The average price lies
Most stock systems answer «how many» and shrug at «how much». They keep one average price per item, and the moment purchase prices drift, that average quietly stops matching reality. The margin on every order is off by a few percent and nobody can say why.
Crate keeps the layers. A receipt creates a lot with its own price; a shipment eats the queue from the oldest end and writes down which lots it took. Open any order and you see the arithmetic: four screws from the June lot, one from late June, eight from July, and what the same line would have cost under a weighted average.
What is inside
Real FIFO
Lots are read with row locks and consumed in receipt order, so two simultaneous orders can never spend the same stock twice. A shipment beyond the balance is never half-posted: the whole document rolls back.
Cost, line by line
Every shipment line stores its layers: lot, date, quantity taken, purchase price. Beside it sits the weighted-average alternative and the difference between them, so you can see exactly how much the average would have lied.
An immutable ledger
A posted document is never edited. A mistake is fixed by a reversal that returns the goods to the very lots they came from. A receipt whose lot has already been shipped cannot be reversed: that would be a lie about the cost of orders already made.
Transfers keep the age
Moving stock between warehouses recreates the same layers with their price and receipt date. Otherwise the FIFO queue would reset on every transfer and six-month-old stock would look fresh.
Stock counts that mean something
The sheet snapshots the expected quantity when it opens, because the warehouse keeps working while somebody walks the aisles. Surplus is received at the average cost of the remainder, shortage is written off through the queue. Two documents instead of one with a sign.
Integers and verification
Money in kopecks, quantity in thousandths of a unit, with no floats in a system whose whole job is that the numbers add up. The crate:verify command recomputes balances from the ledger and compares them with the lots every night.
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