# Design a Mobile Reading App: Kindle-Style Sync and Annotations

Source: https://www.techinterview.org/post/3233475098/design-mobile-reading-app-kindle/
Updated: 2026-06-30 · techinterview.org

Designing a Kindle-style mobile reading app is a layered system design problem. Format rendering, position sync across devices, offline downloads, annotations and highlights, library organization — and the unique constraint of preserving battery during multi-hour reading sessions on a phone or e-reader.

## Functional requirements

- Browse a personal library of purchased and free ebooks

- Read ePub or proprietary format with reflowable text

- Sync reading position across devices

- Highlights, notes, bookmarks, and dictionary lookup

- Search within a book and across the library

- Adjust font size, brightness, theme

## Architecture

Three modules: **library** (purchases, downloads), **reader**, **sync**.

## Format and rendering

Most reading apps use ePub (open format) or proprietary formats derived from it. ePub is essentially HTML + CSS in a zip, with metadata.

Rendering on mobile:

- Parse ePub into chapters and assets

- Apply user-controlled CSS (font, size, line height)

- Lay out into pages — page boundaries shift as user changes settings

- Cache rendered pages for fast scrolling

## Position sync

The challenge: page numbers are not stable across devices. A page on a small phone is different from a page on a tablet. Solution: track position by character offset within the book, not by page.

Sync flow:

- Reader periodically (every 30s or on app background) pushes position to server

- On app launch, fetch latest positions across all devices

- If multiple devices show different positions, default to the most-recent (with HLC for clock-skew tolerance)

- Surface a UI prompt: "Continue from page X on iPad?"

## Annotations and highlights

Each annotation is anchored to a character range. Stored as:

- Book ID

- Start offset, end offset

- Highlighted text (for display when range fails to resolve)

- Color, note text

- Created/updated timestamps

Synced across devices like position. Server is the source of truth; local SQLite caches recent annotations.

## Offline downloads

Books are downloaded once, stored on disk encrypted with a per-device key (DRM). User can have multiple devices in sync. Cloud-side library lists all owned books; the device chooses what to download.

Storage budget: typical user has 50–500 books. Most are 0.5–10MB. Reasonable footprint.

## Search

Within a book: local FTS5 (SQLite full-text). Within the entire library: server-side search with snippets returned. Combined UI shows local + server results.

## Reading-time battery

Reading sessions are long. Battery optimizations:

- Pre-render upcoming pages off-thread; render visible page from cache

- Disable background sync during reading

- Lower display brightness with auto-detect of ambient light

- For e-readers: e-ink draws nearly zero power between page flips

## The "where am I in the book" UX

Page numbers are unstable; books often show:

- Percentage complete

- Time-to-end (based on user reading speed)

- Chapter title

- Location ("Loc 2547" — Kindle's position-based addressing)

## Frequently Asked Questions

### Why do annotations sometimes shift on Kindle?

Updates to the book file change byte offsets. Annotations may be re-anchored heuristically; rarely they end up off by a page or two.

### How does dictionary lookup work?

Bundled dictionary database (often Webster's). On long-press, lookup is local — no network needed.

### Why is e-ink different from LCD for reading?

E-ink uses ambient light, displays static images with zero power, easier on eyes. LCD emits light, faster refresh, full color. Tradeoffs are real — many serious readers own both.