Defining Events

This makes the struct an Event:

#[derive(Serialize, Deserialize, Event)]
#[event(type = "io.eventsourcingdb.library.book-acquired")]
struct BookAcquired {
  title: String,
  author: String,
  isbn: String,
}

#[derive(Serialize, Deserialize, Event)]
#[event(type = "io.eventsourcingdb.library.book-borrowed")]
#[serde(rename_all = "camelCase")]
struct BookBorrowed {
  borrowed_by: String,
  borrowed_until: String,
}

#[derive(Serialize, Deserialize, Event)]
#[event(type = "io.eventsourcingdb.library.book-returned")]
struct BookReturned {}

The struct becomes the event's data, as serde writes it, so rename_all gives the fields of BookBorrowed the names borrowedBy and borrowedUntil, and BookReturned is written as {}. 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 derive derives from the fields that serde writes, unless the event names a function of its own with schema_with, once the schema is registered (see Describing Events with Schemas and Registering Event Schemas).

The event type is a constant of the trait, so BookAcquired::EVENT_TYPE is io.eventsourcingdb.library.book-acquired. Which events a state evolves by, it lists in an enum of its own, with one variant per event type (see Defining State).