When event-driven systems grow past a handful of services, the biggest failures usually are not infrastructure failures. They are contract failures . A producer adds a field and a consumer crashes. A team renames an enum value and downstream processing silently misclassifies events. A “minor change” ships without coordination and turns into a production incident. In this post, I will walk through how I design a contract-first event-driven architecture on AWS with a focus on: Event versioning strategies Schema registry usage Consumer tolerance patterns Breaking vs non-breaking changes Governance for event contracts I will also include an end-to-end walkthrough , implementation discussion , architecture , and code examples that show how I typically structure this in practice.…