EPUB CFI & Annotation Sync

Highlights that disappear when a reader switches devices. Bookmarks that reopen at the wrong paragraph. Last-read positions that jump backward every time a book is opened. These are not rare edge cases. They are what happens when annotation systems are built without a solid understanding of how EPUB actually anchors content. EPUB CFI & Annotation Sync is a practical production handbook for engineers building reading applications, annotation platforms, and ebook distribution systems. It explains the EPUB Canonical Fragment Identifier from first principles: what a CFI is, how it is generated and resolved, why it fails, and how to design systems that remain reliable under real-world conditions such as reflow, DOM changes, content updates, offline use, and multi-device synchronization. Inside, you’ll find: The complete CFI grammar: steps, assertions, offsets, indirection, and ranges Generation and resolution algorithms with working code Stability analysis: what actually invalidates a CFI — and what does not Production-ready data models for highlights, bookmarks, and last-read positions Cross-device synchronization using G-Set CRDTs and LWW Registers Offline-first architectures with outbox queues, retries, and backoff Conflict-resolution policies designed around real user expectations Storage, indexing, and query strategies capable of scaling to 100 million annotations Testing techniques including round-trip, property-based, and chaos testing A complete case study of a production-grade annotation service This is not an introductory tutorial. It is written for engineers who already ship software and need to understand the trade-offs, failure modes, and architectural decisions behind robust EPUB annotation systems. Every chapter ends with a production checklist. Technical…