Balance tracked the cash; Position tracks the shares — the same conserving, checked shape, applied to holdings. How balances, positions, orders, users, and markets connect, and why a position deliberately stores no cost basis.

The first brick of the engine, built test-first: a Balance type that splits spendable from reserved funds and, by construction, can never make money appear or vanish. Why the account model comes first, and how it's built.

Where a matching engine's time actually goes, why it needs a write-ahead log, the latency numbers worth burning into your head, and a step-by-step ladder from tens of thousands of orders per second to a million — with real, runnable benchmarks.

The architecture, and why I landed on a single-process, write-ahead-logged, deterministic engine instead of microservices or a Kafka log — plus the MVP-to-production roadmap that grows by adding, not rebuilding.

Prediction markets, YES/NO shares, the order book, and price-time priority — explained the way I wish someone had explained them to me, plus how every piece maps onto Polymarket's on-chain stack.

The opening note of a series where I build a production-grade prediction market backend in Rust, one module at a time — and open-source every line of code and every design decision, completely free, no strings attached.