Retained messages in IOT Protocols - Step-by-Step Execution

Process Flow - Retained messages

Client publishes message with retain flag

↓

Broker stores message as retained

↓

New client subscribes to topic

↓

Broker sends retained message immediately

↓

Client receives last retained message

↓

Client processes message

When a client sends a message with the retain flag, the broker saves it. New subscribers get this saved message right away.

Execution Sample

IOT Protocols

PUBLISH topic='home/temp' payload='22C' retain=true
SUBSCRIBE topic='home/temp'
RECEIVE message='22C' (retained)

Client publishes a retained message; new subscriber immediately receives it.

Process Table

Step	Action	Topic	Payload	Retain Flag	Broker State	Client Output
1	Client publishes message	home/temp	22C	true	Stores '22C' as retained for 'home/temp'	None
2	New client subscribes	home/temp	N/A	N/A	Retained message exists	Sends retained message '22C'
3	Client receives message	home/temp	22C	retained	No change	Receives '22C' immediately
4	Client processes message	home/temp	22C	retained	No change	Processes '22C'
5	No new messages	N/A	N/A	N/A	No change	No further output

💡 No new messages published; retained message delivered to subscriber; process ends.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Broker Retained Message for 'home/temp'	None	'22C'	'22C'	'22C'	'22C'
Client Output	None	None	'22C' sent	'22C' received	'22C' processed

Key Moments - 3 Insights

Why does the new subscriber get the message immediately even though it was published before?

Because the broker saved the message as retained (see Step 1 and Step 2 in execution_table), it sends it instantly to new subscribers.

What happens if a message is published without the retain flag?

The broker does not save it, so new subscribers won't get any previous message immediately (not shown here, but inferred from Step 1 retain flag).

Does the retained message get deleted after sending to a new subscriber?

No, the broker keeps the retained message until a new retained message replaces it or it is cleared (see Broker State in variable_tracker).

Visual Quiz - 3 Questions

Look at the execution table, what is the broker's retained message after Step 1?

A '22C'

B None

C '22F'

D Empty string

💡 Hint

Check the 'Broker State' column at Step 1 in the execution_table.

At which step does the client receive the retained message?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Client Output' column to see when the message is received.

If the retain flag was false in Step 1, what would change in the execution table?

A Broker would store the message as retained

B Client would receive the message immediately on subscription

C Broker would not store the message; new subscriber gets no immediate message

D Client output would be '22C' processed at Step 4

💡 Hint

Refer to the 'Retain Flag' and 'Broker State' columns in Step 1 and Step 2.

Concept Snapshot

Retained messages:
- Client publishes with retain=true
- Broker saves last message per topic
- New subscribers get saved message immediately
- Retained message stays until replaced or cleared
- Useful for last known state delivery

Full Transcript

Retained messages in IoT protocols work by having a client send a message with a retain flag set to true. The broker then saves this message as the last known message for that topic. When a new client subscribes to the topic, the broker immediately sends the saved retained message to the client. This ensures the new subscriber gets the latest state without waiting for a new message. The retained message remains stored until a new retained message replaces it or it is cleared. This mechanism helps devices know the last known value instantly upon subscription.

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