Design a large-scale news app with caching

Q: Design a large-scale news app with caching

This question evaluates a candidate's ability to design a scalable news application with emphasis on caching strategies and cross-device read-state synchronization, assessing skills in system architecture, API design, data modeling, distributed caching, and consistency handling.

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Question

Scenario

Design a simplified news app (mobile + web) for a very large user base.

Users can:

open the app and see a personalized news feed
open an article to read it
see which articles are read/unread

The interviewer wants you to focus on read tracking and caching.

Requirements

Feed API: return a ranked list of articles for a user.
Article API: return article content (text + images) quickly.
Read state:
- when a user reads an article, it should eventually show as “read” on other devices
- read state should not be lost if offline briefly
Scale assumptions:
- 50M DAU, peak 1M requests/sec globally
- P95 latency: Feed < 200 ms, Article < 300 ms

Deliverables

APIs and data model
Caching strategy (client, CDN, server)
How you store and sync read state
Handling invalidation, personalization, and hot content

Design a large-scale news app with caching

Quick Overview

Scenario

Requirements

Deliverables

Solution

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