MQTT broker role in IOT Protocols - Step-by-Step Execution

Process Flow - MQTT broker role

Client connects to Broker

↓

Broker authenticates Client

↓

Client subscribes to Topics

↓

Client publishes Messages

↓

Broker receives Messages

↓

Broker forwards Messages to Subscribers

↓

Clients receive Messages

↓

End

This flow shows how clients connect to the MQTT broker, subscribe to topics, publish messages, and how the broker routes messages to subscribers.

Execution Sample

IOT Protocols

Client1 -> Broker: Connect
Client1 -> Broker: Subscribe to 'home/temperature'
Client2 -> Broker: Publish '22C' to 'home/temperature'
Broker -> Client1: Forward '22C'
Client1: Receives '22C'

This example shows a client subscribing to a topic and another client publishing a message that the broker forwards to the subscriber.

Process Table

Step	Action	Source	Destination	Broker Role	Result
1	Connect	Client1	Broker	Authenticate client	Client1 connected
2	Subscribe	Client1	Broker	Register subscription to 'home/temperature'	Subscription registered
3	Connect	Client2	Broker	Authenticate client	Client2 connected
4	Publish	Client2	Broker	Receive message on 'home/temperature'	Message received
5	Forward	Broker	Client1	Send message to subscribers of 'home/temperature'	Message forwarded
6	Receive	Client1	Client1	N/A	Client1 receives '22C'
7	Disconnect	Client1	Broker	Remove subscriptions	Client1 disconnected
8	Disconnect	Client2	Broker	Remove subscriptions	Client2 disconnected

💡 All clients disconnected, broker stops forwarding messages

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
Connected Clients	[]	['Client1']	['Client1']	['Client1', 'Client2']	['Client1', 'Client2']	['Client1', 'Client2']	['Client1', 'Client2']	[]
Subscriptions	{}	{'home/temperature': ['Client1']}	{'home/temperature': ['Client1']}	{'home/temperature': ['Client1']}	{'home/temperature': ['Client1']}	{'home/temperature': ['Client1']}	{'home/temperature': ['Client1']}	{}
Messages Received	[]	[]	[]	[]	['22C']	['22C']	['22C']	[]

Key Moments - 3 Insights

Why does the broker need to authenticate clients before allowing subscriptions or publishing?

The broker authenticates clients (see Step 1 and Step 3 in execution_table) to ensure only trusted devices connect and exchange messages, preventing unauthorized access.

How does the broker know which clients should receive a published message?

The broker keeps track of subscriptions (Step 2) and forwards messages only to clients subscribed to the topic (Step 5), ensuring targeted message delivery.

What happens to subscriptions when a client disconnects?

When clients disconnect (Steps 7 and 8), the broker removes their subscriptions to avoid sending messages to offline clients.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does Client1 subscribe to the topic 'home/temperature'?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Check the 'Action' and 'Source' columns in execution_table row for Step 2.

According to variable_tracker, what is the state of 'Connected Clients' after Step 3?

A ['Client1']

B ['Client2']

C ['Client1', 'Client2']

D []

💡 Hint

Look at the 'Connected Clients' row under 'After Step 3' in variable_tracker.

If Client1 disconnects at Step 7, what happens to its subscriptions?

A Subscriptions remain active

B Subscriptions are removed

C Subscriptions are transferred to Client2

D Subscriptions are paused

💡 Hint

Refer to the 'Broker Role' and 'Result' columns in execution_table at Step 7.

Concept Snapshot

MQTT Broker Role:
- Accepts client connections and authenticates them
- Manages client subscriptions to topics
- Receives published messages from clients
- Forwards messages to all subscribed clients
- Removes subscriptions when clients disconnect
Broker acts like a post office routing messages to correct recipients.

Full Transcript

The MQTT broker acts as a central hub in the MQTT messaging system. Clients first connect to the broker and are authenticated to ensure security. Once connected, clients can subscribe to topics they want to receive messages from. Other clients publish messages to these topics. The broker receives these messages and forwards them only to clients subscribed to the relevant topics. When clients disconnect, the broker removes their subscriptions to avoid sending messages to offline clients. This process ensures efficient and secure message delivery between devices.