MQTT with QoS levels in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

MQTT QoS 0 Message Delivery

What will be the output on the subscriber console when a message is published with QoS 0?

Raspberry Pi

import paho.mqtt.client as mqtt

# Callback when a message is received
def on_message(client, userdata, msg):
    print(f"Received message: {msg.payload.decode()} with QoS {msg.qos}")

client = mqtt.Client()
client.on_message = on_message
client.connect("test.mosquitto.org", 1883, 60)
client.subscribe("test/qos0", qos=0)
client.loop_start()

publisher = mqtt.Client()
publisher.connect("test.mosquitto.org", 1883, 60)
publisher.publish("test/qos0", payload="Hello QoS0", qos=0)

import time
time.sleep(2)
client.loop_stop()

A Received message: Hello QoS0 with QoS 0

B Received message: Hello QoS0 with QoS 1

C No message received

D RuntimeError: MQTT connection failed

💡 Hint

QoS 0 means the message is delivered at most once without acknowledgment.

QoS 0 delivers the message once without confirmation. The subscriber receives the message with QoS 0.

❓ Predict Output

intermediate

MQTT QoS 1 Message Acknowledgment

What output will the subscriber show when a message is published with QoS 1?

Raspberry Pi

import paho.mqtt.client as mqtt

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

client = mqtt.Client()
client.on_message = on_message
client.connect("test.mosquitto.org", 1883, 60)
client.subscribe("test/qos1", qos=1)
client.loop_start()

publisher = mqtt.Client()
publisher.connect("test.mosquitto.org", 1883, 60)
publisher.publish("test/qos1", payload="Hello QoS1", qos=1)

import time
time.sleep(2)
client.loop_stop()

A TimeoutError: Message not acknowledged

B Received message: Hello QoS1 with QoS 0

C No message received

D Received message: Hello QoS1 with QoS 1

💡 Hint

QoS 1 guarantees message delivery at least once with acknowledgment.

With QoS 1, the subscriber receives the message and the QoS level is 1, confirming acknowledgment.

🧠 Conceptual

advanced

MQTT QoS 2 Message Flow

Which sequence correctly describes the MQTT QoS 2 message delivery handshake between publisher and broker?

A PUBLISH -> PUBCOMP -> PUBREL -> PUBREC

B PUBLISH -> PUBREC -> PUBREL -> PUBCOMP

C PUBLISH -> PUBREL -> PUBREC -> PUBCOMP

D PUBLISH -> PUBACK -> PUBREC -> PUBREL

💡 Hint

QoS 2 ensures exactly once delivery with a four-step handshake.

The correct order is PUBLISH, PUBREC, PUBREL, then PUBCOMP to guarantee exactly once delivery.

❓ Predict Output

advanced

Effect of QoS Level on Message Duplication

If a publisher sends a message with QoS 1 and the subscriber receives it twice, what is the likely cause?

A QoS 1 can cause duplicate messages due to retransmission until acknowledgment

B QoS 1 guarantees no duplicates, so this is impossible

C Subscriber is ignoring messages with QoS 1

D Broker is dropping messages silently

💡 Hint

QoS 1 ensures delivery at least once, so duplicates can happen.

QoS 1 may deliver duplicates if acknowledgments are lost, causing retransmission.

🔧 Debug

expert

Debugging MQTT QoS 2 Message Loss

Given this code snippet, why might the subscriber never receive messages published with QoS 2? client.subscribe("test/qos2", qos=2) # on_message callback is missing client.loop_forever()

Raspberry Pi

import paho.mqtt.client as mqtt

client = mqtt.Client()
client.connect("test.mosquitto.org", 1883, 60)
client.subscribe("test/qos2", qos=2)

client.loop_forever()

A The client must call loop_start() instead of loop_forever() for QoS 2

B QoS 2 is not supported by the broker, so messages are dropped

C No on_message callback is set, so messages are received but not processed or printed

D Subscription QoS must be 0 for messages to be received

💡 Hint

Without an on_message callback, messages arrive but are ignored silently.

The client subscribes correctly but does not define on_message, so no output or processing occurs.