Non-functional Requirements

1. Storage

Non-Functional Requirement: Maximum file size NFR-1_1
status: open
priority: P0

Maximum allowed file size shall not exceed 2048 MB (2 GB) per book file.

Acceptance criteria:

  • Files up to 2 GB upload successfully

  • Files exceeding 2 GB are rejected with clear error message

  • Large file uploads show progress indicator

  • Upload can be resumed after interruption (for files > 100 MB)
Non-Functional Requirement: Storage backend support NFR-1_2
status: open
priority: P0

The system shall support multiple storage backends.

Supported Backends:

  1. Local storage (device file system)

  2. Cloud storage: Google Drive, OneDrive, Dropbox

  3. Self-hosted server (MinIO, local file system)

  4. Network-attached storage (NAS via SMB/NFS)

Acceptance criteria:

  • Each backend can be configured independently

  • User can switch between backends without data loss

  • Backend credentials are stored securely (encrypted)

  • Connection status is visible to user
Non-Functional Requirement: Multi-backend configuration NFR-1_3
status: open
priority: P1

The system shall support multiple storage backends per user with automatic failover.

Acceptance criteria:

  • User can configure up to 3 storage backends

  • One backend is designated as primary

  • Automatic failover to secondary if primary is unavailable

  • Manual override to force specific backend
Non-Functional Requirement: File encryption NFR-1_4
status: open
priority: P2

File encryption shall be available for sensitive content with user-controlled keys.

Acceptance criteria:

  • AES-256 encryption for files at rest

  • User-managed encryption keys

  • Encrypted files are decrypted on-device only

  • Key recovery mechanism available

2. Synchronization

Non-Functional Requirement: Online/offline parity NFR-2_1
status: open
priority: P0
links incoming: UC-1_4, UC-7_3

The system shall work in online and offline mode with full feature parity for core reading features.

Acceptance criteria:

  • All reading features work offline

  • Library management works offline

  • Annotations and notes work offline

  • Only sync and cloud features require connectivity

  • Clear indicator of online/offline status
Non-Functional Requirement: Offline change indicators NFR-2_2
status: open
priority: P0

Any changes made offline shall have an appropriate visual indicator.

Acceptance criteria:

  • Unsynced items show sync-pending icon

  • Last sync timestamp visible in settings

  • Number of pending changes displayed

  • User can manually trigger sync when online
Non-Functional Requirement: Conflict resolution NFR-2_3
status: open
priority: P1

Synchronization conflicts shall be resolved with user-selectable strategies.

Resolution Strategies:

  1. Server wins (default)

  2. Client wins

  3. Manual resolution (user chooses)

  4. Merge (where possible)

Acceptance criteria:

  • User can set default strategy in settings

  • Per-conflict override option

  • Conflict history log

  • No silent data loss
Non-Functional Requirement: Sync performance NFR-2_4
status: open
priority: P0

Cross-device synchronization shall complete within 30 seconds under normal network conditions (>= 1 Mbps).

Acceptance criteria:

  • Reading position syncs within 30 seconds

  • Metadata changes sync within 1 minute

  • Book files sync based on file size and bandwidth

  • Background sync does not impact reading performance

3. Platform support

Non-Functional Requirement: Web browser support NFR-3_1
status: open
priority: P0

The system shall be available in modern web browsers.

Supported Browsers:

  • Chrome 90+

  • Firefox 90+

  • Safari 14+

  • Edge 90+

Acceptance criteria:

  • All core features work in supported browsers

  • Progressive Web App (PWA) installable

  • Offline capability via service worker

  • Responsive design for all viewport sizes
Non-Functional Requirement: Desktop operating system support NFR-3_2
status: open
priority: P0

The system shall be available on major desktop operating systems.

Supported Operating Systems:

  • Windows 10 or later

  • macOS 10.15 (Catalina) or later

  • Major Linux distributions (Ubuntu 20.04+, Fedora 35+, Debian 11+)

Acceptance criteria:

  • Native application available for each platform

  • Platform-specific keyboard shortcuts

  • System tray/menu bar integration

  • File association for supported formats
Non-Functional Requirement: Mobile operating system support NFR-3_3
status: open
priority: P0

The system shall work on mobile devices.

