The observer design pattern describes how an object manages and notifies observers in case of change in the overall state. Observers register themselves at the subject and will be notified later on. The notification of observers can happen in a synchronous or asynchronous way.

As seen before, CDI includes an eventing functionality, which implements the observer pattern. Developers do not need to handle the registration and notification logic themselves; they just declare the loose coupling using annotation. As shown in the topic Core domain components of modern Java EE, the Event<T> type and @Observes annotations declare the event publishing and observation. In the topic Flow of execution, we will cover asynchronous CDI events.