Event-Driven Contracts

An event name can look clear while carrying different meanings to different consumers. Suppose the bookstore publishes an order.accepted event. Its fulfillment service assumes the inventory service has reserved stock, while its notification service assumes only that the order exists. Both parse the same JSON successfully, but they disagree about what happened.

An event-driven contract must settle that disagreement. It defines the fact an event represents, when it becomes publishable, what data consumers can rely on, and how they recover from duplicates, delays, and missing history.

This chapter develops those decisions using a fictional bookstore order lifecycle.