Fail in publish using paho mqtt client

Question

I am trying to send a file through the MQTT protocol using the Paho MQTT as client and the Mosquitto MQTT broker.

Client code to send the message:

import paho.mqtt.client as mqtt

# Define the broker details
broker_address = "127.0.0.1"
port = 1883
topic = "test"
keep_alive_time = 18 * 60 * 60
# Read the file contents in binary mode
file_path = "D:/FSP/OTA//ftpserverpython/public/u2a16_startup.mot"
with open(file_path, 'rb') as file:
    message = file.read()

# Create an MQTT client instance
client = mqtt.Client()

# Connect to the broker
client.connect(broker_address, port, keep_alive_time)

# Publish the message as binary data
client.publish(topic, message, retain=True)

# Disconnect from the broker
client.disconnect()

print("Binary file published successfully.")

Client code to receive the message:

import paho.mqtt.client as mqtt

# Define the broker details
broker_address = "127.0.0.1"
port = 1883
topic = "test"
keep_alive_time = 18 * 60 * 60
# Read the file contents in binary mode
output_file_path = "D:/FSP/OTA/mqttclient//file/b.mot"
def on_message(client, userdata, message):
    # Write the received message payload to a file
    with open(output_file_path, 'wb') as file:
        file.write(message.payload)
    print(f"File saved to {output_file_path}")

# Create an MQTT client instance
client = mqtt.Client()

# Connect to the broker
client.connect(broker_address, port, keep_alive_time)

# Publish the message as binary data
client.publish(topic, message, retain=True)

# Set the on_message callback
client.on_message = on_message

# Subscribe to the topic
client.subscribe(topic)

# Start the MQTT loop to process network traffic and dispatch callbacks
client.loop_forever()

First I run the code from receive side and the log from broker:

However, when running publish code the broker does not respond. The log is below:

Therefore, the file is not transmitted successfully.

I use mosquitto -v to run Mosquitto.

I expect the file to be transmitted successfully

Answer 1

You message is bigger than will fit in one network packet (MTU), so you must run the MQTT client loop in the publisher.

You have 2 choices.

1. use the pre-built function that will publish a single message

import paho.mqtt.publish as publish

publish.single(topic, message, hostname=broker, port=port)

2. Add the start the client loop

def publish_callback(client, userdata, mid):
  client.disconnect()
  print("Binary file published successfully.")

# Create an MQTT client instance
client = mqtt.Client()

# Connect to the broker
client.connect(broker_address, port, keep_alive_time)

client.on_publish = publish_callback

client.loop_start()

# Publish the message as binary data
client.publish(topic, message, retain=True)