Pub/sub pattern in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Pub/Sub pattern in system design?

The Pub/Sub (Publish/Subscribe) pattern is a messaging model where senders (publishers) send messages without knowing who will receive them, and receivers (subscribers) listen for messages on topics they are interested in. It decouples message producers from consumers.

intermediate

How does the Pub/Sub pattern improve system scalability?

By decoupling publishers and subscribers, the system can add more subscribers or publishers independently. Messages are distributed asynchronously, allowing components to scale without waiting for each other.

beginner

What role does a message broker play in the Pub/Sub pattern?

A message broker acts as an intermediary that receives messages from publishers and routes them to the appropriate subscribers based on topics or channels.

intermediate

Name two common challenges when implementing Pub/Sub systems.

1. Ensuring message delivery guarantees (like at-least-once or exactly-once). 2. Handling message ordering and duplicates.

beginner

Give a real-life example that illustrates the Pub/Sub pattern.

Think of a newspaper subscription: The newspaper company (publisher) prints news and sends it out. Subscribers receive newspapers based on their subscription topics without the company knowing each reader personally.

In the Pub/Sub pattern, who decides which subscribers get the messages?

A The message broker based on topics

B The publisher directly

C The subscriber sends a request

D The network router

The message broker routes messages to subscribers based on the topics they subscribe to.

What is a key benefit of using Pub/Sub over direct messaging?

A Tight coupling between components

B Synchronous communication

C Decoupling of message producers and consumers

D Guaranteed message order

Pub/Sub decouples producers and consumers, allowing them to operate independently.

Which component typically stores and forwards messages in a Pub/Sub system?

A Publisher

B Subscriber

C Message broker

D Load balancer

The message broker intermediates message delivery between publishers and subscribers.

What problem might occur if message ordering is not handled in Pub/Sub?

A Messages may be lost

B Subscribers may receive messages out of order

C Publishers get blocked

D Subscribers cannot subscribe

Without ordering, subscribers might process messages in the wrong sequence.

Which of these is NOT a typical use case for Pub/Sub?

A Event notification systems

B Real-time chat applications

C Direct database queries

D Log aggregation

Direct database queries are not a Pub/Sub use case; Pub/Sub is for asynchronous messaging.

Explain the Pub/Sub pattern and how it helps in building scalable systems.

Think about how publishers and subscribers communicate without knowing each other.

Describe common challenges faced when implementing a Pub/Sub system and possible solutions.

Consider what can go wrong when messages are sent asynchronously.

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