Design a TCP Discord-like chat server

Q: Design a TCP Discord-like chat server

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.

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 medium difficulty System Design question, commonly asked during Onsite rounds at Discord.

Q: What role is this question designed for?

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

Question

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.

Design a TCP Discord-like chat server

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