Design a TCP Discord-like chat server
Company: Discord
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Onsite
Design a Discord-like chat server that accepts multiple clients over raw TCP (e.g., test via PuTTY/telnet).
Functional requirements:
- Clients connect over TCP and can send line-based messages.
- Support slash commands:
- `/help` show available commands
- `/nick <name>` set/change nickname
- `/users` list users (at least in current room, or globally—state your choice)
- `/quit` disconnect
- Chat rooms:
- Create rooms, join rooms, and list rooms
- Messages sent to a room are broadcast to all users currently in that room
- Reconnection:
- If a client disconnects and later reconnects (identify how), they should be able to rejoin their previous rooms
- Message history:
- When a user joins a room, show the last 10 messages from that room
- Persistence:
- Data must persist across server restarts (users/rooms/membership/messages as needed)
Non-functional requirements:
- Keep scalability in mind (multiple concurrent clients; discuss how to scale beyond one process/host).
Explain your protocol, core data model, key components, and how you would handle concurrency, durability, and scaling.
Quick Answer: This question evaluates a candidate's skills in system design, TCP-based protocol design, real-time messaging, data modeling, persistence, session/reconnection handling, and concurrency control for backend services.