Registering Event Schemas

To have the database validate events of a type, implement the Schema function on the event and return a JSON schema. This makes the event a SchemaProvider:

func (BookAcquired) Schema() map[string]any {
  return map[string]any{
    "type": "object",
    "properties": map[string]any{
      "title":  map[string]any{"type": "string"},
      "author": map[string]any{"type": "string"},
      "isbn":   map[string]any{"type": "string"},
    },
    "required": []string{
      "title",
      "author",
      "isbn",
    },
    "additionalProperties": false,
  }
}

The Evolve function collects the schemas of all events that implement Schema. To get them as a slice of EventSchema, each with the fields EventType and Schema, call the Schemas function on the state. Then hand them over to the RegisterSchemas function of the store:

err := store.RegisterSchemas(bookState.Schemas())
if err != nil {
  // ...
}

RegisterSchemas accepts the schemas of several states at once. Event types that are already registered count as success, so you can call the function on every start.