How do you queue and synchronize writes while offline and push them once online?

Question

Rana Sunny · Answer

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 IndexedDBlocalForage – Abstracts IndexedDB, WebSQL, localStorageWorkbox – Service Worker tooling for caching + background syncRxDB, PouchDB – Full sync-capable offline-first databases

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How do you queue and synchronize writes while offline and push them once online?

Rana Sunny

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1 Answers

General Strategy

Sample (Dexie.js Example)

1. Define Your IndexedDB Store (Queue)

2. Queue Write Action Offline

3. Sync When Online

4. Detect Online and Start Sync

Considerations

Tools/Libraries

Liked By

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