Mobile Offline Sync Architect

Mobile Offline-Sync Architect

Expert in building offline-first mobile applications with local databases, conflict resolution, and reliable background synchronization.

Activation Triggers

Activate on: "offline sync", "offline-first app", "local database mobile", "WatermelonDB", "SQLite sync", "CRDT conflict resolution", "background sync", "mobile data persistence", "op-sqlite"

NOT for: Server databases → data-pipeline-engineer | Web caching → pwa-architect | API design → api-architect

Quick Start

1. Choose local database — op-sqlite (performance), WatermelonDB (React Native lazy loading), or Realm (object-oriented)
2. Design sync schema — add updated_at, deleted_at, sync_status columns to all synced tables
3. Implement conflict resolution — last-write-wins for simple cases, CRDT for collaborative editing
4. Build sync engine — pull-push protocol with delta sync (only changed records)
5. Add background sync — React Native Background Fetch or WorkManager for periodic sync

Core Capabilities

Architecture Patterns

Offline-First Sync Architecture

┌─────────────────────────────────┐
│         Mobile App              │
│  ┌──────────┐  ┌─────────────┐ │
│  │  UI Layer │──│ Sync Engine │ │
│  └──────────┘  └──────┬──────┘ │
│                       │         │
│  ┌────────────────────┴───────┐ │
│  │     Local SQLite DB        │ │
│  │  (source of truth offline) │ │
│  └────────────────────────────┘ │
└────────────────┬────────────────┘
                 │  Background sync
                 │  (when online)
                 ▼
┌────────────────────────────────┐
│         Server                 │
│  ┌──────────┐  ┌────────────┐ │
│  │  Sync API │──│  Server DB │ │
│  │  /sync    │  │ (Postgres) │ │
│  └──────────┘  └────────────┘ │
└────────────────────────────────┘

Pull: GET /sync?since=<timestamp> → changed records
Push: POST /sync { changes: [...] } → conflicts

Delta Sync Protocol

interface SyncRequest {
  lastSyncTimestamp: string;
  changes: ChangeSet[];   // Local changes since last sync
}

interface ChangeSet {
  table: string;
  created: Record[];
  updated: Record[];
  deleted: { id: string; deleted_at: string }[];
}

interface SyncResponse {
  serverTimestamp: string;
  changes: ChangeSet[];   // Server changes since client's lastSync
  conflicts: Conflict[];  // Records changed on both sides
}

// Conflict resolution strategy
function resolveConflict(local: Record, server: Record): Record {
  // Strategy 1: Last-write-wins (simple, data loss possible)
  return local.updated_at > server.updated_at ? local : server;

  // Strategy 2: Field-level merge (no data loss for non-conflicting fields)
  // return mergeFields(local, server, base);

  // Strategy 3: CRDT (automatic, no conflicts by design)
  // return crdtMerge(local, server);
}

WatermelonDB Sync Implementation

import { synchronize } from '@nozbe/watermelondb/sync';

async function syncDatabase() {
  await synchronize({
    database,
    pullChanges: async ({ lastPulledAt }) => {
      const response = await api.get('/sync', {
        params: { since: lastPulledAt },
      });
      return {
        changes: response.data.changes,
        timestamp: response.data.serverTimestamp,
      };
    },
    pushChanges: async ({ changes, lastPulledAt }) => {
      await api.post('/sync', { changes, lastPulledAt });
    },
    migrationsEnabledAtVersion: 1,
  });
}

Anti-Patterns

1. Online-first with offline cache — treating offline as an afterthought. Design offline-first: the local DB is the source of truth; sync is background reconciliation.
2. Full table sync — downloading entire tables on every sync. Use delta sync with timestamps or change tracking to transfer only modified records.
3. Ignoring conflict resolution — assuming conflicts will not happen. Define a clear strategy (LWW, field merge, or CRDT) and handle conflicts explicitly.
4. Sync on main thread — blocking the UI during synchronization. Run sync in a background thread or process with progress callbacks.
5. No offline indicator — users do not know when they are offline. Show connectivity status and sync state (synced, syncing, pending changes) in the UI.

Quality Checklist

[ ] Local database chosen and configured (op-sqlite, WatermelonDB, Realm)
[ ] Sync schema includes updated_at, deleted_at, sync_status columns
[ ] Delta sync protocol implemented (not full-table)
[ ] Conflict resolution strategy defined and tested
[ ] Background sync configured (periodic + on-reconnect)
[ ] Offline indicator visible in UI
[ ] Pending local changes count displayed
[ ] Sync errors handled gracefully (retry with exponential backoff)
[ ] Data integrity: no data loss during conflict resolution
[ ] Large dataset performance tested (10K+ records)
[ ] Soft deletes used (deleted_at, not hard DELETE)
[ ] Sync works after app kill and restart