Implement Leaderboard via Redis

Sorted set (ZSET)

• https://systemdesignschool.io/problems/leetcode/solution

Using Redis for a leaderboard is a common and efficient pattern, especially when you need fast reads, writes, and ranking operations. Redis provides a built-in data structure perfect for this: Sorted Sets (ZSET).

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🧠 Redis Sorted Set Primer

A Sorted Set in Redis is a collection of unique strings (like user IDs or usernames), each associated with a floating-point score. The set is automatically ordered by score.

For a leaderboard:

• The user/player ID is the member.
• The score (e.g. points, kills, rating) is the score in the sorted set.

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✅ Basic Leaderboard Operations with Redis

1. Add or Update a Player’s Score

ZADD leaderboard <score> <user_id>

✅ Adds the user if not present, or updates the score if already in the set.

Example:

ZADD leaderboard 1500 "user:123"
ZADD leaderboard 1800 "user:456"

2. Get Top N Players

ZREVRANGE leaderboard 0 N-1 WITHSCORES

• Use ZREVRANGE for descending order (highest score first).
• WITHSCORES returns both user IDs and scores.

Example:

ZREVRANGE leaderboard 0 9 WITHSCORES  # Top 10 players

3. Get Player Rank

ZREVRANK leaderboard <user_id>

• Returns the 0-based rank of the player in descending order.

Example:

ZREVRANK leaderboard "user:123"  # e.g., returns 5

4. Get Player Score

ZSCORE leaderboard <user_id>

Example:

ZSCORE leaderboard "user:123"  # e.g., returns 1500.0

5. Remove a Player

ZREM leaderboard <user_id>

Example:

ZREM leaderboard "user:123"

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🧱 Optional Enhancements

🕐 Time-based Leaderboards

For weekly/monthly leaderboards:

• Use separate keys per time period:

  • leaderboard:weekly:2025-W20
  • leaderboard:monthly:2025-05

• Use a scheduled job to archive or delete old leaderboards.

🧾 Metadata Support

You can’t directly store extra data (e.g., username, avatar) in Redis Sorted Sets. Solutions:

• Store just the user ID in the sorted set.

• Store user metadata in a Redis hash or another DB:

  HGETALL user:123  # e.g., {"username": "Alice", "avatar": "..."}
  

🔁 Score Incrementation

ZINCRBY leaderboard 100 "user:123"

Adds 100 to the existing score of user:123.

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🧪 Sample Use Case (Gaming)

ZADD game:leaderboard 2000 "player:alice"
ZADD game:leaderboard 1900 "player:bob"
ZREVRANGE game:leaderboard 0 4 WITHSCORES  # top 5
ZREVRANK game:leaderboard "player:alice"   # alice's rank

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🧩 Scalability Notes

• Redis is in-memory: super fast, but data is volatile unless persistence (RDB/AOF) is enabled.
• For very large user bases, consider sharding leaderboards or using Redis Cluster.
• If precision isn’t critical, you can batch updates or use approximate leaderboards (e.g., with periodic sync to a database).

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