simulations

Draw a system.
See if it survives.

A whiteboard where the boxes are real. Sketch an architecture, flip one switch, and a deterministic simulator runs it — real queueing, real saturation, a real bill.

Open the canvasTake on a problem

Free · no account needed · everything runs in your browser

50 rpsreadswritesclientsapip99 212mscachehits 91%orders db84% busysimulating

Draw a system

An Excalidraw-fast canvas where the boxes are real: place servers, queues and databases, wire them, and flip one switch to run the drawing.

Open the canvas →

Take on a problem

You inherit a system that works. Then the world turns up — traffic, failures, one enormous customer — and every promise you made still has to hold.

Pick a problem →

Break it on purpose

Inject packet loss, kill a zone, pause a process — with a hypothesis attached, so the run proves something instead of just hurting.

See how it fails →

The problems

Each one is a ladder: the system you are handed passes stage one, and every stage after it turns the world up. What you earn stays earned — or goes red as a regression, in front of you.

The work that happens after you say yesUploads arrive, you accept them immediately, and something behind the scenes does the expensive part. The user is told it worked before it has.Start climbing →Checkout, on the worst day of the yearYou own the checkout path for a marketplace. A shopper hits Pay, you write a capture to your ledger, and you call a payment processor that actually moves the money.Start climbing →One customer, forty times the size of the restYou run a shared API for a few hundred businesses. One of them is enormous, most of them are tiny, and all of them are on the same queue.Start climbing →A feed that everybody reads and almost nobody writesYou serve a public activity feed. Reads outnumber writes a hundred to one, every read hits the same database, and the database is the most expensive thing you own.Start climbing →

The library

every article →

Not everything can be simulated — some things have to be explained. A topic is a map you descend: parts, then articles, then sub-articles, as deep as it takes, with references at the end of every item.

The life of a request25 itemsFrom the moment a request leaves the browser to the moment the response lands: name resolution, the handshakes, the edge, the front door, the cluster, the work, and the way home. Click any box with a ＋ to go deeper.Start reading →Keeping the line open9 itemsRequest/response ends the conversation on purpose. Everything here — polling, long-polling, SSE, WebSockets, gRPC streams, QUIC, webhooks — is a different answer to one question: how does the server speak when it has something to say?Start reading →Nudges, at scaledeepdive21 itemsThe system behind every tooltip, push and "come back": targeting, delivery, templating, measurement — derived from first principles, every approach followed to where it breaks, from one user to a hundred million.Start reading →

Never “failed”. Always the number.

Every claim you make is re-checked on every edit, against a live simulation of the drawing on your screen — nothing is precomputed, and there is no run button. A promise from an earlier stage never leaves the list, so breaking stage one to pass stage three happens where you can see it.

p99 stays under 800 ms412 ms ✓
nobody is charged twice0 duplicates ✓
the bill stays under $68$91 ✗regression — stage one earned this
Deterministic — the same design and the same seed give the same answer, byte for byte. Local — the engine is a 140 KB WebAssembly binary running in your tab, and your drawings live in your browser. Yours — sign in if you like; nothing here is behind it.