How Django's select_for_update Prevents Overselling in E-Commerce Checkouts

Race conditions during checkout can cause multiple orders to be placed for the same last unit of stock, a common problem on moderately trafficked e-commerce platforms. A Django developer addressed this in a single-vendor e-commerce project by separating actual stock from reserved_stock, a field that temporarily holds inventory during an in-progress checkout. Rather than decrementing real stock at checkout, the system only reduces it once payment is confirmed via a Midtrans webhook, preventing phantom availability without risking miscounts from abandoned sessions or failed payments. The solution uses Django's select_for_update() to lock product rows at the database level during validation, forcing concurrent requests to wait rather than read stale data. Consistent row ordering via order_by('id') avoids potential deadlocks, while a single bulk_update call keeps the reservation write efficient.