How the interviewer opens it
“Design the home feed for a social network: when I open the app, I see recent posts from the people I follow.”
The prompt is vague on purpose. The first few minutes are yours: narrow it to the core features and agree on the numbers before you draw anything.
What to clarify first
- ordering
- freshness
- feed latency
- scale
- follower distribution
- media
- duplicates
The answers stay with the interviewer. In a SysRound round, Alex answers them as an interviewer would.
Scope of a strong answer
Core requirements
- Create a post
- Follow users
- Read a reverse-chronological feed of followed accounts
Usually out of scope
- Comments/likes system
- Search
- Ads
What interviewers probe
- Fan-out strategy
- Pull is slow on read; push explodes for celebrities.
- Reliable fan-out pipeline
- Dual writes, worker crashes, backlogs.
Common mistakes
- Storing full post content in every feed list
- Ignoring celebrity skew
- Not caching only active users
- Media bytes in the database
The bar by level
- Mid-level (L4/E4)
- Pull vs push explained, cached feeds, basic data model.
- Senior (L5/E5)
- Hybrid with numbers, reliable async fan-out (outbox, idempotency), hot-post handling, storage choices per table.
- Staff (L6+/E6+)
- Cost/latency trade-offs at global scale, ranking architecture (candidate generation + scoring), multi-region consistency, operational metrics (fan-out lag SLO).