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Home Interviews How do you queue and synchronize writes while offline and push them once online? – MindStick
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1 Answers

Rana Sunny
Rana Sunny 07 Jul 2025
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To queue and synchronize writes while offline and push them once online (commonly used in offline-first web/mobile apps), you typically follow this pattern:

General Strategy

  • Queue Writes Locally (Offline)
    • Store write actions (create/update/delete) in an offline queue, such as:
      • IndexedDB (Web)
      • localStorage (not recommended for complex writes)
      • SQLite (Mobile)
      • Dexie.js or localForage (for easier IndexedDB handling)
  • Detect Online Status
    • Use window.navigator.onLine and listen for:
window.addEventListener("online", syncQueuedWrites);
  • Synchronize (Push) Writes When Online
    • On online event or app load (if online), process queued writes:
      • Read each queued item.
      • Send the write to the server.
      • If successful, remove the item from the queue.
      • Optionally handle retry logic and conflict resolution.

Sample (Dexie.js Example)

1. Define Your IndexedDB Store (Queue)

const db = new Dexie("OfflineWriteDB");
db.version(1).stores({
    items: '++id, type, payload',
});

2. Queue Write Action Offline

async function queueWrite(type, payload) {
    await db.items.add({ type, payload });
}

3. Sync When Online

async function syncQueuedWrites() {
    const allItems = await db.items.toArray();

    for (const item of allItems) {
        try {
            const response = await fetch('/api/sync', {
                method: 'POST',
                headers: { 'Content-Type': 'application/json' },
                body: JSON.stringify(item)
            });

            if (response.ok) {
                await db.items.delete(item.id); // remove if sync successful
            }
        } catch (err) {
            console.error("Sync failed for item", item.id, err);
            // Optionally retry or mark failed
        }
    }
}

4. Detect Online and Start Sync

window.addEventListener("online", syncQueuedWrites);

Considerations

Feature Notes
Conflict Resolution Server should return proper status/errors if conflicts occur (e.g. versioning, timestamps)
Deduplication Ensure you don't double-send operations (e.g., mark with IDs)
Background Sync (PWAs) Use Background Sync API for advanced use
Retries Exponential backoff or retry count in queue schema
Encryption Encrypt local storage if handling sensitive data

Tools/Libraries

  • Dexie.js – Simplified IndexedDB
  • localForage – Abstracts IndexedDB, WebSQL, localStorage
  • Workbox – Service Worker tooling for caching + background sync
  • RxDB, PouchDB – Full sync-capable offline-first databases
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