Design a food news feed and backend flow
Overview
This question evaluates skills in scalable system design, data modeling, API design, and end-to-end backend data flow for high-throughput social feeds, categorized under System Design and emphasizing practical application of distributed architectures alongside conceptual trade-offs.
DoorDash
Jul 26, 2025, 12:00 AM
Software Engineer
Onsite
System Design
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Design a Food Posts News Feed with Detail Modal and Comments
Context
You are building a social, news-feed-like feature inside a food discovery or delivery app. Users follow authors (people, creators, or restaurants). The home feed shows a list of recent and relevant food posts. Tapping a post opens a detail modal with the full post content and comments.
Requirements
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UI behavior
- Feed items show: title, author, and comment count.
- Selecting a feed item opens a detail modal with full post information and a paginated comments list.
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Deliverables
- Data model for users, follows, posts, comments, feed, and counters.
- APIs for: list feed, fetch post detail, and post comment. Include pagination and idempotency where relevant.
- Pagination and ranking strategy for the feed.
- End-to-end backend request flow after the API gateway, including services, message queues, databases, and caches.
- How a new post is processed and propagated to follower feeds (fan-out vs fan-in).
- Discussion of consistency, caching, indexing, rate limiting, and optimizations for latency and read/write throughput.
Assumptions (clarify during interview if needed)
- Users see a personalized home feed from authors they follow, with light ranking over recency.
- Scale target: millions of DAU, thousands of posts per second at peak, tens of thousands of comments per minute.
- Mobile clients; images and videos are stored in object storage and served via CDN.
Solution
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