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:
mosquitto_pub -t 'home/temperature' -m '22°C' -r
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Because retained messages stay on the broker until cleared, outdated or wrong data can be sent to new subscribers if not managed properly.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B Sending outdated data to new subscribers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.

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