The Double-Booking Disaster

CRITICAL — financial ledger mismatch

CFO Office: User alice_99 had $1,000. Two concurrent $600 withdrawals both returned SUCCESS. Final balance: $400 — the company lost $600.

The trap: Read-modify-write without checking version. Both threads read $1000, both write $400.

Your task

Fix `withdraw()` in `ledger.py`: use optimistic locking — pass version from get_account() into db.update_balance(); return FAILED on version conflict.

Book: SELECT FOR UPDATE · Prod: version column optimistic lock without row locks.