Design leaderboard ranking and messenger services
Overview
This question evaluates expertise in large-scale distributed system design, covering competencies in scalable leaderboard and real-time messaging architectures including data modeling, API design, sharding and replication, low-latency read/write paths, ordering and delivery guarantees, presence, and durable storage.
System Design: Leaderboard and Messenger
Context
You are designing services for a large-scale social platform with hundreds of millions of users. Systems must be horizontally scalable, low-latency, and highly available.
Part A — Scalable Leaderboard Service
Design a leaderboard that supports:
- Fetching a user’s global rank.
- Fetching the user’s rank within their friendship circle.
- Returning the k users immediately before and after the user in both scopes (global and friends).
Include APIs, data modeling, read/write paths, handling ties, and scaling across shards.
Part B — Real-Time Messenger
Design a full-featured messenger application that supports real-time chat, message storage, presence, and scalability. Cover end-to-end flow: client connections, message send/receive, ordering, delivery guarantees, storage, presence, and scale-out.
Solution
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