What went wrong?

When two Kobo devices are registered for the same user account (each with its own device endpoint/token), only the first device to sync receives the library. The second device completes its sync without errors but gets zero books — and never receives any afterwards.

The cause appears to be that Kobo sync progress is tracked per user instead of per device:

• kobo_library_snapshots has a UNIQUE constraint on user_id (server/src/db/schema/kobo.ts), so all devices of a user share a single snapshot.
• Each book in kobo_snapshot_books has one boolean synced flag. When device A syncs, KoboSyncService.getDelta pages through unsynced books and marks them synced = true (server/src/modules/kobo/services/kobo-sync.service.ts). When device B syncs afterwards, the pending query finds nothing and returns only collection tags — no entitlements.
• The incoming x-kobo-synctoken header is logged but never used in getDelta (server/src/modules/kobo/kobo-sync.controller.ts), so the server can't distinguish a fresh device from a fully-synced one. The device token itself is only used for auth and URL building, not for sync state.

There is already partial handling for this: KoboDeviceService.createDevice calls invalidateSnapshot when a second device is registered, which resets all synced flags. But that's a one-shot, user-global reset — whichever device happens to sync first afterwards consumes the entire delta again. Since Kobos auto-sync on wake/Wi-Fi, the already-synced device usually wins this race, and the new device stays empty.

This also affects every later snapshot invalidation (library changes, settings changes): only one of the user's devices ever receives each delta.

Steps to reproduce

1. Create two Kobo devices for the same user under Settings → Kobo, and configure each Kobo e-reader with its own endpoint URL.
2. Sync Kobo A → all books arrive correctly.
3. Sync Kobo B → sync completes without error, but no books appear.
4. Any further syncs on Kobo B also return nothing (verified: GET /v1/library/sync returns an empty entitlement list because all kobo_snapshot_books rows are already synced = true).

Expected vs actual behavior

Expected: Each registered device independently receives the full library on its first sync, and subsequent deltas (added/changed/removed books) reach every device.

Actual: Only the first device to sync after snapshot creation/invalidation receives the books. All other devices get an empty delta, permanently.

Your setup

• BookOrbit: bookorbit:latest (1.9.0)
• Deployment: Docker (docker-compose, official image)
• OS: Ubuntu
• Devices: 2× Kobo e-readers

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