Flash sales. Product drops. Viral moments. They're what every Shopify merchant dreams of — and exactly when race conditions turn that dream into a support ticket nightmare. Let me break down what's actually happening, why Shopify's built-in protections aren't enough, and the specific patterns that fix it. What Is a Race Condition in Shopify? A race condition happens when two or more processes read and write the same resource simultaneously, and the result depends on which one finishes first. In order processing, the classic scenario: Customer A checks stock → 1 unit available → proceeds to checkout Customer B checks stock → 1 unit available → proceeds to checkout Customer A completes purchase → inventory = 0 Customer B completes purchase → inventory = -1 Both reads happened before either write completed. That gap between read and write is where race conditions live. Why Shopify's Defaults Aren't Enough Shopify has built-in inventory tracking and "deny checkout when out of stock" settings.…