We replaced Redis with MySQL for inventory reservations—and it scaled

During checkout, when a buyer clicks "Complete purchase," we need to guarantee the items they're buying are still available. If we get this wrong in one direction, two buyers purchase the same last unit: the merchant has to cancel an order, send an apology email, and eat the support cost. If we get it wrong in the other direction, we tell a buyer something is sold out when it isn't, and the merchant loses a sale they should have made. At Shopify's scale, either failure compounds fast. On Black Friday 2025, merchants on our platform hit a record $5.1 million in sales per minute at peak. Every one of those transactions touches inventory. Our oversell protection system handles this by reserving inventory during payment processing—a short hold that prevents two concurrent checkouts from claiming the same unit. For years, this ran on Redis. When we moved toward a unified database strategy, we had to answer a hard question: could MySQL handle the same scale? Earlier attempts had failed. A single row with a quantity column couldn't handle the contention. MySQL 8's SKIP LOCKED feature introduced a different design: one row per inventory unit instead of one row per item. Inspired…