Multi-device MQTT network in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

MQTT Client Connection Output

What will be the output when this Raspberry Pi MQTT client code runs and successfully connects to the broker?

Raspberry Pi

import paho.mqtt.client as mqtt

def on_connect(client, userdata, flags, rc):
    if rc == 0:
        print("Connected successfully")
    else:
        print(f"Connection failed with code {rc}")

client = mqtt.Client("client1")
client.on_connect = on_connect
client.connect("broker.hivemq.com", 1883, 60)
client.loop_start()

A Connected successfully

B Connection failed with code 1

C Connection failed with code 5

D No output, program hangs

💡 Hint

Look at the on_connect callback and the return code rc.

The on_connect callback prints 'Connected successfully' only if rc is 0, which means the connection succeeded.

❓ Predict Output

intermediate

MQTT Message Reception

What will this Raspberry Pi MQTT subscriber print when it receives a message with payload 'Hello' on topic 'test/topic'?

Raspberry Pi

import paho.mqtt.client as mqtt

def on_message(client, userdata, msg):
    print(f"Received message: {msg.payload.decode()}")

client = mqtt.Client("client2")
client.on_message = on_message
client.connect("broker.hivemq.com", 1883, 60)
client.subscribe("test/topic")
client.loop_start()

# Simulate receiving a message with payload 'Hello' on 'test/topic'

A Received message: b'Hello'

B Received message: Hello

C Received message: None

D No output printed

💡 Hint

Check how the payload is decoded before printing.

The payload is bytes, decoded to string with decode(), so output is 'Received message: Hello'.

🔧 Debug

advanced

MQTT Publish Failure Debugging

This Raspberry Pi MQTT publisher code does not send messages as expected. What error will it raise?

Raspberry Pi

import paho.mqtt.client as mqtt

client = mqtt.Client()
client.connect("broker.hivemq.com", 1883, 60)
client.publish("test/topic", "Hello")
client.disconnect()

# Missing client.loop_start() or client.loop_forever()

A No error, message sent successfully

B RuntimeError: start_loop() called twice

C AttributeError: 'Client' object has no attribute 'publish'

D Message not sent, but no error raised

💡 Hint

Consider how MQTT client processes network events.

Without calling loop_start() or loop_forever(), the client does not process network events, so the message is not sent but no error occurs.

📝 Syntax

advanced

Identify Syntax Error in MQTT Callback

Which option contains a syntax error in defining the MQTT on_connect callback function?

A

def on_connect(client, userdata, flags, rc):
    print("Connected")
    return None

B

def on_connect(client, userdata, flags, rc):
    print("Connected")

C

def on_connect(client, userdata, flags, rc)
    print("Connected")

D

)"detcennoC"(tnirp    
:)cr ,sgalf ,atadresu ,tneilc(tcennoc_no fed

💡 Hint

Check function definition syntax carefully.

Option C is missing the colon ':' at the end of the function definition line, causing a syntax error.

🚀 Application

expert

Multi-device MQTT Network Topic Filtering

In a multi-device MQTT network, which topic subscription will allow a Raspberry Pi device to receive messages only from devices in the 'sensors' group located in 'room1'?

A sensors/room1/#

B room1/sensors/+

C sensors/+/room1

D #/sensors/room1

💡 Hint

Remember that '#' matches any number of levels and '+' matches exactly one level.

The subscription 'sensors/room1/#' matches all topics starting with 'sensors/room1/', so it receives messages from all devices in 'sensors' group in 'room1'.