A4Idempotency keys

The problem

1 s on screen = 100 ms

A payment times out, so the shopper pays again. Were they charged twice?

Pay, wait, pay again

Shoppers pay again if they hear nothing within 0.4 s. The ledger records every charge it's sent.

Live metrics for each version of the scene
Metric	No key	With a key	Change
	0	0	—
	—	—	—
target ≤ 400 ms	—	—	—

A4Idempotency keys

A retry is only safe if doing the thing twice has the same effect as doing it once.