Domain events represent events that are relevant to the business domain. They usually emerge from business use cases and have specific domain semantics. Examples for domain events are UserLoggedIn, ActiclePurchased, or CoffeeBrewFinished.

Domain events are typically implemented as value objects containing the required information. In Java, we realize events as immutable POJOs. Events happened in the past and can't be changed later on, so it is highly recommended to make them immutable. As seen before, we can use the CDI events functionality to publish and observe events with loose coupling. In CDI, all Java types can be used to be published as events. The concept of domain events is therefore a business definition rather than a technical one.

Domain events are particularly important for event sourcing and event-driven architectures, which we will extensively discuss in Chapter 8, Microservices and System Architecture.