever() is a “driver” that must keep running is counter-intuitive. They view loop_forever() as a “start” button rather than the engine itself.