Interview Simulator · System Design
Design a Chat App
A full 45 minute round, one phase at a time. Each phase has a time budget, follow-up questions from the interviewer, and a checklist for your level. Reveal a phase's model answer whenever you need it; the report shows which phases you earned.
Design a chat app with 1:1 and group messages, delivery and read receipts, online presence, and offline delivery. It has 500 million daily users who each send about 40 messages a day. Messages arrive in order, and a message is never lost once the sender sees it as sent.