# 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`:

```go
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:

```go
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.