Defining Events

An event describes what has happened. Define it as a struct with JSON annotations and implement the EventType function, which returns the event type.

This makes the struct an Event:

type BookAcquired struct {
  Title  string `json:"title"`
  Author string `json:"author"`
  ISBN   string `json:"isbn"`
}

func (BookAcquired) EventType() string {
  return "io.eventsourcingdb.library.book-acquired"
}

type BookBorrowed struct {
  BorrowedBy    string `json:"borrowedBy"`
  BorrowedUntil string `json:"borrowedUntil"`
}

func (BookBorrowed) EventType() string {
  return "io.eventsourcingdb.library.book-borrowed"
}

type BookReturned struct{}

func (BookReturned) EventType() string {
  return "io.eventsourcingdb.library.book-returned"
}

The struct becomes the event's data. The subject is taken from the command, and the source from the store. The database checks every event against the JSON schema of its type, which the kit derives from the struct, unless the event has a Schema function of its own, once the schema is registered (see Describing Events with Schemas and Registering Event Schemas).