StockFlowStockFlow

The problem

Tracking sales, stock and outstanding balances for several sellers by hand falls apart quickly: the stock no longer matches reality, totals depend on who does the maths, and nobody knows exactly who owes what. I wanted an application where those numbers cannot drift apart — including when someone actively tries to make them drift.

1. 01

   Authorisation lives in the database, not the interface

   Every table carries its own access policies, every write function opens with a guard, and direct writes are revoked. The test I apply before adding any check: could the whole interface be deleted without opening a hole? If the answer is no, the check sits in the wrong place.

2. 02

   No business calculation in TypeScript

   Totals, costs, margins, debts, stock checks: all computed in SQL. Amounts shown while typing are indicative only — the one that counts is recalculated by the database. Without that rule a seller could submit a total that suits them, and two implementations of the same calculation would drift apart anyway.

3. 03

   No access key in the browser

   The environment variables carry no public prefix: they are read at runtime and never baked into the code sent to the client. Verified rather than assumed — the 36 scripts loaded by the page were inspected one by one, and the Docker image holds no secret.

4. 04

   An access rule that called itself

   The policy protecting the profiles table calls a function that must read that very table. Read normally, the policy re-triggers inside itself and the query spins forever. The function is therefore declared to run with its owner's rights, letting it read the table without going back through the access check.