Retained messages in IOT Protocols - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a retained message in MQTT?

A retained message is a special message that the MQTT broker stores and sends immediately to any client that subscribes to the topic. It keeps the last message sent on that topic.

beginner

How does a retained message help new subscribers?

New subscribers get the last retained message right after subscribing, so they immediately have the latest information without waiting for the next update.

intermediate

How do you publish a retained message using MQTT command line?

Use the option to mark the message as retained. For example: <br>mosquitto_pub -t 'home/temperature' -m '22°C' -r <br>This publishes '22°C' as a retained message on the topic 'home/temperature'.

intermediate

What happens if you publish a retained message with an empty payload?

Publishing a retained message with an empty payload clears the retained message on that topic. The broker deletes the stored message.

intermediate

Why should you be careful when using retained messages?

Because retained messages stay on the broker until cleared, outdated or wrong data can be sent to new subscribers if not managed properly.

What does a retained message do in MQTT?

A Deletes all messages on a topic

B Encrypts the message content

C Sends messages only to one client

D Stores the last message on a topic for new subscribers

A retained message stores the last message on a topic so new subscribers get it immediately.

How do you clear a retained message on a topic?

A Publish a message without the retained flag

B Unsubscribe from the topic

C Publish a message with an empty payload and retained flag

D Restart the MQTT broker

Publishing an empty retained message clears the stored retained message on that topic.

Which MQTT command publishes a retained message?

A mosquitto_pub -t 'topic' -m 'message' -r

B mosquitto_sub -t 'topic' -r

C mosquitto_pub -t 'topic' -m 'message'

D mosquitto_sub -t 'topic'

The '-r' flag in mosquitto_pub marks the message as retained.

When does a new MQTT subscriber receive a retained message?

A Immediately after subscribing to the topic

B Only after the next message is published

C Only if the subscriber requests it

D Never, retained messages are for the broker only

New subscribers get the retained message immediately after subscribing.

What is a risk of using retained messages carelessly?

A Losing messages permanently

B Sending outdated data to new subscribers

C Overloading the broker with messages

D Preventing clients from subscribing

Retained messages stay until cleared, so old data can be sent if not updated.

Explain what retained messages are and how they help MQTT clients.

Think about how new clients get the latest info without waiting.

Describe how to publish and clear retained messages using MQTT commands.

Focus on the command line options and what happens on the broker.