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One Service Goes Down and Everything Breaks? Go Event-Driven.
A progressive guide to event-driven systems — starting from domain events and building up to Kafka, event sourcing, CQRS, and eventual consistency with full Spring Boot implementations at every stage.
Why Events Matter
Your inventory service hiccups for 30 seconds and suddenly nobody can place an order — even though placing an order has nothing to do with checking inventory. That’s the wall every tightly coupled system eventually slams into. In 17 years of enterprise Java in Singapore, every significant system I’ve built ended up event-driven, not because it was trendy, but because the synchronous version stopped scaling and started paging people.
That wall looks like this: your order service directly calls inventory, payment, notification, and analytics services. One service goes down and the entire order flow breaks. You want to add a loyalty points service, and now you’re modifying the order service — a service owned by a different team, deployed on a different cadence, with its own test suite you don’t fully understand.
Events solve this by inverting the dependency. The order service publishes what happened. Everyone…
