Multi-device MQTT network in Raspberry Pi - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the MQTT client library.

Raspberry Pi

import [1]

Drag options to blanks, or click blank then click option'

Amqtt

Bmqtt.client

Cpaho.mqtt.client

Dpaho.mqtt

Attempts:

3 left

💡 Hint

Common Mistakes

Using just 'mqtt' or 'mqtt.client' which are not valid imports.

Importing 'paho.mqtt' without '.client' which does not provide the client class.

The paho.mqtt.client module is the standard Python library for MQTT communication.

2fill in blank

medium

Complete the code to create an MQTT client instance named 'client'.

Raspberry Pi

client = [1]()

Drag options to blanks, or click blank then click option'

Amqtt.client.Client

Bpaho.Client

Cmqtt.Client

Dpaho.mqtt.client.Client

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'mqtt.Client' which is not imported directly.

Using 'paho.Client' which is incomplete.

The MQTT client is created by calling paho.mqtt.client.Client().

3fill in blank

hard

Fix the error in the code to connect the client to the MQTT broker at 'broker.hivemq.com' on port 1883.

Raspberry Pi

client.[1]('broker.hivemq.com', 1883)

Drag options to blanks, or click blank then click option'

Aconnect

Bconnect_to

Cstart_connect

Dopen_connection

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent methods like 'connect_to' or 'start_connect'.

Trying to use 'open_connection' which is not part of the MQTT client.

The correct method to connect the client to the broker is connect().

4fill in blank

hard

Fill both blanks to subscribe the client to the topic 'home/room1/temperature' with QoS level 1.

Raspberry Pi

client.[1]('[2]', 1)

Drag options to blanks, or click blank then click option'

Asubscribe

Bpublish

Chome/room1/temperature

Dhome/room1/humidity

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'publish' instead of 'subscribe' to listen to messages.

Subscribing to the wrong topic like humidity.

Use subscribe method with the topic 'home/room1/temperature' to receive messages on that topic.

5fill in blank

hard

Fill all three blanks to publish the message '22.5' to the topic 'home/room2/humidity' with QoS 0.

Raspberry Pi

client.[1]('[2]', [3], qos=0)

Drag options to blanks, or click blank then click option'

Asubscribe

B'22.5'

Chome/room2/humidity

Dpublish

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'subscribe' instead of 'publish' to send messages.

Swapping topic and message arguments.

Use publish method with the topic 'home/room2/humidity' and message '22.5' to send data.

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