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Mobile System Design Interview Template

Use this template to structure your thoughts and notes during the interview.

1. Requirement Gathering (5 min)

Goal: Clarify scope and identify the "MVP".

Questions to Ask:

  • Users: DAU/MAU? Spike traffic patterns?
  • Geography: Target market? (Network speed, data costs, device capabilities).
  • Platform: iOS, Android, or both? Minimum OS version?
  • Team: Team size? (Influences modularity).
  • Features: What are the top 3-5 critical features?

Functional Requirements:

Non-Functional Requirements:

  • Offline Support: (Yes/No/Partial)
  • Performance: (Latency, startup time)
  • Security: (E2E encryption, local storage encryption)
  • Reliability: (Data consistency, conflict resolution)

Out of Scope:

  • (e.g., Auth, Settings, Analytics, Web version)

2. High-Level Diagram (10-15 min)

Goal: visual "Big Picture". Identify key components.

Core Components:

  • UI Layer: (View, ViewModel/Presenter)
  • Navigation: (Coordinator/Router)
  • Domain Layer: (Use Cases/Interactors)
  • Data Layer: (Repository)
    • Remote: (API Service -> Retrofit/Alamofire)
    • Local: (Persistence -> Room/CoreData)
  • DI Graph: (Dependency Injection)
  • Image Loader: (Coil/SDWebImage)

Sketch: (Draw boxes and arrows here. Focus on data flow: UI -> ViewModel -> Repository -> API/DB)

3. Deep Dives (20-30 min)

Select 1-2 topics to explore.

A. API Design

  • Protocol: REST vs GraphQL vs WebSocket vs gRPC.
  • Pagination: Offset vs Cursor vs PageNumber.
  • Endpoints:
    • GET /resource
    • POST /resource
  • Payloads: (JSON structure, field types)

B. Data Storage

  • Schema: (Tables, Columns, Relationships)
  • Sensitive Data: (Keychain/Keystore)
  • File Storage: (Images/Videos -> Internal vs External cache)

C. Synchronization (Offline Support)

  • Strategy: Write-Through vs Write-Back?
  • Conflict Resolution: Last-Write-Wins vs Server-Side vs User-Manual?
  • Queue: Request Queue / Worker Manager for background sync.

D. Media Handling

  • Images: Caching (Memory/Disk), Resizing, Formats (WebP).
  • Uploads: Resumable/Chunked uploads?

4. Signal Checklist

  • Did I mention Trade-offs? (e.g., "WebSockets are faster but harder to scale than polling").
  • Did I mention Edge Cases? (e.g., "What if the network drops in the middle of an upload?").
  • Did I mention Platform Specifics? (e.g., "On Android, we need to handle process death").
  • Did I check assumptions with the interviewer?