The query side

So far, we have focused on state transitions in the system that are represented as events. When our domain model emits a new event, by definition, it means that we have put the system in a new state. State transitions are triggered by commands—that's why we have two APIs that are purposefully named ClassifiedAdCommandApi and UserProfileCommandApi. That shouldn't come as a surprise, since I have mentioned CQS and CQRS several times before in this book. Commands trigger methods of the aggregates, and following the CQS principle, none of those methods return anything, but instead change the system state. But no system can have only commands. Our users want to see something on the screen and it can't just be static texts and pictures. For our marketplace, the main purpose of the application is not that people can place classified ads in it; quite...