A system design interview evaluates structured reasoning more than memorized diagrams. Begin by clarifying users, core use cases, scale, latency targets, availability, consistency, privacy, and constraints. Select a small set of core flows and state what is out of scope.

For “design a messaging app,” clarify one-to-one versus group chat, message history, online delivery, offline delivery, media, read receipts, end-to-end encryption, and target scale. Different answers create materially different architectures.

Summarize the requirements aloud before proceeding. This gives the interviewer a chance to correct assumptions and demonstrates that architecture follows product needs. If a requirement is unknown, state a reasonable assumption and show how it affects the design.