Why Pub/sub pattern in HLD? - Purpose & Use Cases

The Big Idea

What if you could send a message once and have it instantly reach only those who care, without lifting the phone?

The Scenario

Imagine a busy office where every employee must call each other individually to share updates. If one person has news, they must dial every colleague's number and explain it one by one.

The Problem

This manual calling is slow, tiring, and easy to forget someone. Important messages get delayed or lost, and the office becomes chaotic with repeated calls and confusion.

The Solution

The Pub/sub pattern acts like a central bulletin board where anyone can post messages, and those interested can read them instantly. No need to call everyone; messages reach the right people automatically.

Before vs After

✗ Before

for subscriber in subscribers:
    subscriber.receive(message)

✓ After

subscribers = broker.get_subscribers(topic)
publisher.publish(topic, message)

What It Enables

It enables efficient, scalable communication where senders and receivers stay loosely connected, making systems faster and easier to manage.

Real Life Example

News apps use pub/sub to send breaking news alerts only to users interested in specific topics, without bothering everyone with all updates.

Key Takeaways

Manual direct messaging is slow and error-prone.

Pub/sub centralizes message delivery for efficiency.

It supports scalable and flexible communication.

Practice

1. What is the main purpose of the Pub/Sub pattern in system design?

easy

A. To decouple message senders from receivers for flexible communication

B. To tightly couple components for faster data exchange

C. To store data permanently in a database

D. To encrypt messages for security

Solution

1. Step 1: Understand the role of Pub/Sub

   Pub/Sub pattern separates the sender (publisher) and receiver (subscriber) so they do not depend on each other directly.

2. Step 2: Identify the main benefit

   This separation allows flexible communication and easier scaling of systems.

3. Final Answer:

   To decouple message senders from receivers for flexible communication -> Option A

4. Quick Check:

   Decoupling = A [OK]

Hint: Pub/Sub means senders and receivers don't know each other [OK]

Common Mistakes:

• Thinking Pub/Sub stores data permanently
• Confusing Pub/Sub with encryption
• Assuming tight coupling between components

2. Which of the following is the correct component in a Pub/Sub system?

easy

A. Sender, Receiver, Firewall

B. Client, Server, Database

C. Publisher, Subscriber, Message Broker

D. Producer, Consumer, Cache

Solution

1. Step 1: Recall Pub/Sub components

   Pub/Sub systems have publishers (senders), subscribers (receivers), and a message broker that routes messages.

2. Step 2: Match components to options

   Only Publisher, Subscriber, Message Broker lists these three correctly.

3. Final Answer:

   Publisher, Subscriber, Message Broker -> Option C

4. Quick Check:

   Pub/Sub components = D [OK]

Hint: Remember the broker routes messages between publishers and subscribers [OK]

Common Mistakes:

• Confusing Pub/Sub with client-server terms
• Mixing cache or firewall as core components
• Using generic terms like producer/consumer without broker

3. Consider a Pub/Sub system where a publisher sends messages to a topic. If three subscribers are subscribed to that topic, how many copies of each message does the broker send?

medium

A. One copy shared among all subscribers

B. Three copies, one for each subscriber

C. Zero copies, subscribers pull messages themselves

D. Depends on the message size

Solution

1. Step 1: Understand message delivery in Pub/Sub

   Each subscriber receives its own copy of the message from the broker to ensure independent processing.

2. Step 2: Calculate copies sent

   With three subscribers, the broker sends three copies, one per subscriber.

3. Final Answer:

   Three copies, one for each subscriber -> Option B

4. Quick Check:

   Copies = Subscribers count = B [OK]

Hint: Broker sends one message per subscriber, not shared [OK]

Common Mistakes:

• Assuming one copy shared by all subscribers
• Thinking subscribers pull messages without broker sending
• Believing message size affects copy count

4. A developer implemented a Pub/Sub system but notices subscribers never receive messages. Which is the most likely cause?

medium

A. Subscribers are using the wrong database

B. Publisher is sending messages too frequently

C. Message broker is encrypting messages

D. Subscribers are not subscribed to the correct topic

Solution

1. Step 1: Analyze why subscribers get no messages

   If subscribers never receive messages, a common cause is they are not subscribed to the topic where messages are published.

2. Step 2: Evaluate other options

   Sending frequency or encryption does not prevent delivery; wrong database is unrelated.

3. Final Answer:

   Subscribers are not subscribed to the correct topic -> Option D

4. Quick Check:

   Subscription mismatch = A [OK]

Hint: Check subscriber topic matches publisher topic first [OK]

Common Mistakes:

• Blaming message frequency for delivery failure
• Assuming encryption blocks delivery
• Confusing database usage with Pub/Sub messaging

5. You need to design a scalable notification system using Pub/Sub. Which approach best ensures messages are delivered even if some subscribers are temporarily offline?

hard

A. Use a message broker with persistent storage and durable subscriptions

B. Send messages directly from publisher to subscribers without broker

C. Use a broker that deletes messages immediately after sending

D. Require subscribers to poll the publisher continuously

Solution

1. Step 1: Identify requirements for offline subscriber support

   To handle offline subscribers, messages must be stored until subscribers can receive them.

2. Step 2: Choose the right Pub/Sub feature

   Persistent storage and durable subscriptions in the broker ensure messages are saved and delivered later.

3. Step 3: Evaluate other options

   Direct sending or immediate deletion loses messages; polling is inefficient and not scalable.

4. Final Answer:

   Use a message broker with persistent storage and durable subscriptions -> Option A

5. Quick Check:

   Durable subscriptions = reliable delivery = C [OK]

Hint: Durable subscriptions keep messages for offline subscribers [OK]

Common Mistakes:

• Skipping message persistence in broker
• Trying direct publisher-to-subscriber without broker
• Relying on polling instead of push delivery