You can't memorize the systems. You can memorize the patterns.
Every "Design X" question is a costume over one of a small set of recurring problems. This guide gives you the nine patterns underneath the prompts, the six-phase loop every interview follows, and a scripted playbook for each canonical question — what to say, what never to say, and how the signal shifts from mid to senior to staff.
A book, not a flashcard deck.
There are dozens of system-design questions, but they reuse the same handful of moves. Memorizing 64 systems is hopeless; memorizing 9 patterns is doable — and once you can name the pattern under a prompt, a question you've never seen becomes one you've already solved. Each chapter is a self-contained playbook: the architect's lens, the phase-by-phase script, the follow-up probes, the red-flag phrases, and a closing scorecard.
Read it in three passes.
Get the map
Start with The Pattern Atlas. Learn the nine lenses and the question-to-pattern index so you can classify any prompt in the first thirty seconds.
Open the Atlas →Learn the loop
Read The Interview Loop (Rate Limiter). It scripts the six phases every interview follows and shows exactly how mid, senior, and staff answers diverge.
Open the loop →Drill the patterns
Work the nine pattern playbooks below. Each one runs the loop end-to-end on a canonical system, so you rehearse the cadence on real designs.
Jump to the library →Table of contents
Read these two first. The Atlas is the map; the Loop is the method. Everything else applies them.
The Pattern Atlas
Nine patterns that underlie every system-design prompt — fan-out, geospatial, collaboration, consistency, search, streaming, blob, realtime, scheduling — each with the questions it unlocks, the metrics to quote, and a question→pattern index.
Open chapter →The Interview Loop
The whole 45-minute loop, scripted phase by phase and probe by probe, on the most classic distributed-systems question. Shows what to say, what never to say, and exactly how the signal changes between mid, senior, and staff.
Open chapter →Each chapter runs the full loop on one canonical system. The number matches its pattern in the Atlas.
Twitter / News Feed
Build each follower's timeline on write, on read, or both — and survive the celebrity with a hundred million followers. The most-asked question in the canon.
Open chapter →Uber
Find the nearest driver among millions of moving cars in under two seconds. A write-heavy location firehose meets a latency-critical proximity search — never a distance scan.
Open chapter →Google Docs
Two cursors type in the same sentence at the same millisecond; both edits land and every screen converges. The interview turns on the conflict-resolution algorithm.
Open chapter →Ticketmaster
A million people hit "buy" the instant on-sale opens, and no two get the same seat. The one cluster where you trade availability for consistency on purpose.
Open chapter →Typeahead / Autocomplete
Ranked suggestions on every keystroke, back in under 100 milliseconds. A latency problem in a search costume — a trie with top-K precomputed at every node.
Open chapter →Ad Click Aggregator
A firehose of clicks counted and deduplicated in near-real-time — and these counts bill advertisers. Think pipelines, not request-response; promise effectively-once, not exactly-once.
Open chapter →YouTube
A huge binary payload that never touches your app servers — straight to object storage and out through a CDN. The depth lives in transcoding and adaptive bitrate streaming.
Open chapter →Messages arrive instantly, in order, without duplicates — and still land when the recipient is offline. Millions of persistent connections, routed to whichever server holds the socket.
Open chapter →Distributed Job Scheduler
Something must happen later — reliably, maybe a billion times, surviving machines that die mid-execution. Reframe before you're cornered: at-least-once plus idempotency is effectively-once.
Open chapter →Eleven chapters · two foundations and nine patterns · one repeating cadence. Pick the pattern, run the loop, sound like you've shipped it. Start with The Pattern Atlas.