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.

Practice

1. What does QoS level 1 guarantee in MQTT communication?

easy

A. At least once delivery of the message

B. Exactly once delivery of the message

C. At most once delivery of the message

D. No delivery guarantee

Solution

1. Step 1: Understand QoS levels in MQTT

   QoS 0 means no guarantee, QoS 1 means at least once, QoS 2 means exactly once.

2. Step 2: Identify QoS 1 guarantee

   QoS 1 ensures the message is delivered at least once, but duplicates may occur.

3. Final Answer:

   At least once delivery of the message -> Option A

4. Quick Check:

   QoS 1 = At least once delivery [OK]

Hint: QoS 1 means message arrives at least once [OK]

Common Mistakes:

• Confusing QoS 1 with exactly once delivery
• Thinking QoS 1 has no delivery guarantee
• Mixing QoS 0 and QoS 1 guarantees

2. Which of the following is the correct way to publish a message with QoS 2 using the Paho MQTT Python client?

easy

A. client.publish(topic, payload, qos=1)

B. client.publish(topic, payload, qos=3)

C. client.publish(topic, payload, qos=2)

D. client.publish(topic, payload, qos=0)

Solution

1. Step 1: Recall valid QoS values in MQTT

   MQTT QoS levels are 0, 1, and 2 only; 3 is invalid.

2. Step 2: Check syntax for publishing with QoS 2

   The Paho client uses qos parameter in publish method; qos=2 is valid.

3. Final Answer:

   client.publish(topic, payload, qos=2) -> Option C

4. Quick Check:

   QoS 2 publish syntax = client.publish(..., qos=2) [OK]

Hint: QoS must be 0, 1, or 2; 2 is highest reliability [OK]

Common Mistakes:

• Using qos=3 which is invalid
• Omitting qos parameter defaults to 0
• Confusing qos=1 with qos=2 syntax

3. Consider this Python code snippet using Paho MQTT client:

client.publish('sensor/data', 'temp=22', qos=0)
client.publish('sensor/data', 'temp=23', qos=1)
client.publish('sensor/data', 'temp=24', qos=2)

What is the expected behavior regarding message delivery?

medium

A. All messages are delivered exactly once

B. First message may be lost, second delivered at least once, third exactly once

C. All messages may be lost without retry

D. First message delivered at least once, others may be duplicated

Solution

1. Step 1: Analyze QoS 0 message delivery

   QoS 0 means message sent once, no retry, may be lost.

2. Step 2: Analyze QoS 1 and QoS 2 messages

   QoS 1 guarantees at least once delivery (may duplicate), QoS 2 guarantees exactly once delivery.

3. Final Answer:

   First message may be lost, second delivered at least once, third exactly once -> Option B

4. Quick Check:

   QoS 0=may lose, QoS 1=at least once, QoS 2=exactly once [OK]

Hint: Remember QoS 0 no retry, QoS 1 retry, QoS 2 handshake [OK]

Common Mistakes:

• Assuming QoS 0 always delivers
• Confusing QoS 1 with exactly once
• Ignoring possible duplicates at QoS 1

4. You wrote this code to publish a message with QoS 2:

client.publish('home/lights', 'ON', qos=2)

But the message is sometimes duplicated on the subscriber side. What is the likely cause?

medium

A. The client.publish syntax is incorrect

B. QoS 2 does not guarantee no duplicates

C. QoS 2 is not supported by the broker

D. The subscriber is not handling duplicate messages properly

Solution

1. Step 1: Understand QoS 2 guarantees

   QoS 2 guarantees exactly once delivery, so duplicates should not occur in transmission.

2. Step 2: Identify cause of duplicates

   If duplicates appear, likely the subscriber application is not filtering duplicates properly.

3. Final Answer:

   The subscriber is not handling duplicate messages properly -> Option D

4. Quick Check:

   QoS 2 = no duplicates in network, subscriber must handle duplicates [OK]

Hint: Duplicates often come from subscriber logic, not QoS 2 itself [OK]

Common Mistakes:

• Blaming QoS 2 for duplicates
• Assuming broker always supports QoS 2
• Thinking publish syntax is wrong

5. You want to design a Raspberry Pi sensor system that sends temperature data reliably without duplicates, but network is unstable. Which MQTT QoS level should you choose and why?

hard

A. QoS 2, to guarantee exactly once delivery with no duplicates

B. QoS 1, to ensure message arrives at least once

C. QoS 1, because QoS 2 is not supported on Raspberry Pi

D. QoS 0, for fastest delivery without retries

Solution

1. Step 1: Analyze requirements for reliability and no duplicates

   The system needs reliable delivery and no duplicate messages despite unstable network.

2. Step 2: Match QoS levels to requirements

   QoS 0 is fastest but unreliable, QoS 1 may duplicate, QoS 2 guarantees exactly once delivery.

3. Step 3: Confirm Raspberry Pi supports QoS 2

   MQTT clients on Raspberry Pi support QoS 2, so it is suitable.

4. Final Answer:

   QoS 2, to guarantee exactly once delivery with no duplicates -> Option A

5. Quick Check:

   Reliable + no duplicates = QoS 2 [OK]

Hint: Choose QoS 2 for no duplicates and reliable delivery [OK]

Common Mistakes:

• Choosing QoS 1 and ignoring duplicates
• Thinking QoS 0 is reliable
• Assuming Raspberry Pi can't use QoS 2