Design a Real-Time Leaderboard

Requirements

A leaderboard ranks players by score and answers questions like "who is in the top 10?" and "what is my rank?" while scores keep changing.

Functional:

• Submit or increase a player's score.
• Get the top K players (K is often 10 to 100).
• Get one player's rank and score.
• Get the players just above and below me (my neighbourhood).
• Separate boards: daily, weekly, all-time, per game or region.

Non-functional:

• Updates appear on the board within about a second.
• Reads are fast (under 50 ms) even with 100 million players.
• Rank must be consistent: two different players never share a rank unless the rules say ties are allowed.
• Scores are never lost; the board can always be rebuilt.