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.