Requirements & Scoping
Turn a vague one-line prompt into an agreed, finishable problem: scope, functional and non-functional requirements, and the API sketch.
Clarifying a Vague Prompt
Turn a one-line prompt into a scoped problem with three to five sharp questions and explicit out-of-scope.
Functional Requirements
Pick the three user capabilities that define the core loop, phrase them as 'users can X', and defer the rest with a reason.
Non-Functional Requirements, Quantified
Turn 'scalable and reliable' into quantified, testable targets (p99, nines, QPS) and name the design lever each one forces.
Core Entities & the API Sketch
Turn requirement nouns into design-relevant entities and one endpoint per requirement, with protocol, idempotency, and auth chosen deliberately.
Back-of-the-Envelope Estimation
Turn a vague prompt into defensible numbers for QPS, storage, bandwidth, and cache size in under five minutes, and use each number to justify a concrete architecture decision.
The Fermi Estimation Method
Decompose an unknown into stated assumptions, round to powers of ten, convert average to peak, and compute only numbers that change a decision.
QPS and Read-vs-Write Modeling
Derive read and write QPS separately, use the ratio to pick which path to optimize, and model where fan-out multiplies the real load.
Storage, Bandwidth & Cache Sizing
Size storage with replication and overhead multipliers, compute ingress and egress separately, and size the cache from the hot working set.
Latency Numbers Every Engineer Should Know
Memorize the latency ladder, unit conversions, object sizes, and single-machine ceilings, and turn each constant into a design decision.
The Structured Walkthrough
Run the round on a fixed clock: a repeatable 6-phase plan, a boxes-and-arrows design proven by tracing one real request, then an NFR-driven deep dive and an operational wrap-up.
Phased Delivery & the Interview Clock
Budget the 45 minutes across six phases with exit criteria, reach a complete simple design before adding complexity, and narrate every transition.
High-Level Architecture & Data-Flow
Start from the minimal skeleton, justify every added box against a requirement, label arrows, and prove the design by tracing one request end to end.
Deep Dives & the Operational Wrap-Up
Let the tightest NFR pick the bottleneck, compare two options and commit with a quantified reason, then close on break point, failure mode, monitoring, and cost.
Driving the Conversation & Tradeoffs
Frame every major choice as an explicit committed tradeoff, calibrate depth to the level you are interviewing for, run the round from a reusable template, and narrate so the interviewer stays inside your head.
Trade-off Articulation & Decision Framing
Frame choices through a principled lens (CAP/PACELC, push/pull, sync/async), commit on a stated assumption, quantify, and name what you gave up.
Level Calibration: Junior vs Senior vs Staff
Know the four rubric axes and aim your depth at the target level: complete design (junior), unaided deep dives (senior), ambiguity plus org/cost/evolution (staff).
A Reusable Template & the Top Pitfalls
Carry a phase backbone, stock questions, an estimation checklist, a component palette, and tradeoff lenses, and actively counter the five classic pitfalls.
Communication, Whiteboarding & Reading the Interviewer
Narrate assumption-option-choice continuously, keep the board in fixed zones with a parking lot, and treat interviewer hints as intentional steering.