01 — The problem
What has to stop going wrong.
The numbers stop agreeing. Somewhere between two systems, or two concurrent requests, a value goes missing and nobody finds out until an audit.
02 — What we build
The system behind the surface.
Money as decimals end to end and as strings across the API. Ledgers as the source of truth with balances as a cache. Row locking where concurrency actually bites. Immutable posted entries enforced by database triggers.
01
Bound it
Define the failure modes, permissions and consistency rules before the interface hides them.
02
Build it
Ship production code in your repository, in phases with a written exit state.
03
Try to break it
Test the paths that carry money, identity, isolation and irreversible decisions.
03 — Decision material
Know whether it fits before a call.
A strong fit
- Payments, inventory, balances or other coupled records
- Migrations where historical data must remain explainable
- Systems exposed to retries, concurrency or partial failure
What leaves with you
- Consistency and ownership map
- Ledger, reconciliation or invariant design
- Reversible migrations and recovery path
- Concurrency, idempotency and failure tests
04 — Evidence
A claim should lead somewhere.
A checkout that writes the sale, the stock, the cash, the tax and the ledger together or not at all.
Shown in Nexora AI — Retail and trading