Retained messages in IOT Protocols - Mini Project: Build & Apply

Understanding Retained Messages in MQTT

📖 Scenario: You are working with MQTT, a messaging protocol used in IoT devices. Retained messages help new subscribers get the last known message immediately after subscribing.Imagine you have a smart home system where sensors send temperature updates. You want new devices to get the latest temperature right away without waiting for the next update.

🎯 Goal: Build a simple simulation of MQTT retained messages using a Python dictionary. You will store messages with a retained flag, update the retained messages, and retrieve the last retained message for a topic.

📋 What You'll Learn

Create a dictionary to store MQTT topics and their retained messages

Add a variable to represent a new message with a retained flag

Write logic to update the retained message only if the retained flag is true

Print the retained message for a specific topic

💡 Why This Matters

🌍 Real World

MQTT retained messages are used in IoT devices to ensure new subscribers get the latest state immediately, such as the last temperature reading or device status.

💼 Career

Understanding retained messages is important for IoT developers and DevOps engineers managing MQTT brokers and device communication to ensure reliable and efficient data delivery.

1

Create the initial retained messages dictionary

Create a dictionary called retained_messages with these exact entries: 'home/temperature': '22C', 'home/humidity': '45%'

IOT Protocols

# Create the retained_messages dictionary with initial topics and messages
# Your code here

Hint

Use curly braces to create a dictionary with keys as topic strings and values as message strings.

2

Add a new MQTT message with retained flag

Create a variable called new_message as a dictionary with keys 'topic', 'payload', and 'retained'. Set 'topic' to 'home/temperature', 'payload' to '23C', and 'retained' to true.

IOT Protocols

retained_messages = {'home/temperature': '22C', 'home/humidity': '45%'}
# Create the new_message dictionary with topic, payload, and retained flag
# Your code here

Hint

Create a dictionary with the exact keys and values as specified.

3

Update retained messages if retained flag is true

Write an if statement that checks if new_message['retained'] is true. If yes, update retained_messages for the key new_message['topic'] with the value new_message['payload'].

IOT Protocols

retained_messages = {'home/temperature': '22C', 'home/humidity': '45%'}
new_message = {'topic': 'home/temperature', 'payload': '23C', 'retained': true}
# Update retained_messages if new_message['retained'] is true
# Your code here

Hint

Use an if statement to check the retained flag and update the dictionary accordingly.

4

Print the retained message for 'home/temperature'

Write a print statement to display the retained message for the topic 'home/temperature' from the retained_messages dictionary.

IOT Protocols

retained_messages = {'home/temperature': '22C', 'home/humidity': '45%'}
new_message = {'topic': 'home/temperature', 'payload': '23C', 'retained': true}
if new_message['retained']:
    retained_messages[new_message['topic']] = new_message['payload']
# Print the retained message for 'home/temperature'
# Your code here

Hint

Use print() with the dictionary key to show the updated retained message.

Practice

1. What is the main purpose of a retained message in MQTT?

easy

A. To delete all messages from the broker

B. To store the last message on a topic for new subscribers

C. To encrypt messages during transmission

D. To limit the message size to 256 bytes

Solution

1. Step 1: Understand retained message concept

   Retained messages keep the last message sent on a topic stored on the broker.

2. Step 2: Identify the purpose for new subscribers

   New subscribers receive the last retained message immediately upon subscribing, ensuring they have the latest state.

3. Final Answer:

   To store the last message on a topic for new subscribers -> Option B

4. Quick Check:

   Retained message = last stored message [OK]

Hint: Retained means broker keeps last message for new subscribers [OK]

Common Mistakes:

• Confusing retained messages with message encryption
• Thinking retained messages delete old messages
• Assuming retained messages limit message size

2. Which MQTT publish option must be set to true to create a retained message?

easy

A. retain

B. qos

C. cleanSession

D. willFlag

Solution

1. Step 1: Identify publish options related to message retention

   The retain flag controls whether the broker stores the message as retained.

2. Step 2: Confirm correct flag for retention

   Setting retain=true tells the broker to keep the message for future subscribers.

3. Final Answer:

   retain -> Option A

4. Quick Check:

   retain flag true = retained message [OK]

Hint: Set retain flag true to keep message on broker [OK]

Common Mistakes:

• Confusing retain with cleanSession flag
• Using qos instead of retain for message storage
• Mistaking willFlag for retention control

3. Given the following MQTT publish code snippet, what will a new subscriber receive immediately after subscribing to topic home/temperature?

client.publish('home/temperature', '22°C', retain=True)
client.publish('home/temperature', '23°C', retain=False)

medium

A. The message '22°C' because it was retained

B. The message '23°C' because it was the last published

C. No message because retain is false on last publish

D. Both messages '22°C' and '23°C'

Solution

1. Step 1: Analyze retained flag on each publish

   The first message '22°C' is published with retain=True, so it is stored on the broker.

2. Step 2: Understand effect of second publish with retain=False

   The second message '23°C' is published without retain, so it is sent but not stored as retained. Importantly, publishing with retain=False on the same topic clears the retained message.

3. Step 3: Determine what new subscribers receive

   New subscribers get no retained message because the last retained message was cleared by the second publish.

4. Final Answer:

   No message because retain is false on last publish -> Option C

5. Quick Check:

   Retain=False on last publish clears retained message [OK]

Hint: Retain=False on last publish clears retained message [OK]

Common Mistakes:

• Assuming last published message is always retained
• Thinking retain=False messages overwrite retained messages
• Expecting multiple retained messages per topic

4. You want to clear a retained message on topic device/status. Which publish command will correctly clear it?

medium

A. client.publish('device/status', '', retain=True)

B. client.publish('device/status', '', retain=False)

C. client.publish('device/status', None, retain=True)

D. client.publish('device/status', None, retain=False)

Solution

1. Step 1: Understand how to clear retained messages

   Publishing an empty payload with retain=True clears the retained message on that topic.

2. Step 2: Analyze the options

   Publishing with retain=False does not update or clear the retained message. Using None as payload is invalid. Only publishing an empty string ('') with retain=True clears the retained message.

3. Final Answer:

   client.publish('device/status', '', retain=True) -> Option A

4. Quick Check:

   Empty payload + retain=True clears retained message [OK]

Hint: Publish empty message with retain=True to clear retained message [OK]

Common Mistakes:

• Using retain=False when trying to clear retained message
• Sending None as payload which causes error
• Expecting retain=False to remove retained message

5. You have multiple IoT devices publishing their status to devices/+/status with retained messages. A new device subscribes to devices/device123/status. What will it receive and why?

hard

A. It receives only non-retained messages published after subscription

B. It receives all retained messages from all devices under devices/+/status

C. It receives no messages until a new status is published

D. It receives the last retained status message for device123 immediately

Solution

1. Step 1: Understand MQTT wildcard subscription

   The subscription to devices/device123/status matches only that specific topic, not all devices.

2. Step 2: Understand retained message delivery

   The broker sends the last retained message on that exact topic immediately upon subscription.

3. Step 3: Determine what the new device receives

   The new device receives the last retained status message for device123 immediately, not messages from other devices.

4. Final Answer:

   It receives the last retained status message for device123 immediately -> Option D

5. Quick Check:

   Retained message sent on exact subscribed topic [OK]

Hint: New subscriber gets last retained message on exact topic [OK]

Common Mistakes:

• Thinking wildcard subscription delivers all retained messages
• Expecting no retained messages on subscription
• Confusing retained with non-retained message delivery