Why MQTT is the IoT standard in Raspberry Pi - Challenge Your Understanding

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this MQTT message publishing code?

Consider this Python code running on a Raspberry Pi that publishes a message to an MQTT broker. What will be printed when the message is successfully published?

Raspberry Pi

import paho.mqtt.client as mqtt

client = mqtt.Client()
client.connect('test.mosquitto.org', 1883, 60)

result = client.publish('home/temperature', '22.5')
print(result.rc)

A 1

B 0

C None

D Exception raised

💡 Hint

The publish() method returns an MQTTMessageInfo object with an rc attribute. 0 means success.

In MQTT, a result code of 0 means the message was accepted for delivery by the client library.

🧠 Conceptual

intermediate

Why is MQTT preferred for IoT devices?

Which of the following reasons best explains why MQTT is widely used as the IoT communication standard?

A It requires devices to maintain constant TCP connections without keep-alive.

B It uses a heavy protocol with large message sizes for reliability.

C It supports a lightweight publish-subscribe model with low bandwidth use.

D It only works with wired Ethernet connections.

💡 Hint

Think about IoT devices often having limited power and network.

MQTT uses a lightweight publish-subscribe model that minimizes bandwidth and power consumption, ideal for IoT.

🔧 Debug

advanced

Identify the error in this MQTT subscriber code

This Python code is intended to subscribe to an MQTT topic and print received messages. What error will it raise when run?

Raspberry Pi

import paho.mqtt.client as mqtt

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

client = mqtt.Client()
client.on_message = on_message
client.connect('broker.hivemq.com', 1883)
client.subscribe('home/temperature')
client.loop_start()

# Missing loop or wait here

A No messages printed because program exits immediately

B RuntimeError because loop_start() is called without loop_forever()

C No error, prints messages as they arrive

D SyntaxError due to missing colon

💡 Hint

Think about what happens after starting the loop in a script that ends immediately.

The script starts the network loop in the background but then exits immediately, so no messages are received or printed.

📝 Syntax

advanced

Which MQTT topic filter is valid for subscribing to all sensors?

You want to subscribe to all topics under 'sensors' using MQTT wildcards. Which subscription topic string is syntactically correct?

A sensors/*

B sensors/+/#

C sensors/+/+/#

D sensors/#

💡 Hint

MQTT uses '#' for multi-level and '+' for single-level wildcards.

The '#' wildcard must be at the end and matches all subtopics. '+' matches exactly one level. '*' is not valid in MQTT.

🚀 Application

expert

How many messages will be received by this MQTT subscriber?

A subscriber subscribes to 'devices/+/status' and the broker receives messages published to these topics: - devices/device1/status - devices/device2/status - devices/device1/temperature - devices/device3/status - devices/device3/battery How many messages will the subscriber receive?

A 3

B 5

C 4

D 2

💡 Hint

The '+' wildcard matches exactly one topic level.

The subscription matches topics with exactly one level between 'devices' and 'status'. Only three topics match: device1/status, device2/status, device3/status.