Real-Time Gaming Leaderboard

Phase 1 — Clarify & Scope

Interviewer: Design a real-time gaming leaderboard — think a global leaderboard for a mobile game or an esports tournament ranking system. What do you want to clarify?

Candidate: A few questions:

1. What's the leaderboard scope — global (all players), regional (per country), or per-game-session (temporary)?
2. What triggers a score update — end of match, continuous score during gameplay, or daily aggregation?
3. What's the read pattern — top-N query, or arbitrary rank lookup (where am I ranked among 100M players)?
4. How many players and score updates per second?
5. Does the rank need to be exact, or can it be approximate?

Interviewer: Global leaderboard for a mobile game. 100 million players, each with a cumulative lifetime score. Score updates happen at match end — about 10 million matches completed per day. Reads: display top-100 globally, plus the requesting player's rank and score. Exact ranks required. Near real-time updates — rank should reflect the latest completed match within 30 seconds.

Candidate: Let me size this:

$\text{Score updates/sec} = \frac{10\text{M matches/day}}{86{,}400} \approx 116 \text{ updates/sec average}$

$\text{Peak updates (10× average)} = 1{,}160 \text{ updates/sec}$

$\text{Read queries} = 100\text{M DAU} \times 5 \text{ leaderboard views/day} / 86{,}400 \approx 5{,}787 \text{ reads/sec}$

For rank calculation: 100M players sorted by score. A naive SQL COUNT(*) WHERE score > player_score takes O(N) — unacceptable at 100M rows.

Interviewer: What are the core requirements?

Candidate:

• Update player score (atomic increment, not absolute…