Offline Sync Engine

Offline Sync Engine

Overview

Manages offline data storage and synchronization when connectivity returns.

Decision Framework

When to use

• App needs to work without internet
• Data must sync across devices
• Users edit data while offline

When NOT to use

• Real-time collaborative systems
• Read-only data displays

Trigger Phrases

• "Work offline"
• "Sync data when back online"
• "Offline support"
• "Cache data locally"

Sync Patterns

Storage Layers

┌─────────────────────────────────────┐
│ Application Layer                    │
├─────────────────────────────────────┤
│ Sync Queue (pending changes)         │
├─────────────────────────────────────┤
│ Local Cache (IndexedDB/SQLite)       │
├─────────────────────────────────────┤
│ Conflict Resolution                  │
├─────────────────────────────────────┤
│ Remote API                           │
└─────────────────────────────────────┘

Implementation

Sync Queue

class SyncQueue {
  constructor() {
    this.pending = [];
    this.failed = [];
  }

  add(change) {
    this.pending.push({
      id: generateId(),
      operation: change.operation,
      data: change.data,
      timestamp: Date.now(),
      retries: 0
    });
  }

  async flush() {
    while (this.pending.length > 0) {
      const change = this.pending[0];
      try {
        await this.sendToServer(change);
        this.pending.shift();
      } catch (error) {
        change.retries++;
        if (change.retries >= 3) {
          this.failed.push(this.pending.shift());
        }
        break;
      }
    }
  }
}

Conflict Resolution

class ConflictResolver {
  resolve(local, remote, strategy = 'last-write-wins') {
    switch(strategy) {
      case 'last-write-wins':
        return local.updatedAt > remote.updatedAt ? local : remote;
      case 'merge':
        return this.deepMerge(local, remote);
      case 'manual':
        throw new ConflictError(local, remote);
    }
  }
}

Escalation Chain

1. Retry - Immediate retry (3 attempts)
2. Backoff - Exponential backoff (1s, 2s, 4s, 8s)
3. Queue - Add to offline queue for later sync
4. Notify - Alert user of sync failure
5. Manual - User intervention required

Error Classification