02IdempotencyArticle →
Retry storm
The phone lost signal and retried. The customer bought one ticket.
tap to buy
Buy one ticket. The signal is bad.
Where moneygoes missing.
Every way a payment system loses money, as a game you can break and an article that shows the real fix.
01ConcurrencyArticle →
Double-spend
A $100 balance. Fire payments at it faster than it can count.
Never existed$0
03Money mathArticle →
Ten cents
Add 10¢ a thousand times in floating point. Count how often the total is wrong.
hold to pour dimes
Drop a dime in the jar. Then another.
04State machineArticle →
Refund twice
The button was disabled in the UI. The API was not.
tap to refund
Pay first. Then have both agents refund.
05LedgerArticle →
The scale
Two UPDATEs. The server dies between them.
tap to pay $30
Pay a few times. Then crash.
06WebhooksArticle →
Late mail
Providers deliver events late, twice and out of order. You pick the order.
Deliver envelopes in any order. Make the paid order look unpaid.
07Money mathArticle →
Split the bill
$100 three ways. Where does the last cent go?
Split the bill. Watch the last cent.
3 ppl
08ConcurrencyToy only
Two workers
The payout job was slow, so someone ran a second copy.
tap to run the job
$300 due to six people. Run the job.
These are toys
Read the articles The real versions hit a real database.
Same bugs, run against Postgres with real transactions, real locks and the SQL on screen.