# event-schema-already-registered

The event type **has a schema already**, which can be neither replaced nor removed.

- **Routes:** `register-event-schema` and `restore-backup`
- **Description:** `event schema is already registered: a schema can be neither replaced nor removed, so a different one needs a new event type`
- **Details:** When registering a schema, **`value`**, the event type as the request gave it. When restoring a backup, **`part`**, the part of the backup that brings the second schema, counting from 0, with the full backup as part 0, and **`value`**, the event type.

## Why It Happens

An event type has at most one **[schema](/learn/tutorials/first-steps-with-eventsourcingdb/registering-event-schemas)**, which applies to all of its events and can be **neither replaced nor removed** once it is registered. Registering a schema for a type that has one is refused, **even if it is the same schema**, for example when a service registers its schemas each time it starts.

When restoring a backup, the backup brings **two different schemas** for the same event type. Every part of a backup brings every schema again, and the same schema is taken once.

## How to Fix It

To change the schema, introduce a **[new event type](/docs/eventsourcingdb/event-types#versioning)**, such as `io.eventsourcingdb.library.book-acquired.v2`, with a schema of its own. To make sure that a schema is registered, read the event type with `read-event-type`, which answers with its schema, and register one only if it has none. Sending the same request again fails the same way.

## Example

To register a schema for `io.eventsourcingdb.library.book-acquired`, which has one already, EventSourcingDB answers with this error, as a line of the stream, and then ends the stream with its `end` item:

```json
{
  "type": "error",
  "data": {
    "code": "https://docs.eventsourcingdb.io/errors/event-schema-already-registered",
    "description": "event schema is already registered: a schema can be neither replaced nor removed, so a different one needs a new event type",
    "details": {
      "value": "io.eventsourcingdb.library.book-acquired"
    }
  }
}
```