Each lesson is the complete system design of one real company — Netflix, WhatsApp, Ticketmaster, Visa — drawn piece by piece, diagram by diagram.
a typical system · every lesson draws a real one, piece by piece
The complete system design of one real company per lesson, sector by sector — every piece drawn and explained.
Everything an AI engineer needs to build solid apps — AI engineering, architecture, backend, data, infra, reliability, security — one concept per topic, in diagrams.
Chapter 1 of every lesson is open — no account needed. New systems every week.
DNS, TCP, HTTPS, CDNs and servers: one request followed from your phone to a data center and back.
The complete design: one open connection per phone, store-and-forward mailboxes, receipts as a state machine, a media road, end-to-end encryption — and how a team of about 50 ran it for 900 million people.
Two systems in one: a brain on AWS that decides what you watch and where it comes from, and its own CDN inside your internet provider, filled every night — plus per-title encoding, failover practiced on purpose, and 65 million live streams.
A network, not a bank: four parties and VisaNet get your bank's yes or no to the shop in seconds — with a ten-second deadline and a stand-in when the bank is silent — while the real money moves later, in files, netted and guaranteed. Plus tokens, and the day one switch broke Europe's payments.
A read-mostly machine: over a thousand reads for every edit, so seven data centers answer from cache while MediaWiki and sharded MariaDB only build what changed — plus how an edit purges every copy, and PoolCounter, born the day Michael Jackson died.
A funnel in front of one record per seat: Verified Fan filters fans days before, Smart Queue lets a few hundred in at a time, the core engine holds your seat on a timer and sells it once, and SafeTix makes the ticket impossible to copy — with the Eras Tour presale as the day it was all tested.
Pick a sector to see its companies — or start with the fundamentals.
The building blocks every system is made of, from the basics to the advanced.
Chat, calls and real-time delivery to billions of phones.
Feeds, follower graphs and the celebrity problem.
Streaming, encoding and getting bytes close to every viewer.
Millions of players, live, at the same time.
Moving money: authorizations, ledgers and never charging twice.
Exchanges, banks and systems where a millisecond is money.
Carts, inventory and surviving the biggest sale day of the year.
Selling a fixed number of seats to millions of people at once.
Search, availability and bookings across the world.
Matching riders, drivers and orders in real time, on a map.
Indexing the world and answering in milliseconds.
The storage, databases and schedulers everything else runs on.
Protecting systems — and what happens when protection breaks.
Collaboration, sync and the tools developers live in.
Each system in 4–8 short chapters, one screen at a time — the whole picture first, then each part. A bar shows how far you've come.
Hand-drawn diagrams you play with Next: follow a message, a payment or a click through the system. Tap any piece to see why it's there.
A quick question closes every chapter. Finish the lesson to see what you now know — and what to open next. Your progress is saved.
AI can write a service in minutes. It can't tell you which pieces you need, where your system will break at a million users, or what each choice will cost you a year from now.
Learn from systems that already work at the largest scale on Earth, with the same five questions every time: what must it do, how much load, where does it break, what do you give up, what happens next.
Open your first system now — chapter 1 is free, no account needed.