The 8 Questions of System Design

Interviewers don't have a secret list of systems they expect you to have memorized. They have one question — 'design X' — and a rubric that is remarkably stable underneath. Once you see the rubric, memorizing 30 different architectures stops being the point.

The eight lenses

• —Requirements — what must this system actually do? Write functional + non-functional requirements out loud.
• —Architecture — the boxes and arrows. Services, databases, caches, queues, load balancers.
• —Components — the nouns: what is each box, and why does it exist?
• —Interactions — the verbs: which box calls which, synchronously or asynchronously?
• —Data — schema, cardinality, access patterns, retention, consistency budget.
• —Scale — QPS, storage, bandwidth, memory. Every number changes the architecture.
• —Failures — what happens when each box dies, each call hangs, each message duplicates?
• —Trade-offs — every decision's cost, stated as a sentence, not a slogan.

Why the checklist wins

A candidate who rambles about load balancers for ten minutes and never says 'the write path is the bottleneck' has learned diagrams, not design. The eight lenses force you to touch the write path, the failure path, and the price tag — which is exactly where senior interviewers steer.

You earn points for constraints questioned, numbers justified, and trade-offs named — not for the picture being beautiful.

Your move

Take any product you used today. Walk it through the eight lenses in writing. Then do the same for a payment system — and notice how the answers change even though the lenses don't. That's the whole trick: the questions are fixed, the answers are not.