Supported Platforms:

  • Android 8.0 (API level 26) or later

  • iOS 12.0 or later

Acceptance criteria:

  • App available in Google Play Store and Apple App Store

  • Touch-optimized interface

  • Support for device orientation changes

  • Background sync support

  • Push notifications (optional)
Non-Functional Requirement: E-ink device support NFR-3_4
status: open
priority: P1

The system shall support e-ink readers and tablets with touch interfaces.

Target Devices:

  • Kobo readers (via sideloading)

  • Android-based e-ink devices (Boox, Hisense)

  • reMarkable (future consideration)

Acceptance criteria:

  • E-ink optimized UI mode (high contrast, grayscale)

  • Reduced/disabled animations

  • Larger touch targets (minimum 48px)

  • Support for hardware page-turn buttons

  • Reduced screen refresh for battery efficiency

  • Dark-on-light and light-on-dark themes optimized for e-ink

4. Performance

Non-Functional Requirement: Application startup NFR-4_1
status: open
priority: P0

Application startup time shall not exceed 3 seconds on supported devices.

Acceptance criteria:

  • Cold start < 3 seconds on mid-range devices (2020+)

  • Warm start < 1 second

  • Library view loads within 2 seconds of app start

  • Progress indicator shown if startup exceeds 1 second
Non-Functional Requirement: Book opening NFR-4_2
status: open
priority: P0

Book opening time shall not exceed 2 seconds for files up to 50 MB.

Acceptance criteria:

  • EPUB < 50 MB opens in < 2 seconds

  • PDF < 50 MB opens in < 3 seconds

  • Progress indicator for larger files

  • Last reading position restored automatically

  • Opening performance measured on mid-range devices
Non-Functional Requirement: Search performance NFR-4_3
status: open
priority: P0

Search results shall be returned within 1 second for libraries up to 10,000 books.

Acceptance criteria:

  • Metadata search returns in < 500ms

  • Full-text search returns in < 1 second

  • Results are paginated for large result sets

  • Search is cancelable

  • Incremental results shown as available
Non-Functional Requirement: Library scalability NFR-4_4
status: open
priority: P1

The system shall support libraries with up to 50,000 books without performance degradation.

Acceptance criteria:

  • Library view loads in < 3 seconds with 50,000 books

  • Search remains under 2 seconds

  • Scrolling remains smooth (60 FPS on capable devices)

  • Memory usage scales linearly with library size

  • Virtual scrolling for large lists
Non-Functional Requirement: E-ink performance NFR-4_5
status: open
priority: P1

The system shall be optimized for e-ink display refresh characteristics.

Acceptance criteria:

  • Page turns complete in < 500ms

  • Minimal full-refresh operations

  • Partial refresh used where possible

  • UI updates batched to reduce flicker

  • Animation duration configurable (default: 0 for e-ink)

5. Usability

Non-Functional Requirement: Design system NFR-5_1
status: open
priority: P0

The user interface shall follow Material 3 design guidelines.

Acceptance criteria:

  • Consistent use of Material 3 components

  • Dynamic color theming support

  • Consistent typography scale

  • Proper use of elevation and surfaces

  • Dark/light theme support
Non-Functional Requirement: Accessibility NFR-5_2
status: open
priority: P0

The application shall be accessible to users with disabilities (WCAG 2.1 AA compliance).

Acceptance criteria:

  • Screen reader compatibility (TalkBack, VoiceOver)

  • Minimum color contrast ratio 4.5:1

  • All interactive elements keyboard accessible

  • Focus indicators visible

  • Text scalable up to 200%

  • No information conveyed by color alone

  • Touch targets minimum 44x44 points
Non-Functional Requirement: RTL and internationalization NFR-5_3
status: open
priority: P1

The system shall support right-to-left (RTL) languages and internationalization.

Acceptance criteria:

  • UI mirrors correctly for RTL languages

  • Book content renders correctly for RTL text

  • Date/time formats localized

  • Number formats localized

  • Bidirectional text support
Non-Functional Requirement: Localization NFR-5_4
status: open
priority: P1

All user-facing text shall be localizable.

Acceptance criteria:

  • English as primary language

  • Localization framework in place for additional languages

  • No hardcoded user-facing strings

  • Locale can be overridden in settings

  • Pluralization rules supported
Non-Functional Requirement: E-ink usability NFR-5_5
status: open
priority: P1

The system shall provide optimized usability for e-ink displays.

