Retained messages in IOT Protocols - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the main purpose of retained messages in MQTT?

In MQTT, retained messages serve a specific function. What is the main purpose of using retained messages?

A To guarantee message delivery even if the subscriber is offline

B To limit the message size to reduce bandwidth usage

C To encrypt messages for secure transmission

D To store the last message on a topic so new subscribers receive it immediately upon subscribing

💡 Hint

Think about what happens when a new device connects and wants the latest data.

Retained messages keep the last message on a topic so that new subscribers get the most recent data right away without waiting for the next update.

💻 Command Output

intermediate

What output does the MQTT broker produce after publishing a retained message?

Consider this MQTT publish command with the retain flag set:

mosquitto_pub -t 'home/temperature' -m '22' -r

What is the expected behavior of the broker regarding this message?

A The broker stores '22' as the last retained message for 'home/temperature' and sends it to new subscribers immediately

B The broker deletes any previous messages on 'home/temperature' and does not store the new message

C The broker sends the message only to currently connected subscribers and discards it afterwards

D The broker queues the message until the subscriber acknowledges receipt

💡 Hint

Retain flag means the broker keeps the message for future subscribers.

When a message is published with the retain flag, the broker saves it as the last retained message for that topic and delivers it immediately to any new subscribers.

❓ Troubleshoot

advanced

Why might a subscriber not receive the retained message after subscribing?

A subscriber connects to the MQTT broker and subscribes to a topic with a retained message, but does not receive the retained message. Which of the following is the most likely cause?

A The broker does not support retained messages and ignores the retain flag

B The subscriber used a QoS level of 0, which disables retained messages

C The retained message was cleared by publishing an empty retained message to the topic

D The subscriber did not set the clean session flag to false

💡 Hint

Think about how retained messages can be removed from the broker.

Publishing an empty retained message to a topic clears the retained message. If this happens before the subscriber connects, the subscriber will not receive any retained message.

🔀 Workflow

advanced

Order the steps for setting a retained message and receiving it as a new subscriber

Arrange the following steps in the correct order to successfully publish a retained message and have a new subscriber receive it immediately.

A 1,3,2,4

B 1,2,3,4

C 2,1,3,4

D 3,1,2,4

💡 Hint

Think about what happens first: publishing, storing, subscribing, then delivery.

The correct order is: publish with retain flag, broker stores the message, subscriber connects and subscribes, broker sends the retained message immediately.

✅ Best Practice

expert

What is the best practice to update a retained message with new sensor data?

You have a temperature sensor publishing data to an MQTT topic with retained messages enabled. What is the best practice to update the retained message correctly?

A Publish new readings with retain flag only if the temperature changes by more than 5 degrees.

B Publish each new temperature reading with the retain flag set to overwrite the previous retained message.

C Publish new readings with retain flag set to false and rely on subscribers to request the latest data.

D Publish new readings without the retain flag and periodically clear the retained message manually.

💡 Hint

Consider how retained messages keep the last known good value.

Publishing each new reading with the retain flag updates the retained message so new subscribers always get the latest sensor data immediately.

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