IndexedDB version change and custom rollback procedure integration

When to trigger the backup

The safest moment is inside the versionchange transaction’s oncomplete (or success) handler, which fires after the transaction is granted but before any schema modifications are applied. At that point you have a consistent snapshot of all existing object stores.

Steps to capture a backup

1. In the onupgradeneeded handler (or the versionchange transaction’s oncomplete), open a separate backup database (different name or origin).
2. Iterate each object store in the source database using a cursor (to avoid memory spikes) and put each record into the corresponding store in the backup database.
3. When the backup transaction completes, allow the upgrade to proceed.

Interpreting a VersionError for restoration

A VersionError should be treated as a rollback cue only when the error originates from an attempt to open the database with a version number lower than the current one. Check event.target.error.name === 'VersionError' and compare event.target.result?.version (or the requested version passed to indexedDB.open) against the known current version. If the requested version is lower, invoke your restore routine; otherwise treat the error as a concurrent upgrade conflict and retry or abort.

Reliable snapshot mechanisms across browsers

• Export each store to a separate IndexedDB database (different name or same origin but distinct name) – works in Chrome, Firefox, Safari, Edge.
• Serialize records to JSON and store them in localStorage or sessionStorage as a fallback; be aware of the 5 MB quota per origin.
• For larger datasets, use cursor‑based chunked copying or compress the JSON before writing to localStorage or to a backup IndexedDB database.

These mechanisms give you a durable snapshot that can be read back and re‑inserted after clearing the original stores.