News feed generation in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Architecture

intermediate

Design components for a scalable news feed system

Which of the following component sets best supports a scalable news feed generation system that handles millions of users with low latency?

A User service, Feed generator, Cache layer, Database, Message queue

B User service, Monolithic feed generator, Single database, No cache

C User service, Feed generator, Cache layer, Multiple databases without queue

D User service, Feed generator, Cache layer, Message queue, No database

💡 Hint

Think about components that help with asynchronous processing and fast data retrieval.

Option A includes a message queue for asynchronous feed generation, a cache for fast access, and a database for persistence, which together support scalability and low latency.

❓ scaling

intermediate

Handling high write throughput in news feed updates

When millions of users post updates simultaneously, which approach best handles the write load for news feed generation?

A Batch updates once a day to reduce write frequency

B Directly write all updates synchronously to the main database

C Store updates only in cache without persistence

D Use a distributed message queue to buffer updates and process them asynchronously

💡 Hint

Consider how to smooth out spikes in write traffic.

Using a distributed message queue buffers incoming updates and allows asynchronous processing, preventing database overload and improving scalability.

❓ tradeoff

advanced

Tradeoffs between push and pull models in news feed generation

Which statement correctly describes a tradeoff between push and pull models for generating user news feeds?

A Pull model reduces latency but increases storage cost; push model reduces storage but increases latency

B Push model reduces latency but increases storage cost; pull model reduces storage but increases latency

C Push model increases latency and storage cost; pull model reduces both latency and storage

D Push and pull models have identical latency and storage costs

💡 Hint

Think about when feed data is prepared and stored.

Push model precomputes and stores feeds, reducing latency but using more storage. Pull model computes feeds on demand, saving storage but increasing latency.

🧠 Conceptual

advanced

Impact of fan-out on news feed system design

What is the main challenge caused by high fan-out (users with millions of followers) in news feed generation?

A It causes excessive read load on the database

B It reduces the number of messages in the queue

C It causes excessive write amplification and storage overhead

D It simplifies feed generation logic

💡 Hint

Consider what happens when one user’s post must be delivered to many followers.

High fan-out means one post must be copied to many feeds, increasing write operations and storage needs, known as write amplification.

❓ estimation

expert

Estimating storage for a news feed system

Estimate the daily storage needed if 10 million users each post 2 updates per day, and each update is stored in the feeds of 500 followers. Each feed entry is 1 KB.

A Approximately 10 TB per day

B Approximately 5 TB per day

C Approximately 1 TB per day

D Approximately 20 TB per day

💡 Hint

Calculate total updates, multiply by followers, then by size.

10M users * 2 updates = 20M updates/day. Each update stored in 500 feeds: 20M * 500 = 10B entries. Each 1 KB = 10B KB = 10 TB.

Practice

1. What is the main purpose of a news feed generation system in social media platforms?

easy

A. To store user passwords securely

B. To manage user account settings

C. To show personalized and timely content to users

D. To handle payment transactions

Solution

1. Step 1: Understand the role of news feed generation

   The news feed system is designed to deliver content that is relevant and timely to each user.

2. Step 2: Identify the correct purpose among options

   The other options relate to security, settings, and payments, which are unrelated to news feed generation.

3. Final Answer:

   To show personalized and timely content to users -> Option C

4. Quick Check:

   News feed = personalized timely content [OK]

Hint: News feed = personalized content display [OK]

Common Mistakes:

• Confusing news feed with user authentication
• Mixing news feed with payment processing
• Thinking news feed manages account settings

2. Which of the following is a common method used in news feed generation to deliver updates efficiently?

easy

A. Manual refresh by users only

B. Push model where updates are sent to users proactively

C. Storing all data in a single database without caching

D. Using FTP to transfer news feed data

Solution

1. Step 1: Identify common delivery methods in news feed systems

   Push model proactively sends updates to users, improving latency and experience.

2. Step 2: Evaluate other options for efficiency

   Manual refresh is user-driven and less efficient; no caching slows performance; FTP is unrelated to real-time feed delivery.

3. Final Answer:

   Push model where updates are sent to users proactively -> Option B

4. Quick Check:

   Push model = efficient update delivery [OK]

Hint: Push model proactively sends updates [OK]

Common Mistakes:

• Assuming manual refresh is efficient
• Ignoring caching benefits
• Confusing FTP with real-time data delivery

3. Consider a news feed system using a pull model where users request their feed on demand. What is a likely outcome when many users request feeds simultaneously?

medium

A. Users receive outdated feeds only

B. Instant delivery with no server load

C. System automatically caches all feeds without delay

D. High latency and increased load on backend servers

Solution

1. Step 1: Understand pull model behavior under load

   Pull model requires backend to generate feeds on request, causing high load if many users request simultaneously.

2. Step 2: Analyze other options for feasibility

   Instant delivery with no load is unrealistic; outdated feeds depend on caching, not pull model alone; automatic caching without delay is ideal but not guaranteed.

3. Final Answer:

   High latency and increased load on backend servers -> Option D

4. Quick Check:

   Pull model + many requests = high load [OK]

Hint: Pull model causes backend load spikes [OK]

Common Mistakes:

• Assuming pull model has no latency
• Confusing caching with pull model behavior
• Believing feeds are always instantly cached

4. A news feed system uses a push model but users report seeing stale content. Which is the most likely cause?

medium

A. Cache not invalidated after new content is pushed

B. Users are not refreshing their browsers

C. Backend servers are down

D. Users have slow internet connections

Solution

1. Step 1: Identify push model behavior and caching role

   Push model sends updates, but if cache is not invalidated, users see old content.

2. Step 2: Evaluate other options for staleness cause

   Browser refresh is less relevant in push; backend down causes no updates; slow internet delays but does not cause stale cached data.

3. Final Answer:

   Cache not invalidated after new content is pushed -> Option A

4. Quick Check:

   Push + stale feed = cache invalidation issue [OK]

Hint: Stale feed often means cache invalidation failed [OK]

Common Mistakes:

• Blaming user refresh instead of cache
• Ignoring cache invalidation importance
• Assuming slow internet causes stale cache

5. You are designing a news feed system for a platform with 100 million users. Which approach best balances scalability and freshness of content?

hard

A. Hybrid model: push important updates and pull less critical content

B. Pure pull model: generate feed on every user request

C. Pure push model: push all updates to all users immediately

D. No caching: always fetch fresh data from database

Solution

1. Step 1: Analyze scalability and freshness needs for large user base

   Pure pull causes high load; pure push is costly and complex; no caching is inefficient.

2. Step 2: Evaluate hybrid model benefits

   Hybrid model pushes critical updates for freshness and uses pull for less urgent content, balancing load and latency.

3. Final Answer:

   Hybrid model: push important updates and pull less critical content -> Option A

4. Quick Check:

   Hybrid model balances scale and freshness [OK]

Hint: Hybrid push-pull balances scale and freshness [OK]

Common Mistakes:

• Choosing pure pull causing backend overload
• Choosing pure push causing network overload
• Ignoring caching and load balancing