Fan-out on write vs fan-out on read in HLD - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Fan-out on Write

In a social media system, when a user posts a new update, the system immediately copies this update to all followers' timelines. What is this approach called?

A Fan-out on read

B Fan-out on write

C Lazy loading

D Batch processing

💡 Hint

Think about when the copying happens: at write time or read time?

Fan-out on write means the system distributes data to all relevant places immediately when data is written, such as copying a post to followers' timelines right away.

🧠 Conceptual

intermediate

Understanding Fan-out on Read

In a news feed system, when a user opens their feed, the system fetches the latest posts from all the people they follow at that moment. What is this approach called?

A Fan-out on write

B Data sharding

C Pre-computation

D Fan-out on read

💡 Hint

Consider when the system gathers data: during reading or writing?

Fan-out on read means the system gathers or computes data only when a user requests it, such as fetching posts from all followed users when the feed is opened.

❓ tradeoff

advanced

Trade-offs Between Fan-out on Write and Fan-out on Read

Which of the following is a true trade-off when choosing fan-out on write over fan-out on read in a large-scale social network?

A Fan-out on write reduces write latency but increases read latency.

B Fan-out on write decreases storage costs but increases read latency.

C Fan-out on write increases storage costs but reduces read latency.

D Fan-out on write reduces both storage and read latency.

💡 Hint

Think about when data is duplicated and how it affects storage and reading speed.

Fan-out on write duplicates data to many places at write time, increasing storage but making reads faster since data is pre-assembled.

❓ Architecture

advanced

Choosing Fan-out Strategy for a High-Traffic Messaging App

You are designing a messaging app where users can have thousands of contacts. Which fan-out strategy is better to handle message delivery efficiently and why?

A Fan-out on read, because messages are fetched from the sender's storage when a contact opens the chat, saving storage space.

B Fan-out on write, because it reduces storage by not duplicating messages.

C Fan-out on write, because messages are copied to all contacts immediately to ensure fast reads.

D Fan-out on read, because it duplicates messages to all contacts at read time.

💡 Hint

Consider the number of contacts and storage implications for immediate copying.

Fan-out on read is better here because copying messages to thousands of contacts immediately would be expensive in storage and write load. Fetching on read saves resources.

❓ scaling

expert

Estimating Storage Impact of Fan-out on Write

A social network has 10 million users. Each user has on average 500 followers. If each user posts 2 updates per day, estimate how many total timeline entries are created daily using fan-out on write.

A 10 million * 2 * 500 = 10 billion timeline entries

B 10 million * 500 = 5 billion timeline entries

C 10 million * 2 = 20 million timeline entries

D 10 million * 2 / 500 = 40,000 timeline entries

💡 Hint

Multiply users, posts per user, and followers per user to find total timeline entries.

Each post is copied to all followers' timelines. So total entries = users * posts per user * followers per user.