MQTT with QoS levels in Raspberry Pi

Introduction

MQTT with QoS levels helps control how messages are sent and received reliably between devices.

When you want to make sure a message is delivered at least once.

When you want to avoid duplicate messages.

When you want the fastest message delivery without guarantees.

When you need reliable communication between sensors and a server.

When you want to balance speed and reliability in message delivery.

Syntax

Raspberry Pi

client.publish(topic, message, qos=level)

# qos levels:
# 0 - At most once (fast, no guarantee)
# 1 - At least once (guaranteed delivery, possible duplicates)
# 2 - Exactly once (guaranteed delivery, no duplicates)

The qos parameter controls message delivery quality.

Higher QoS means more reliability but slower delivery.

Examples

Sends temperature once, no guarantee it arrives.

Raspberry Pi

client.publish('home/temperature', '22C', qos=0)

Sends temperature and retries until confirmed received.

Raspberry Pi

client.publish('home/temperature', '22C', qos=1)

Sends temperature exactly once with full handshake.

Raspberry Pi

client.publish('home/temperature', '22C', qos=2)

Sample Program

This program connects to a public MQTT broker and sends three messages with QoS levels 0, 1, and 2. It then disconnects and prints a confirmation.

Raspberry Pi

import paho.mqtt.client as mqtt

# Define MQTT broker address
broker = 'test.mosquitto.org'

# Create MQTT client
client = mqtt.Client()

# Connect to broker
client.connect(broker, 1883, 60)

# Publish messages with different QoS levels
client.publish('test/qos', 'Message with QoS 0', qos=0)
client.publish('test/qos', 'Message with QoS 1', qos=1)
client.publish('test/qos', 'Message with QoS 2', qos=2)

# Disconnect
client.disconnect()

print('Messages sent with QoS 0, 1, and 2')

Important Notes

QoS 0 is fastest but may lose messages.

QoS 1 retries sending until acknowledged, so duplicates can happen.

QoS 2 ensures message is received exactly once but is slower.

Summary

MQTT QoS controls message delivery reliability.

Use QoS 0 for speed, QoS 1 for guaranteed delivery, QoS 2 for no duplicates.

Choose QoS based on your application's needs for speed and reliability.

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