High-Level Design

Design a real-time messaging service: users exchange 1:1 and group messages that arrive within milliseconds, in order, exactly once to the eye, and survive the recipient being offline.

In scope: sending and receiving 1:1 and group messages, offline delivery on reconnect, per-conversation ordering, receipts (sent/delivered/read), and presence (online/last-seen). End-to-end encryption is a variant.