One chapter a day from System Design Interview, rebuilt. Every claim is tied to a number, every diagram can be stepped through, and the key trade-offs are calculators you can drag rather than conclusions you have to accept. The quizzes are interview scenarios, not recall.
Part A — Foundations
One request end to end, what DNS actually does, and why statelessness is the reason the tiers get split.
Five moves in order, each with the bottleneck it clears and the bill it creates. Includes the replication-lag bug.
The rounding shortcuts, latency on human scale, and the availability arithmetic almost everyone gets backwards.
The clock, the eight clarifying questions, and what to say when you are behind or do not know.
Part B — Building blocks
Five algorithms drawn, the exact moment the cheap one leaks, and atomic counting across a fleet.
Drive the ring yourself: add and remove servers and watch the moved-key counter against plain modulo.
CAP as a concrete three-node scenario, the W + R > N overlap proof, and three repair mechanisms.
64 bits drawn to scale, three IDs minted field by field, and what a backwards clock actually costs.
Why the redirect code is a one-way decision, and why base 62 beats hashing.
Front queues pick what, back queues pick when — and a burst against one host becomes unrepresentable.
Take the SMS vendor down and watch what happens to push and email. Then find the crash that beats dedupe.
Built on System Design Interview — An Insider's Guide, Vol. 1 by Alex Xu, chapters 1–10. Diagrams, simulators, worked arithmetic and quiz scenarios are added here; sources are cited per lesson.