Design leaderboard ranking and messenger services

Q: Design leaderboard ranking and messenger services

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.

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Question

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.

Design leaderboard ranking and messenger services

System Design: Leaderboard and Messenger

Context

Part A — Scalable Leaderboard Service

Part B — Real-Time Messenger

Solution

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