Background Sync Failed — Service Worker Offline Data Sync Errors

Fix Background Sync API failures where offline actions queued by Service Workers fail to sync when the browser regains network connectivity. This guide covers everything you need to know about this topic, including common causes, step-by-step solutions, and answers to frequently asked questions.

Here are the key things to understand: Background Sync allows Service Workers to defer actions until the user has stable network connectivity. The sync event fires in the Service Worker when the browser detects a stable network connection. If the sync handler fails, the browser retries with exponential backoff. After a maximum number of retries (browser-dependent), the sync is abandoned. Background Sync requires a registered Service Worker and user has visited the site recently. Understanding these fundamentals will help you diagnose and resolve this issue more effectively.

The most common reasons this occurs include: Service Worker sync event handler throwing an error or rejecting the promise. Network connection detected but the target API server is unreachable. Sync data stored in IndexedDB was deleted or corrupted before the sync could complete. Browser killed the Service Worker before the sync event could complete (battery saver, memory pressure). Identifying the root cause is the first step toward finding the right solution.

To resolve this, follow these recommended steps: Check Service Worker logs: DevTools > Application > Service Workers > check for sync event errors. Verify IndexedDB data: DevTools > Application > IndexedDB > check if queued data is intact. Test sync handler manually: in Service Worker DevTools, simulate the sync event. Implement proper error handling: catch errors in the sync handler and re-queue failed operations. Notify the user if sync fails permanently so they can retry the action manually. If these steps do not resolve the issue, consider consulting additional resources or a qualified professional.

This article is part of our Browser Errors collection on Error Codes Wiki. We provide comprehensive, up-to-date information to help you find solutions quickly.