Design a Follow-Based Social News Feed With Paginated Home Timelines
Company: Meta
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Technical Screen
Design a news feed system for a social application. Users publish posts and follow other users; each user's home feed shows recent posts from the accounts they follow, and loads more posts page by page as the user scrolls. The report gives only the name of the problem, so scope and scale are open: settle them with the interviewer before designing.
```hint Work at write time or at read time
Compare assembling a follower's feed when a post is published with assembling it when the follower opens the app, and ask which kinds of accounts make each choice expensive.
```
### Clarifying Questions
- Is the feed in reverse-chronological order, or ranked by predicted interest?
- How many daily active users are there, how many accounts does a typical user follow, and how many followers can the largest accounts have?
- Is following one-way, or are connections mutual friendships?
- Do posts include images or video, and must like and comment counts appear in the feed?
- How soon after publishing must a post appear in followers' feeds?
### What a Strong Answer Covers
- Requirements and estimates that make the read and write volumes explicit
- A data model for users, follow relationships, posts and precomputed feeds
- Fan-out on write versus fan-out on read, and a hybrid for accounts with very large followings
- A read path with caching, cursor-based pagination and hydration of post content
- Media storage and delivery, and where ranking fits if it is in scope
- Consistency for deletes and unfollows, failure handling, scaling and monitoring
### Follow-up Questions
- An account with a very large number of followers publishes a post. What does your design do, and how long until every follower can see it?
- A post is deleted, or a user unfollows an account. How do the stale entries stop appearing?
- How do you paginate so that posts published while the user scrolls cause neither duplicates nor gaps?
- How would you add a ranking model without making the feed slow to load?
Overview: Design a social news feed in which users post and follow other accounts, and each user sees recent posts from the accounts they follow, loaded page by page. It probes fan-out on write versus read, accounts with very large followings, feed caching, cursor pagination and keeping feeds consistent with deletes and unfollows.
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