You publish the OrderPlaced event. On day one, there is one consumer, and it’s yours. Two years later, there are eleven, owned by four teams, two of which you have never spoken to, and one of which is a third party with a contract. Now change the event.

Event-driven architecture is sold on decoupling, and it delivers - that’s exactly the problem. With an HTTP API, you can see your callers, measure them, deprecate with a header, and find the two who didn’t migrate. On an event bus, your consumers are invisible by design: the property you bought is the property that stops you from changing anything, and nobody mentions that at the point where you’re drawing the boxes. This session covers the part of event-driven architecture that only shows up in year three - what a breaking change actually is when the contract is an event, four versioning strategies and what each one really costs you and more.