Publish-subscribe architecture in IOT Protocols - Step-by-Step Execution

Process Flow - Publish-subscribe architecture

Publisher sends message

↓

Message broker receives message

↓

Broker checks topic

↓

Broker forwards message to subscribers

↓

Subscribers receive message

↓

Subscribers process message

This flow shows how a publisher sends a message to a broker, which then forwards it to all subscribers of the topic.

Execution Sample

IOT Protocols

Publisher -> Broker: Publish 'temp/room1' = 22°C
Broker -> Subscribers: Forward 'temp/room1' = 22°C
Subscriber: Receive and process message

A publisher sends a temperature message to the broker, which forwards it to all subscribers of 'temp/room1'.

Process Table

Step	Action	Topic	Message	Broker State	Subscribers Notified
1	Publisher sends message	temp/room1	22°C	Message received	None yet
2	Broker checks topic	temp/room1	22°C	Topic found with 2 subscribers	None yet
3	Broker forwards message	temp/room1	22°C	Message forwarded to subscribers	Subscriber A, Subscriber B
4	Subscribers receive message	temp/room1	22°C	No change	Subscriber A processes, Subscriber B processes
5	Subscribers process message	temp/room1	22°C	No change	Processing complete

💡 Message delivered to all subscribers; process complete.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
Message	None	22°C	22°C	22°C	22°C	22°C
Broker Queue	Empty	22°C received	Checked topic	Forwarded message	Empty	Empty
Subscribers Notified	None	None	None	Subscriber A, Subscriber B	Received message	Processed message

Key Moments - 3 Insights

Why does the broker need to check the topic before forwarding?

The broker checks the topic to know which subscribers are interested. Without this, it wouldn't know where to send the message. See execution_table step 2.

Do subscribers get the message directly from the publisher?

No, subscribers get messages only from the broker, not directly from the publisher. The broker manages message delivery. See execution_table steps 1 and 3.

What happens if no subscribers exist for a topic?

The broker receives the message but finds no subscribers to forward to, so the message is discarded or stored depending on configuration. This is implied in step 2 if subscriber count is zero.

Visual Quiz - 3 Questions

Look at the execution table, at which step does the broker forward the message to subscribers?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Check the 'Action' and 'Subscribers Notified' columns in execution_table row for step 3.

According to variable_tracker, what is the state of the broker queue after step 3?

A Empty

B Checked topic

C Forwarded message

D 22°C received

💡 Hint

Look at the 'Broker Queue' row and the column 'After Step 3' in variable_tracker.

If a new subscriber joins after step 3, how would the broker's behavior change?

A Broker ignores new subscriber for past messages

B Broker deletes all messages

C Broker forwards message to the new subscriber immediately

D Broker sends message back to publisher

💡 Hint

Consider that messages are forwarded only to subscribers registered at the time of forwarding (see execution_table step 3).

Concept Snapshot

Publish-subscribe architecture:
- Publishers send messages to a broker on topics.
- Broker forwards messages to all subscribers of those topics.
- Subscribers receive messages only from the broker.
- Decouples senders and receivers for flexible communication.
- Broker manages message routing and delivery.

Full Transcript

In publish-subscribe architecture, a publisher sends a message to a broker on a specific topic. The broker receives the message and checks which subscribers are registered for that topic. Then, the broker forwards the message to all those subscribers. Subscribers receive and process the message. This system allows many publishers and subscribers to communicate without knowing each other directly. The broker acts as a middleman managing message delivery based on topics.

Practice

1. What is the main role of a broker in a publish-subscribe architecture?

easy

A. To receive messages from publishers and forward them to subscribers

B. To directly connect publishers and subscribers

C. To store all messages permanently

D. To encrypt messages between devices

Solution

1. Step 1: Understand the broker's function

   The broker acts as a middleman that receives messages from publishers.

2. Step 2: Identify message forwarding role

   The broker forwards these messages to subscribers interested in specific topics.

3. Final Answer:

   To receive messages from publishers and forward them to subscribers -> Option A

4. Quick Check:

   Broker forwards messages [OK]

Hint: Broker forwards messages, it doesn't connect devices directly [OK]

Common Mistakes:

• Thinking broker connects devices directly
• Assuming broker stores messages permanently
• Confusing broker with encryption service

2. Which of the following is the correct way to describe a topic in publish-subscribe?

easy

A. A label used to organize messages for subscribers

B. A direct connection between two devices

C. A security protocol for message encryption

D. A device that publishes messages

Solution

1. Step 1: Define what a topic is

   A topic is a label or category that organizes messages so subscribers receive only relevant ones.

2. Step 2: Eliminate incorrect options

   Topics are not connections, security protocols, or devices.

3. Final Answer:

   A label used to organize messages for subscribers -> Option A

4. Quick Check:

   Topic organizes messages [OK]

Hint: Topics organize messages, not devices or connections [OK]

Common Mistakes:

• Confusing topics with device connections
• Thinking topics are security features
• Mixing up topics with publishers

3. Given this scenario: A device publishes messages to topic home/temperature. Which device will receive the messages?

medium

A. All devices on the network

B. Any device subscribed to home/humidity

C. Any device subscribed to home/temperature

D. Only the publishing device

Solution

1. Step 1: Identify the topic published to

   The device publishes to home/temperature.

2. Step 2: Determine which subscribers get the message

   Only devices subscribed to home/temperature receive the messages.

3. Final Answer:

   Any device subscribed to home/temperature -> Option C

4. Quick Check:

   Subscribers to topic get messages [OK]

Hint: Subscribers to the exact topic get messages [OK]

Common Mistakes:

• Assuming all devices get messages
• Thinking publishing device receives its own messages
• Confusing similar topic names

4. A subscriber is not receiving messages from the broker. Which of these is a likely cause?

medium

A. Publisher is sending messages to the subscriber directly

B. Subscriber is publishing messages instead of subscribing

C. Broker is encrypting messages incorrectly

D. Subscriber is not subscribed to the correct topic

Solution

1. Step 1: Check subscription correctness

   If the subscriber is not subscribed to the right topic, it won't get messages.

2. Step 2: Rule out other causes

   Publishers don't send messages directly; encryption issues or wrong roles are less common causes here.

3. Final Answer:

   Subscriber is not subscribed to the correct topic -> Option D

4. Quick Check:

   Correct topic subscription needed [OK]

Hint: Check subscriber's topic subscription first [OK]

Common Mistakes:

• Thinking publisher sends messages directly
• Blaming encryption without evidence
• Confusing subscriber and publisher roles

5. In a large IoT system using publish-subscribe, how can you ensure that only devices in the 'factory/floor1' area receive temperature alerts without affecting others?

hard

A. Publish all alerts to factory/temperature and let devices filter messages locally

B. Publish temperature alerts to topic factory/floor1/temperature and have devices subscribe to it

C. Send alerts directly to each device's IP address

D. Use a single topic temperature for all devices

Solution

1. Step 1: Use topic hierarchy for targeted messaging

   Publishing to factory/floor1/temperature targets only devices subscribed to that specific area.

2. Step 2: Avoid broad topics or direct device addressing

   Broad topics or direct IP messaging reduce scalability and flexibility.

3. Final Answer:

   Publish temperature alerts to topic factory/floor1/temperature and have devices subscribe to it -> Option B

4. Quick Check:

   Use specific topics for targeted delivery [OK]

Hint: Use detailed topic paths for precise message delivery [OK]

Common Mistakes:

• Using broad topics causing unwanted message delivery
• Trying direct device IP messaging in pub-sub
• Ignoring topic hierarchy benefits