Domain event represents an meaningful business event that occurred in the system. It is a message that back-end system knows how to process and that will change the state of the system.

They are preferred way of manipulating data instead of simple CUD operations (create, update, delete). Unlike aggregate domain event which is tied to a change in a single aggregate root, domain event should be used when an action will result in modifications to multiple aggregates, external call (like sending an email) or some other action.

By default event will be applied immediately. If async is used, event will be stored immediately but applied later.

DomainEvent is defined in DSL with keyword event.

module Todo {
  aggregate Task;
  event MarkDone {
    Task task;
  }
}