Design a one-to-one and group chat system

Setting

You're building the messaging backbone for a consumer chat app: one-to-one conversations, small group chats, delivery/read receipts, and a client that's frequently offline (mobile, spotty connectivity).

The ask

Design the system that lets a user send a message, delivers it to every recipient's device (online or not), and shows accurate delivery and read state.

Constraints & scale

• ~50M daily active users, ~10B messages/day, groups up to 250 members
• End-to-end delivery latency < 1 s p99 when both parties are online
• Messages must never be lost and must arrive in a sane order per conversation, even across reconnects and multiple devices per user
• Offline users must receive every message once they reconnect, without duplicates
• Read receipts and typing indicators are best-effort and may be dropped under load — message delivery may not

What a strong answer covers

Requirements → estimates → API/data model → high-level diagram (connection layer, message store, fan-out) → 1–2 deep dives → trade-offs. Likely probes: how you fan out a group message, how a client resumes after being offline for days, and where exactly-once vs. at-least-once delivery applies.

Graded on

Every session — this problem included — is scored against the same 10-dimension senior/staff rubric: