SKUantify

Inventory tools almost universally let somebody type over a quantity. That one decision is why stock figures stop being trusted: once a number can be edited, nobody can say how it got there, and the argument about whether the shelf or the screen is wrong has no evidence on either side. We wanted a system where the number is an account of itself.

1. 1

   Made the transaction ledger the source of truth and the stock level a cache derived from it, so a quantity cannot be set — only moved.

2. 2

   Handled concurrency in the database rather than in application logic: partial unique indexes stop two open counts on one location, and row locks serialise role changes so a business cannot lose its last owner.

3. 3

   Scoped staff to their own locations on reads as well as writes, so a location filter is a boundary rather than a convenience.

4. 4

   Made stock counts set balances to what was counted rather than applying a difference, so a sale partway through a count cannot be charged twice.

5. 5

   Wrote the test suite as verification scripts that run against a real database — fifteen suites covering the ledger, authorisation, concurrency and plan limits.

6. 6

   Built the Android app around the jobs that happen standing up, and left reports and administration on the web where the keyboard is.