Real-time inventory sync is an architectural system that propagates stock changes instantly (or nearly instantly, within milliseconds to seconds) across all sales channels, fulfillment centers, and inventory databases. When one system updates inventory (warehouse receives shipment, customer purchases), every connected system sees the update immediately—preventing double-selling and ensuring consistent stock views. Implementation involves message queues (Kafka, RabbitMQ), event streaming, database replication, caching layers (Redis), and idempotency patterns to handle retries without duplication.