Shopify replaced Redis with MySQL for inventory reservations–and it scaled

Shopify successfully migrated inventory reservation systems from Redis to MySQL by redesigning around MySQL 8's SKIP LOCKED feature, enabling them to handle record-breaking scale (5.1M transactions/minute on Black Friday 2025) while maintaining ACID compliance and eliminating cross-system consistency failures. This unified database approach reduces operational complexity, improves data integrity between reservations and inventory ledgers, and demonstrates that traditional databases can match specialized systems when architecturally optimized. The key lesson extends beyond technology: the real bottleneck often isn't what teams initially measure, requiring deeper observability and willingness to challenge assumptions.