System design interview question

Design a like/dislike counter at YouTube scale

Medium · System design (HLD)

The prompt

"Design the like/dislike counter for YouTube. Every video shows a like count. Users can like, unlike, or switch to dislike. A viral video gets 100,000 likes per minute while being viewed 10 million times an hour. Design just this — the counter."

What this round really tests

A deceptively small scope that exposes exactly how well someone understands write contention, idempotency, and consistency trade-offs. There is nowhere to hide behind boxes and arrows. It is one of the best signal-per-minute questions in the set, and it is frequently used as a 30-minute screen.

Clarifying questions worth asking

Must the displayed count be exact?
Can a user like the same video twice?
Do we need to know if I liked it?
Historical counts / analytics?
How many videos?
Do we need the count in real time?

How the round usually runs

0–4 min: Scoping
4–8 min: Estimates
8–15 min: Naive design + why it fails
15–28 min: Deep dive
28–38 min: Deep dive
38–45 min: Wrap

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