Mobile System Design¶
Worked case studies for mobile system design interviews and real projects: payments, wearables, chat, feeds, live tracking, uploads and safe releases.
Most system design material is about servers. Mobile rounds ask different questions: what happens when the app is killed mid-payment, how a wearable's data reaches the cloud exactly once, how a chat stays in order on a train with no signal. This guide answers them with diagrams, data models, APIs and failure modes, the way you'd explain them in an interview.
Start here¶
- The framework: the seven steps to follow in any mobile design round, and the mistakes to avoid.
- Building blocks: offline-first reads, the outbox, conflicts, real-time updates, pagination, caching, tokens and background work. Every case study reuses these.
- Cheat sheet: one page for the night before.
Case studies¶
| Case study | Difficulty | You'll practise |
|---|---|---|
| UPI payments app | 🔴 Senior | Payment state machines, idempotency, reconciliation, device binding |
| Wearable health-data sync | 🔴 Senior | Bluetooth LE, background limits, deduplication, time zones, HealthKit and Health Connect |
| Offline-first chat | 🟡 Mid – 🔴 Senior | Outbox, ordering with sequence numbers, receipts, sockets and push |
| Infinite news feed | 🟢 Junior – 🟡 Mid | Cursor pagination, images, video, smooth lists, optimistic likes |
| Live ride tracking | 🔴 Senior | Location streams, trip state, smooth maps, battery |
| Resumable video upload | 🟡 Mid – 🔴 Senior | S3 multipart upload, presigned URLs, background transfers |
| Feature flags and staged rollouts | 🟡 Mid – 🔴 Senior | Remote config, kill switches, experiments, forced updates |
Every case study has the same sections: requirements, estimates, architecture, data model, API, deep dives, failure modes, trade-offs and interview follow-ups. Read the requirements, close the page, sketch your own design, then compare.
Who it's for¶
- Mobile engineers (Flutter, Android, iOS, React Native) preparing for senior and staff interviews
- Interviewers who want consistent questions and a reference answer
- Teams designing features like these for real
The designs are platform-neutral, with notes for Android, iOS and Flutter where the platforms differ.
Related¶
- Flutter Architect Handbook: 67 interview questions from Dart basics to architecture and leadership
- flutter-production-starter: a template app that applies many of these patterns