Acceptance criteria:

  • High contrast theme option

  • Simplified UI with fewer visual elements

  • Large, clear typography

  • Reduced reliance on hover states

  • Clear button/touch feedback without animations

6. Security

Non-Functional Requirement: Password security NFR-6_1
status: open
priority: P0

User passwords shall be hashed using industry-standard algorithms.

Acceptance criteria:

  • Passwords hashed with bcrypt or Argon2

  • Minimum password length: 8 characters

  • Password strength indicator during registration

  • No plaintext password storage or transmission

  • Secure password reset mechanism
Non-Functional Requirement: Transport security NFR-6_2
status: open
priority: P0

All data transmission shall use TLS 1.3 or higher encryption.

Acceptance criteria:

  • HTTPS required for all API communication

  • Certificate pinning for mobile apps (optional)

  • No fallback to unencrypted connections

  • TLS 1.2 minimum (1.3 preferred)
Non-Functional Requirement: Two-factor authentication NFR-6_3
status: open
priority: P2

The system shall support two-factor authentication (TOTP).

Acceptance criteria:

  • TOTP-based 2FA (Google Authenticator compatible)

  • Backup codes available

  • 2FA can be enabled/disabled by user

  • Graceful recovery if 2FA device lost
Non-Functional Requirement: Session management NFR-6_4
status: open
priority: P0

Session management shall include automatic timeout and secure token handling.

Acceptance criteria:

  • JWT tokens with configurable expiration

  • Refresh token rotation

  • Sessions can be revoked remotely

  • Automatic logout after inactivity (configurable)

  • Secure token storage on device
Non-Functional Requirement: Privacy by default NFR-6_5
status: open
priority: P0

The application shall collect no analytics by default; telemetry is opt-in.

Acceptance criteria:

  • No analytics without explicit consent

  • Clear privacy policy accessible in-app

  • Telemetry settings easily accessible

  • No third-party tracking SDKs by default

  • Data collection scope clearly explained

7. Reliability

Non-Functional Requirement: System uptime NFR-7_1
status: open
priority: P1

The system shall have 99.5% uptime for self-hosted deployments under normal conditions.

Acceptance criteria:

  • Maximum planned downtime: 4 hours/month

  • Health check endpoints available

  • Graceful degradation during partial outages

  • Automatic recovery from transient failures
Non-Functional Requirement: Data integrity NFR-7_2
status: open
priority: P0

Data corruption shall be prevented through checksums and integrity verification.

Acceptance criteria:

  • File checksums verified on upload/download

  • Database transactions are atomic

  • Corrupted files detected and reported

  • No silent data corruption
Non-Functional Requirement: Network resilience NFR-7_3
status: open
priority: P0

The system shall gracefully handle network interruptions without data loss.

Acceptance criteria:

  • Operations are idempotent where possible

  • Uploads/downloads resumable

  • No data loss on network failure

  • Clear error messages for network issues

  • Automatic retry with exponential backoff
Non-Functional Requirement: Backup and restore NFR-7_4
status: open
priority: P1

Automatic backup and restore capabilities shall be available for user data.

Acceptance criteria:

  • Export all user data in portable format

  • Import/restore from backup

  • Scheduled automatic backups (optional)

  • Backup integrity verification

8. Extensibility

Non-Functional Requirement: Plugin architecture NFR-8_1
status: open
priority: P3

The architecture shall support modular plugins without compromising core stability.

Acceptance criteria:

  • Plugin API documented

  • Plugins run in sandboxed environment

  • Core functionality unaffected by plugin failures

  • Plugin enable/disable without restart

  • Version compatibility checking

9. Maintainability

Non-Functional Requirement: Code quality NFR-9_1
status: open
priority: P1

The codebase shall maintain high quality standards.

Acceptance criteria:

  • Automated linting (dart analyze)

  • Code formatting enforced (dart format)

  • Minimum 70% test coverage for core functionality

  • No critical security vulnerabilities

  • Documentation for public APIs
Non-Functional Requirement: Logging and monitoring NFR-9_2
status: open
priority: P1

The system shall provide comprehensive logging and monitoring capabilities.

Acceptance criteria:

  • Structured logging for all components

  • Log levels configurable (debug, info, warning, error)

  • No sensitive data in logs

  • Log rotation and retention policies

  • Error tracking integration (optional)