System Design

1. Approaching Design Questions

Q: How do you approach a mobile system design question?

Follow this structure:

1. Clarify Requirements (2-3 min)

   • Functional: What features?
   • Non-functional: Offline? Scale? Battery?
   • Constraints: Platform? API limitations?

2. High-Level Design (5-10 min)

   • Architecture diagram
   • Main components
   • Data flow

3. Deep Dive (10-15 min)

   • Specific component design
   • Data models
   • Key algorithms

4. Trade-offs & Extensions (5 min)

   • What you'd do differently
   • Scalability considerations
   • Edge cases

Q: What non-functional requirements matter for mobile?

2. News Feed / Social Feed

Q: Design a news feed like Twitter or Instagram.

Key Components:

• Feed Repository: Single source of truth, coordinates remote/local
• Paging: Infinite scroll with Paging 3
• Caching: Room database as cache
• Real-time: WebSocket or polling for new items

Architecture:

UI (LazyColumn) → ViewModel → Repository
                                 ├── RemoteMediator → API
                                 └── PagingSource → Room

Key Decisions:

• Cursor vs Offset pagination: Cursor is stable (items don't shift)
• Cache invalidation: Time-based or on pull-to-refresh
• Optimistic updates: Show locally before server confirms
• Image loading: Preload next page images

Trade-offs:

• More caching = better UX, more storage/complexity
• WebSocket = real-time, more battery/complexity

3. Messaging App

Q: Design a chat/messaging app like WhatsApp.

Key Components:

• Message Repository: Handles send/receive/sync
• WebSocket/XMPP: Real-time message delivery
• Local Database: All messages stored locally
• Background Service: Receive messages when app closed

Data Model:

Conversation: id, participants, lastMessage, unreadCount
Message: id, conversationId, sender, content, timestamp, status

Key Decisions:

• Delivery guarantees: At-least-once with deduplication
• Message status: Sent → Delivered → Read
• Offline messages: Queue locally, sync when online
• End-to-end encryption: Signal Protocol pattern

Sync Strategy:

1. On app start, fetch missed messages since last sync
2. WebSocket for real-time during session
3. Push notification triggers sync when app closed

4. Image Loading Library

Q: Design an image loading library like Coil or Glide.

Key Components:

• Request Queue: Prioritize visible images
• Memory Cache: LruCache for decoded bitmaps
• Disk Cache: DiskLruCache for downloaded files
• Decoder: BitmapFactory with sampling
• Transformations: Resize, crop, rounded corners

Flow:

Request → Memory Cache? → Disk Cache? → Network
    ↓           ↓              ↓            ↓
   Done     Decode         Decode       Download
                ↓              ↓            ↓
          Cache in         Cache in    Cache both
           Memory           Memory

Key Decisions:

• Memory cache size: ~1/8 of available memory
• Disk cache size: ~50-250MB configurable
• Request cancellation: Cancel when view recycled
• Placeholder/Error: Immediate feedback

Optimizations:

• Downsampling: Load at target size, not full resolution
• Request coalescing: Same URL = single request
• Preloading: Load upcoming images in list

5. Offline-First App

Q: How do you design an offline-first application?

Principles:

1. Local database is source of truth
2. All operations work offline
3. Sync when connectivity available
4. Handle conflicts gracefully

Architecture:

UI → ViewModel → Repository (SSOT)
                      ↓
                 Local DB ←→ SyncManager → API

Sync Strategies:

Key Decisions:

• Change tracking: Track local modifications since last sync
• Conflict detection: Compare versions/timestamps
• Partial sync: Only sync changed data
• Background sync: WorkManager with network constraint

6. Video Streaming

Q: Design a video streaming app like YouTube/Netflix.

Key Components:

• Player: ExoPlayer with adaptive streaming
• Caching: Pre-cache upcoming segments
• Download: Background download for offline
• CDN: Multiple quality levels, nearby servers

Streaming Protocol:

• HLS/DASH: Adaptive bitrate streaming
• Segments downloaded progressively
• Quality adjusts to network conditions

Key Decisions:

• Buffer size: Balance memory vs smooth playback
• Quality selection: Auto (ABR) or user choice
• Preloading: Start loading next video early
• Resume position: Save and restore playback position

Offline Viewing:

• Download in background with WorkManager
• Encrypt downloaded content (DRM)
• Track download progress, allow pause/resume
• Expire after license period

7. Location-Based App

Q: Design a ride-sharing or delivery app.

Key Components:

• Location Service: Foreground service with updates
• Map Integration: Google Maps or Mapbox
• Real-time Updates: Driver location via WebSocket
• Geofencing: Trigger events at locations

Battery Considerations:

• Use significant location change (not continuous)
• Batch location updates
• Reduce frequency when stationary
• Foreground service required for background location

Key Decisions:

• Location accuracy: Fused Location Provider, balance power/accuracy
• Update frequency: Every 5-10 seconds active, less in background
• Predictive routing: Update ETA based on traffic
• Offline maps: Cache tiles for core areas

8. E-Commerce App

Q: Design a shopping app like Amazon.

Key Components:

• Product Catalog: Search, browse, filter
• Cart: Local + synced
• Checkout: Payment integration
• Order Tracking: Status updates

Architecture Decisions:

• Search: Debounce input, paginate results
• Cart: Optimistic local updates, sync to server
• Product Cache: Cache popular items, invalidate on changes
• Deep Linking: Open product from notification/share

Key Features:

• Recently Viewed: Local history
• Recommendations: Personalized from server
• Wishlist: Local with sync
• Reviews: Lazy load, paginate

9. Design Patterns for Mobile

Q: What patterns come up repeatedly in mobile system design?

Quick Reference