In an era dominated by subscription models and constant internet connectivity, we wanted to build a reading application that returns ownership to the reader. The digital reading landscape is filled with applications that track your metrics, limit your reading privileges, or require constant syncs to cloud servers.

BookFlow is our response. Developed as a high-performance Chrome extension, BookFlow enables readers to catalog, manage, and read their personal library of eBooks (EPUB, PDF) and digital comics (CBZ, CBR) directly within their browser—fully local, completely offline, and with absolute privacy.

The Architecture of local-first reading

One of the main goals of BookFlow was absolute data sovereignty. Most ePUB and comic viewers rely on server-side processing to render books or unpack compressed files. This means your personal libraries are transmitted and hosted on third-party servers.

BookFlow achieves its 100% offline-first capability through modern browser mechanics:

Chrome's unlimitedStorage API: Allows users to save gigabytes of reading files directly in their local profile folder, avoiding memory limits and ensuring that your library is stored locally.
IndexDB Cataloging: Metadata, book titles, tags, custom reviews, and current reading progress percentages are indexed in a local high-performance database inside your browser profile.

Secure Decompression via WebAssembly (WASM)

A technical challenge arose in the handling of digital comic book files. Comics (.cbr and .cbz formats) are standard archives (RAR and ZIP) containing high-resolution JPEG or PNG page files. While zip files can be decoded in standard Javascript with ease, RAR decompression (CBR format) is computationally expensive and complex.

To resolve this, we compiled the high-performance C libraries libarchive and unrar into WebAssembly (WASM). When a user drops a comic file, it is decompressed inside a sandboxed environment:

// Isolated sandbox execution inside Chrome extensions
const readerWorker = new Worker('libarchive-worker.js');
readerWorker.postMessage({ fileData: rawBuffer });

This keeps all compute-intensive decompression operations away from the main thread, maintaining 60 FPS transitions, page scaling, and page flips. More importantly, it meets the strict Content Security Policy (CSP) rules of Chrome Extension Manifest V3, keeping your browsing experience safe and secure.