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, 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, 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:

{
  "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"
    }
  }
}