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'.

Practice

1.

What is the main role of the MQTT broker in a multi-device network?

easy

A. It routes messages between devices based on topics.

B. It stores all device data permanently.

C. It assigns IP addresses to devices.

D. It encrypts messages automatically.

Solution

1. Step 1: Understand MQTT broker function

   The MQTT broker acts as a central hub that receives messages from devices and forwards them to others subscribed to the same topic.

2. Step 2: Eliminate incorrect roles

   The broker does not store data permanently, assign IPs, or encrypt messages by default; its main job is message routing.

3. Final Answer:

   It routes messages between devices based on topics. -> Option A

4. Quick Check:

   MQTT broker = message router [OK]

Hint: Broker routes messages by topic, not storing or assigning IPs [OK]

Common Mistakes:

• Thinking broker stores all data permanently
• Confusing broker with network router
• Assuming broker assigns IP addresses

2.

Which of the following is the correct way to connect a Raspberry Pi MQTT client using the paho-mqtt Python library?

client = mqtt.Client(____)

easy

A. subscribe_topic

B. broker_address

C. mqtt.connect()

D. "unique_client_id"

Solution

1. Step 1: Identify client ID parameter

   The mqtt.Client() constructor takes a unique client ID string to identify the device to the broker.

2. Step 2: Understand other options

   Broker address is used in connect()</>, not in Client()</>. Methods like connect() or topics are not valid constructor arguments.

3. Final Answer:

   "unique_client_id" -> Option D

4. Quick Check:

   Client ID goes in Client() constructor [OK]

Hint: Client ID string goes inside Client() constructor [OK]

Common Mistakes:

• Passing broker address to Client() instead of connect()
• Using method calls as constructor arguments
• Confusing topic names with client ID

3.

Given this Python code snippet using paho-mqtt on a Raspberry Pi, what will be printed when a message with payload 'ON' is received on topic 'home/livingroom/light'?

def on_message(client, userdata, msg):
    if msg.topic == 'home/livingroom/light':
        print(f"Light status: {msg.payload.decode()}")

client = mqtt.Client("pi1")
client.on_message = on_message
client.connect("broker.local")
client.subscribe("home/livingroom/light")
client.loop_start()

medium

A. Light status: b'ON'

B. Light status: ON

C. Light status: None

D. No output

Solution

1. Step 1: Understand message payload decoding

   The payload is a byte string, so msg.payload.decode() converts it to a normal string, here 'ON'.

2. Step 2: Check print statement output

   The print uses an f-string to show "Light status: ON" exactly when the topic matches.

3. Final Answer:

   Light status: ON -> Option B

4. Quick Check:

   Decoded payload prints as string [OK]

Hint: Decode payload bytes to string before printing [OK]

Common Mistakes:

• Printing raw bytes without decoding
• Not subscribing to the correct topic
• Missing the on_message callback assignment

4.

Identify the error in this MQTT client setup code on a Raspberry Pi:

client = mqtt.Client()
client.connect("broker.local")
client.subscribe("home/kitchen/temperature")
client.loop_forever()

# Missing on_message callback assignment

medium

A. The on_message callback is not assigned, so messages won't be processed.

B. The client ID is missing in mqtt.Client() constructor.

C. The subscribe topic is invalid syntax.

D. loop_forever() should be called before connect().

Solution

1. Step 1: Check callback assignment

   Without assigning client.on_message, received messages have no handler and won't be processed.

2. Step 2: Verify other parts

   Client ID is optional but recommended; subscribe syntax is correct; loop_forever() must be after connect().

3. Final Answer:

   The on_message callback is not assigned, so messages won't be processed. -> Option A

4. Quick Check:

   Missing on_message means no message handling [OK]

Hint: Always assign on_message callback to process messages [OK]

Common Mistakes:

• Ignoring on_message callback assignment
• Thinking client ID is mandatory
• Calling loop_forever() before connect()

5.

You want to control multiple Raspberry Pi devices in different rooms using MQTT. Each device should only receive messages for its room. Which is the best way to organize MQTT topics and client IDs?

• Device 1 in living room
• Device 2 in kitchen

Choose the best topic and client ID setup.

hard

A. Use topics livingroom and kitchen but same client ID pi for both devices.

B. Use the same topic home/all for both and client IDs pi1 and pi2.

C. Use topics like home/livingroom/# and home/kitchen/# with client IDs pi_livingroom and pi_kitchen.

D. Use random topics and client IDs for each device without pattern.

Solution

1. Step 1: Organize topics by room

   Using hierarchical topics like home/livingroom/# and home/kitchen/# lets devices subscribe only to their room's messages.

2. Step 2: Assign unique client IDs

   Unique client IDs like pi_livingroom and pi_kitchen avoid connection conflicts and identify devices clearly.

3. Final Answer:

   Use topics like home/livingroom/# and home/kitchen/# with client IDs pi_livingroom and pi_kitchen. -> Option C

4. Quick Check:

   Unique IDs + room topics = clean multi-device MQTT [OK]

Hint: Use unique client IDs and room-based topics [OK]

Common Mistakes:

• Using same client ID for multiple devices
• Subscribing all devices to a single topic
• Not organizing topics by device location