Where money moves
Who may press a button, what a retry costs the receiver, and one app that runs two shops.
Practice4 lessons26 min
- 1Who May Press This — Hiding a Button Is Not AuthorisationTwo people open the same screen. The void button is visible to both. When the assistant presses it the server refuses, and the refusal goes into the ledger. The screen file is the same file, and we printed its hash into the log.6 min
- 2Told Yes Twice — What One await DecidesTwo people take the same slot at the same moment. Described in words, the two handlers do exactly the same thing — look whether it is free, and if it is, take it. One of them confirms to both people; the other does not. The difference is one await in the middle.7 min
- 3What Retrying Costs — Not the Side Calling, the Side ReceivingWriting about retries is mostly written from the caller's side. This piece counts from the receiving side. Four callers got through a 700-millisecond outage two different ways, and both eventually succeeded. What differed is how many times the gateway was hit meanwhile — 64 and 25.7 min
- 4One App, Two Shops — If the Second Shop Is a Code Change, You Have LostTwo cafés share the same counter screen. Prices, hours and menus differ, and one quotes tax-inclusive while the other adds it at the counter. Not one character of either shop's name appears inside the bundle, and the verification protects that with a single grep.6 min