People who self-host their ebook library with Calibre-Web and read on KOReader can finally sync reading position, but highlights and notes still live on-device or require a separate desktop app plus manual plugins, and progress desyncs across different file formats of the same book. The niche wants one system where annotations and progress follow the reader everywhere and surface back in the web library, without a cloud subscription.
builder note Do not rebuild progress sync, KOSync already won that. The real gap is a normalized annotation model that survives a book changing formats and renders in the web reader. Build the highlights layer on top of the existing KOSync protocol rather than forking it.
landscape (4 existing solutions)
Reading-position sync is basically a solved, commodity piece now. Unified highlights and notes sync that survives format changes and surfaces inside a self-hosted web library is not, so readers still glue together three or four separate tools to approximate it.
KOReader Sync Server (KOSync) Syncs reading position only, and only between KOReader instances. No highlights, no notes, and nothing for Kindle or web readers. Calibre-Web-Automated Recently added KOReader progress sync, but not highlights or notes, and reading position desyncs across epub, kepub, and pdf copies of the same title. KOHighlights A separate desktop app for viewing and exporting KOReader highlights. Manual, not integrated into your web library, and not tied to progress sync. Readwise Polished highlight syncing but cloud-only, subscription-priced, and built around Kindle and web. Not your self-hosted KOReader library. sources (2)
self-hostedebookskoreadercalibre-webannotations
People who self-host Nextcloud want true automatic two-way folder sync on Android, edit a file on either side and both converge with conflicts handled. Nextcloud's own client still only does one-way auto-upload after a decade, so users cobble together a paid app, an opinionated one, or a discontinued peer-to-peer tool. The unmet need is a polished, free or cheap, per-folder-configurable WebDAV sync client people can actually trust with their files.
builder note The trap is treating this as a sync-engine problem. EasySync already syncs fine, people still complain because nobody nailed trustworthy conflict resolution and per-folder direction control. Win on transparency and control over what moves where, not on the sync loop itself.
landscape (4 existing solutions)
Every option is a compromise, paid, opinionated, peer-to-peer, abandoned, or a different protocol entirely. The official Nextcloud client has punted on this for ten years, so demand keeps flowing to imperfect third parties and nobody has shipped the trustworthy, flexible, low-cost version users keep asking for.
EasySync Does true two-way sync but costs 2.99 dollars, is opinionated about what it syncs, gives no control over per-folder direction or custom local-to-remote folder mapping, and users report reliability problems. FolderSync Generic WebDAV tool, not Nextcloud-aware, and genuine automatic two-way sync is locked behind the paid Pro tier. Syncthing (and Syncthing-Fork) Peer-to-peer protocol, not WebDAV or Nextcloud, and the official Android app was discontinued in December 2024 leaving only a community fork. DAVx5 Mounts WebDAV and handles contacts and calendar well, but file access is an on-demand mount, not real offline two-way folder synchronization. sources (2)
self-hostednextcloudfile-syncandroidprivacy