# Background sync — Progressive Web Apps

Source: https://www.geekswithgeeks.com/en/pwa/pwa-background-sync

> Implement background sync to reliably deliver user data even if the connection is lost.

## Reliable data delivery

**Background sync** queues user actions (form submissions, file uploads) and retries them when the connection is restored. The browser handles retries, not your app.

## Register background sync

Queue a task for background sync and handle it in the service worker.

```javascript
// In page
navigator.serviceWorker.ready.then(reg => {
  reg.sync.register('submit-form').then(() => {
    console.log('Sync registered');
  });
});

// In service worker
self.addEventListener('sync', event => {
  if (event.tag === 'submit-form') {
    event.waitUntil(
      fetch('/api/submit', { method: 'POST', body: formData })
        .catch(() => Promise.reject())
    );
  }
});
```

Output:

```
Form submission retried automatically when connection is restored.
```

## A common trap

Background sync only works in the service worker and requires proper error handling. Failed syncs are retried with exponential backoff.

## Handling sync failures and retries

Implement resilient sync with proper retry logic and user feedback.