How Race Conditions Break Stock Updates and Three Ways to Fix Them

Concurrent stock update requests can cause race conditions where multiple operations read the same inventory value simultaneously, pass availability checks, and each record a successful decrement — even when only one unit exists. This flaw means the final stock count may appear correct while the number of successful transactions exceeds actual available inventory. An atomic SQL update, such as decrementing stock directly within a conditional WHERE clause, eliminates the application-level read-check-write gap that creates the vulnerability. PostgreSQL row locking with SELECT FOR UPDATE offers another approach, serializing concurrent transactions so only one can read and modify a row at a time. Testing these scenarios requires validating not just the final stock value but also that the count of successful operations never exceeds initial inventory.