Design a Chat System

Requirements

A chat system lets users send messages to one person or a group and see them arrive in real time.

Functional:

• One-to-one and group chat (up to a few hundred members).
• Message history, available on a new device.
• Delivery to offline users when they return.
• Online or last-seen status, called presence.
• Text first; media goes through object storage.

Non-functional:

• Low latency, a few hundred milliseconds end to end.
• Ordering: messages in one conversation appear in the same order for everyone.
• No lost messages; duplicates are hidden from users.
• Stays up when a server dies.