Pub/sub pattern in HLD - Architecture Diagram

System Overview - Pub/sub pattern

The Pub/Sub pattern is a messaging architecture where publishers send messages without knowing who will receive them. Subscribers express interest in certain message topics and receive only those messages. This decouples message producers from consumers, enabling scalable and flexible communication.

Architecture Diagram

User Publisher
    |
    v
+-----------------+
|  Message Broker |
+-----------------+
    ^       ^
    |       |
Subscriber1 Subscriber2

Components

User Publisher

publisher

Sends messages to the message broker on specific topics

Message Broker

message_broker

Receives messages from publishers and routes them to subscribers based on topic subscriptions

Subscriber1

subscriber

Receives messages from the broker for subscribed topics

Subscriber2

subscriber

Receives messages from the broker for subscribed topics

Request Flow - 3 Hops

User Publisher → Message Broker

Message Broker → Subscriber1

Message Broker → Subscriber2

Failure Scenario

Component Fails:Message Broker

Impact:Messages cannot be routed; publishers cannot deliver messages; subscribers receive no updates

Mitigation:Use broker clustering with failover and replication to ensure availability and message durability

Architecture Quiz - 3 Questions

In the Pub/Sub pattern, who decides which subscribers receive a message?

A Subscribers pull messages from the Publisher

B The Message Broker based on topic subscriptions

C The Publisher directly sends to subscribers

D A Load Balancer distributes messages randomly

Design Principle

The Pub/Sub pattern decouples message producers and consumers by introducing a message broker that routes messages based on topics. This enables scalable, flexible, and asynchronous communication where publishers and subscribers do not need to know about each other directly.