Check Then Charge

Two withdrawal requests for $80 hit the same $100 wallet at the same moment. Each handler reads the balance, sees $100, decides $80 is affordable, and writes the debit. The wallet lands at -$60. What actually fixes this?

Keep a cached copy of the balance in Redis and check that copy before writing, since the cached copy reflects the newest debit before the row does

Put the predicate in the write: UPDATE wallets SET balance = balance - 80 WHERE id = ? AND balance >= 80, and only proceed when a row is returned

Wrap the read and the write in a single database transaction so the pair of statements commits atomically and the second handler cannot see a stale balance

Retry the whole handler with exponential backoff so the losing request re-reads a corrected balance on its second pass through the handler