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.