# event-hash-version-unknown

An event has a **hash of a version** that this version does not know.

- **Routes:** `restore-backup` and `audit-history`
- **Descriptions:** `event hash version is unknown: the backup was written by a newer release, which is needed to restore it` when restoring a backup, and `event hash version is unknown: the event was written by a newer release, which is needed to audit it` when auditing the event store
- **Details:** **`eventId`**, the ID of the event.

## Why It Happens

A **[restore](/docs/eventsourcingdb/backup-and-restore)** checks the **[chain of hashes](/docs/eventsourcingdb/auditing-the-event-store)** of the backup, event by event, and computes the hash of each event again to compare it with the one the event holds. The **`hash`** of an event names its version in a prefix: `esdb:hash:v2:` for version 2, which this version writes, and no prefix for version 1. A hash whose prefix names another version comes from a **newer version of EventSourcingDB**, and this version cannot compute it.

An **[audit](/docs/eventsourcingdb/auditing-the-event-store)** checks the chain of the stored events the same way and refuses such an event as well. It meets one only in a store that came by it otherwise, since EventSourcingDB refuses to start on a data directory that a newer version has written.

Rather than take such an event for one that was changed, which **[`event-chain-broken`](/docs/eventsourcingdb/errors/event-chain-broken)** reports, EventSourcingDB names the version of the hash as the reason. An older version can restore a backup only as long as it can read everything the backup contains, see **[Compatibility](/docs/eventsourcingdb/upgrading#compatibility)**.

## How to Fix It

Restore the backup with the version of EventSourcingDB that created it, or a newer one. EventSourcingDB has removed what it restored so far, so the database is empty again. Audit a store that holds such an event with the version that wrote it, or a newer one. Resending the request to this version fails the same way.

## Example

For a backup whose event `0` has a hash of a version that this version does not know, 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-hash-version-unknown",
    "description": "event hash version is unknown: the backup was written by a newer release, which is needed to restore it",
    "details": {
      "eventId": "0"
    }
  }
}
```