MQTT with QoS levels in Raspberry Pi - Mini Project: Build & Apply

MQTT with QoS levels

📖 Scenario: You have a Raspberry Pi that collects temperature data. You want to send this data to a server using MQTT. MQTT allows you to choose how reliable the message delivery is by setting Quality of Service (QoS) levels.QoS 0 means the message is sent once with no confirmation. QoS 1 means the message is sent at least once, with confirmation. QoS 2 means the message is sent exactly once, with a more complex handshake.

🎯 Goal: Create a simple Python program on your Raspberry Pi that publishes a temperature reading to an MQTT broker using different QoS levels. You will set up the data, configure the QoS level, publish the message, and then print the result.

📋 What You'll Learn

Use the paho-mqtt Python library to connect to an MQTT broker.

Create a variable with a temperature reading.

Create a variable to set the QoS level (0, 1, or 2).

Publish the temperature message with the chosen QoS level.

Print a confirmation message showing the QoS level used.

💡 Why This Matters

🌍 Real World

MQTT is widely used in IoT devices like Raspberry Pi to send sensor data reliably to servers or cloud services.

💼 Career

Understanding MQTT and QoS levels is important for roles in IoT development, embedded systems, and network programming.

1

DATA SETUP: Create a temperature variable

Create a variable called temperature and set it to the float value 23.5 representing the temperature reading.

Raspberry Pi

# Create a variable called temperature and set it to 23.5
# Your code here

Hint

Use a simple assignment like temperature = 23.5.

2

CONFIGURATION: Set the QoS level

Create a variable called qos_level and set it to the integer 1 to represent the MQTT QoS level.

Raspberry Pi

temperature = 23.5
# Create a variable called qos_level and set it to 1
# Your code here

Hint

Set qos_level to 1 using simple assignment.

3

CORE LOGIC: Publish the temperature with QoS

Import the paho.mqtt.client module as mqtt. Create a client called client. Connect to the broker at "test.mosquitto.org" on port 1883. Publish the temperature to the topic "home/temperature" using the qos_level. Disconnect the client after publishing.

Raspberry Pi

temperature = 23.5
qos_level = 1
# Import paho.mqtt.client as mqtt
# Create client, connect to broker, publish temperature with qos_level, then disconnect
# Your code here

Hint

Use client.publish(topic, message, qos=qos_level) to send the message.

4

OUTPUT: Print confirmation message

Write a print statement that outputs: "Published temperature 23.5 with QoS 1" using the variables temperature and qos_level inside an f-string.

Raspberry Pi

import paho.mqtt.client as mqtt

temperature = 23.5
qos_level = 1

client = mqtt.Client()
client.connect("test.mosquitto.org", 1883)
client.publish("home/temperature", str(temperature), qos=qos_level)
client.disconnect()
# Print confirmation message below
# Your code here

Hint

Use print(f"Published temperature {temperature} with QoS {qos_level}").

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