Design a real-time chat system with hot groups

Q: Design a real-time chat system with hot groups

This is a System Design interview question from Airbnb for Software Engineer roles. View the full question and solution on PracHub.

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Question

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Design a real-time chat system (similar to an in-app messaging feature) that supports:

1:1 and group conversations
Real-time delivery over WebSockets (or equivalent)
Message persistence and history sync
Read receipts (at least per-user “last read”)
Multi-device users (same user logged in on multiple clients)
High availability / disaster recovery considerations

Additional requirement:

The system must optimize for the Top N “hottest” group chats (e.g., groups with extremely high message throughput and/or many concurrently online participants). Explain what “hot” means and how you detect it.

The interviewer expects particular attention to:

A clear high-level architecture
A concrete data schema (tables/collections, keys, indexes)
How messages get routed when you have multiple WebSocket gateway servers
Scalability and performance trade-offs

Design a real-time chat system with hot groups

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