Design leaderboard and messenger systems

Q: Design leaderboard and messenger systems

This question evaluates system design and distributed-systems competencies—including scalability, data modeling, consistency, real-time messaging guarantees, security, and operational resilience—and is in the System Design domain.

Q: How do I approach System Design interview questions?

System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

Q: What difficulty level is this interview question?

This is a hard difficulty System Design question, commonly asked during Onsite rounds at Meta.

Q: What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Meta during technical interviews.

Question

System Design: Leaderboard Service (A) and Real-Time Messenger (B)

A) Leaderboard Service for a Large-Scale Game

Design a service to:

Compute a user’s global rank and their rank within their friendship circle.
Retrieve the k users immediately before and after a given user, both globally and within the friend circle ("around me").
Handle high write rates, pagination windows, anti-cheat, seasonal resets, and multiple game modes.
Detail:
- APIs (read/write, pagination, friend-scope queries).
- Data models (e.g., sorted sets, secondary indexes) and schema.
- Sharding/partitioning strategy.
- Caching strategy.
- Consistency model and rank computation approach.
- Friend-graph integration at read time and/or via materialization.
- Backfill/batch workflows (rebuilds, seasonal archives).

Assume tens of millions of active players per season and high write rates (hundreds of thousands of updates per second) with low-latency reads.

B) Real-Time Messenger Application

Design a system to support:

1:1 and group chat with correct per-conversation message ordering.
Delivery guarantees: choose among at-least-once, at-most-once, or exactly-once; justify the choice.
Online presence, typing indicators, read receipts.
Media sharing (images/video), push notifications, end-to-end encryption (E2EE), offline sync, and fanout for large groups.
Spam/abuse controls and disaster recovery.
Provide an end-to-end architecture, storage choices, protocols, scale estimates, and operational considerations.

Assume hundreds of millions of DAU, multi-region deployment, and peak traffic in the hundreds of thousands of messages per second.

Design leaderboard and messenger systems

Quick Overview

System Design: Leaderboard Service (A) and Real-Time Messenger (B)

A) Leaderboard Service for a Large-Scale Game

B) Real-Time Messenger Application

Solution

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