Keeping inventory accurate across Shopify, warehouses, and marketplaces sounds simple. At scale, it is one of the hardest engineering problems in ecommerce. A single API call after each sale works fine at 200 orders a day. At 20,000 concurrent transactions, it collapses. Here is what breaks first: Overselling when two orders hit the same SKU simultaneously Stale counts when a warehouse update takes minutes to reflect Silent failures when a sync call times out with no retry Duplicate decrements when a webhook fires twice These are predictable failure modes of monolithic sync. A distributed architecture fixes all of them. The Four Layers You Need 1. Event Producer Layer Captures inventory change events from Shopify webhooks, WMS, POS, and marketplaces. 2. Message Queue Layer Events land in a durable queue (Kafka, RabbitMQ, SQS). Nothing gets lost. 3. Microservices Processing Layer Dedicated services consume events, apply business logic, push updates downstream. 4.…