Try to breakbreakbreak my backends.

Two ATMs withdraw 60 from an account holding 100 at the same instant. Correct result: one succeeds, one is refused. The database interleaves their statements however it likes.

1. Race with Read, then write. Usually 120 comes out of a 100 account.
2. Pick either my code option and race again. Never more than 100.

How the withdrawal is written

common bugRead, then writeSELECT the balance, check it, UPDATE with the value you read my codeSELECT … FOR UPDATELock the row, so the second reader waits for the commit my codeConditional UPDATECheck and write in one atomic statement

The server charges the card, then the response is lost on the way back. The client times out and retries, exactly as a well-behaved client should. Correct result: one charge.

1. Turn the key off (common bug) and send. You get charged three times.
2. Turn it back on (my code) and send. One charge.

Send an Idempotency-Key my codeOff is the common bug: every retry looks like a new payment Change the amount on retryReuse the same key with a different body Responses lost in transit

2

Split a bill, then add the shares back up. The left column stores money as a JavaScript number, the common bug. The right column is my code: whole cents as integers. Correct result: the shares always add up to the total.

1. Pick an example. The common bug column loses or invents a cent.
2. My code column always adds up exactly, and refuses amounts like 1.005 instead of rounding them.

Every entry stores the SHA-256 of the one before it. You are an attacker with database access: edit an amount, delete the newest row, or re-hash the whole chain. Correct result: every change is detected.

1. Turn the checkpoint off (common bug), delete the newest row. Verification still says OK.
2. Turn it back on (my code). The same tampering is caught.

Verify against a published checkpoint my codeOff is the common bug: a truncated or re-hashed log still verifies

Alice opened INC-1042. The rule: two people other than Alice must approve the current evidence before it can be resolved. You can act as anyone.

1. With enforcement off (common bug), as alice: investigate, attach evidence, contain, approve, resolve. She closes it alone.
2. Turn enforcement on (my code) and try the same. The server refuses.

Enforce the rules on the server my codeOff is the common bug: “resolved” is just a status field the UI guards

Act as

Aaliceauthor Bbobon call CcarolSRE lead Mmalloryinsider