Interview Question: Design a Chat API

The interview question is:

Design a Chat API.

The main requirements are consistent message ordering and efficient access to a growing history.

The difficult cases include simultaneous sends, incorrect phone clocks, many readers, and reconnecting without showing duplicate messages.

Scope

"I will design one-to-one and group conversations, ordered text messages, cursor-paged history, read receipts, live delivery, reconnect, and deletion. I will not design media upload, end-to-end encryption, search, moderation, voice calls, or the internal message-storage topology."