Sheldon Wang

Sheldon Wang

Seasoned iOS Veteran, Mentor

Design an Offline-First Sync System for Notes App

Published:

🎯 Problem

Users can create/edit notes while offline. The app syncs later when network returns.

Constraints:

  • Low battery/network usage
  • No data loss
  • Reasonable conflict behavior
  • Works across multiple devices

🧱 Architecture

flowchart LR; App["Mobile App"]-->LocalDB["Local DB"]; App-->Queue["Sync Queue"]; Queue-->SyncWorker["Background Sync Worker"]; SyncWorker-->API["Sync API"]; API-->ServerDB["Server DB"]; API-->Conflict["Conflict Resolver"];

🔑 Key Principles

  1. Local write is source of UX truth
    User action succeeds immediately after local commit.

  2. Durable operation queue
    Every mutation becomes a queue entry with retry metadata.

  3. Idempotent sync API
    Each operation carries request ID to avoid duplicate side effects.

🗂 Data Model Fields

  • entity_id
  • updated_at
  • version
  • sync_state (pending, synced, failed)
  • op_id (idempotency key)

⚔️ Conflict Strategy

Recommended interview default:

  • last-write-wins for low-risk fields
  • server-side merge for structured content
  • explicit user resolution only for high-value conflicts

Mention tradeoff: strict CRDT gives stronger merges but adds complexity.

🚨 Failure Handling

  • Network timeout -> exponential backoff + jitter
  • 409 conflict -> mark needs_merge, fetch latest, merge policy
  • 5xx spike -> pause queue and retry with capped backoff

📈 Operational Metrics

  • sync success rate
  • queue depth and age
  • conflict rate
  • median time to consistency

✅ Interview Wrap-Up

Highlight:

  • local-first UX
  • reliable queue semantics
  • deterministic conflict policy
  • observability and rollback controls