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Design Gaming Leaderboard
EasyReal-time Systemsrankingreal-timecachingsorted-sets
Question Overview
Design a live game leaderboard that shows the global top 100, any player's exact rank out of 100M, and a friends-only board. The interesting parts are choosing a data structure with logarithmic rank queries, handling ties and period resets, and scaling past one node.…
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Requirements
- Update a player's score from server-validated match results and reflect the new rank within 1 second
- Return the top 100 for the season, weekly, or daily board with p99 latency under 50 ms
- Return any player's exact rank and score plus the five players directly above and below them
- Return a friends-only leaderboard ranked by the same season score
- Break ties in favor of the player who reached the score first; reset boards on schedule
- Never lose a score update; the board can be rebuilt from durable storage
Back-of-the-envelope numbers
- Score updates: 50M DAU × 10 matches/day = 500M/day ÷ 86,400 s ≈ 5.8K writes/s average, ~17K/s at 3× peak
- Board reads: 50M × 20 views/day = 1B/day ÷ 86,400 s ≈ 11.6K reads/s average, ~35K/s at 3× peak
- Redis writes: one ZINCRBY on each of the season, weekly, and daily boards → ~17.4K × 3 ≈ 52K writes/s at peak
- Sorted set memory: 100M players × ~100 bytes per entry (member, score, skip list and hash overhead) ≈ 10 GB per season board
- Rank lookup: ZREVRANK is O(log N), roughly log2(100M) ≈ 27 comparisons on a span-annotated skip list, so a rank query costs microseconds, not a table scan
- Friends view: if a third of peak reads are friends boards, ~12K/s × 200 friends ≈ 2.3M score lookups/s, so cache results briefly
- Durable log: 500M match results/day × ~50 bytes ≈ 25 GB/day of append-only score history for rebuilds and audits
Key components
- Score service: accepts results only from trusted game servers, validates them, writes them durably, and emits an event that updates Redis
- Redis sorted set per board key (e.g. lb:season:12, lb:daily:2026-09-13) with ZINCRBY for updates and ZREVRANGE 0 99 for the top 100
- My-rank path: ZREVRANK and ZSCORE for the player, then ZREVRANGE from rank−5 to rank+5 for the neighbors
- Friends board: fetch friend IDs from the social graph, ZMSCORE them against the season board, sort in the app, and cache ~30 s
- Top-100 cache: every player reads the same list, so recompute it once per second and serve it from an in-memory or edge cache
- Durable store (e.g. DynamoDB or Cassandra) of per-player totals and a match log, used to rebuild Redis after a failure
- Period resets: write each period to a new key with a TTL instead of clearing a 100M-member set in place
Common mistakes
- Computing rank with SELECT COUNT(*) WHERE score > mine on every request, a scan over up to 100M rows
- Trusting scores sent directly from the game client, which makes cheating trivial
- Assuming Redis Cluster spreads one sorted set across nodes; a single key always lives on one shard
- Packing a millisecond tie-break timestamp into the score without noticing Redis scores are doubles with 53 bits of integer precision
- Maintaining a per-user friends leaderboard with fan-out on write, turning one score update into 200 writes
- Running DEL on a 100M-member sorted set at reset time, which blocks Redis; use per-period keys with UNLINK or lazy expiry
- Treating Redis as the only copy of scores, so a failover with replication lag silently loses recent updates
Likely follow-ups
- How would you scale when one sorted set no longer fits on a single Redis node or its write rate is too high?
- How would you show an approximate percentile such as top 3% for players far from the top more cheaply?
- How would you remove a banned cheater and correct everyone's ranks afterward?
- How would you rebuild the leaderboard after losing the Redis primary and its replica?
- How would you add per-country and per-guild boards without multiplying write cost excessively?
- How would you push live rank changes to players who are watching the leaderboard screen?
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