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Service worker background sync lets a progressive web app save an operation while offline and ask the browser to retry it later. It is useful for deferred work such as sending a message, but it does not guarantee immediate delivery or a fixed retry schedule. For content refresh, Periodic Background Sync is a separate, browser-controlled feature—not a timer.
How do I sync data in the background in a PWA?
A service worker runs separately from the page, and the browser can start it when background work is eligible. With one-off Background Sync, the page stores the intended operation, registers a sync tag, and the browser later fires a sync event in the service worker’s scope when it considers connectivity available. MDN describes this event flow in its Offline and background operation guide.
A practical sequence is:
- Register a service worker. Ensure the app has an active service worker controlling the relevant pages.
- Persist the operation while the page is available. Save the user’s intended action, such as a message and its destination, in durable storage such as IndexedDB. Do not rely on a page’s in-memory state surviving closure.
- Register a sync tag. From the page, check that the registration supports
sync, then request a tag for the queued work. A tag identifies work for the service worker; it is not a promise of execution at a particular time. - Handle the matching event in the service worker. Listen for
sync, check the tag, and perform the request insideevent.waitUntil()so the browser knows the asynchronous work is still in progress. - Update the queue and the interface. Remove an operation only after the server confirms success. Show whether work is queued, sent, or needs the user’s attention.
Microsoft’s PWA background sync guide uses offline message sending and article refresh to illustrate the pattern. Treat the queue as application data: use idempotency keys or another server-supported duplicate-prevention strategy, and define how to handle expired authentication, permanent validation errors, and operations that cannot safely be replayed. Those details depend on the server contract.
Will background sync work when the app is closed?
It can run without the page being open: the browser may restart the service worker and dispatch the event when it deems the work eligible. MDN states that when network connectivity returns, the browser can restart the service worker if necessary and fire a sync event. The same guidance cautions that retries and execution time are limited. See the MDN background operation guide and the Background Synchronization API reference.
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That makes Background Sync a best-effort delivery mechanism, not a guaranteed delivery service. The browser controls when and whether it runs; do not promise that a queued item will be sent immediately after connectivity returns. Keep the pending state visible, and provide a way to retry when the app is open.
How do I retry failed requests when a user comes back online?
Workbox’s Background Sync plugin can queue requests when the fetch operation throws and replay them on a later sync event. In browsers without native Background Sync, Workbox attempts replay when the service worker starts; that fallback is less dependable because it needs the worker to start, typically in connection with a page being used. The plugin’s behavior and configuration are documented in Workbox Background Sync.
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One important distinction: an HTTP response such as 503 is still a completed fetch, not a thrown network failure. By default, the plugin’s queueing hook does not treat ordinary HTTP error responses as failures. If your app should retry selected server responses, explicitly classify those responses as retryable in its Workbox strategy.
- Keep a foreground retry path for queued work when the app opens; do not make a background event the only way to recover.
- Use
navigator.onLineonly as a connectivity hint. It does not prove that your API endpoint is reachable or that a request will succeed. - Separate transient failures from permanent ones. Authentication expiry or validation rejection may need a user action rather than endless replay.
- Make replay safe against duplicates, ideally using idempotency support on the server.
Can a service worker periodically refresh data?
Periodic Background Sync is a distinct API for opportunistic content refresh, such as updating cached content between visits. It is not the same as retrying a specific queued operation. A registration can request a minInterval, but that value is a request to the browser, not a guaranteed cadence. Scheduling can depend on browser policy and app engagement; support and permission are constrained. See MDN’s Periodic Background Synchronization API reference, PeriodicSyncManager, and Chrome’s overview of Periodic Background Sync.
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Feature-detect the periodic registration capability and check permission where the API exposes it. Handle missing support or denied permission without breaking the app, and refresh on app open as a dependable user-facing fallback. MDN notes that availability is limited; avoid browser-by-browser assumptions without checking current compatibility information.
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Which background-sync option fits the job?
| Option | Best for | Trigger and permission | Reliability limit |
|---|---|---|---|
| One-off Background Sync | Retrying a specific deferred operation after connectivity returns. | The page registers work; the browser fires sync when it considers the work eligible. MDN says explicit permission is not required. |
Retries and execution time are limited; no precise delivery time is promised. MDN and Background Synchronization API. |
| Periodic Background Sync | Opportunistically refreshing content between app visits. | The browser schedules periodicsync under its policy; permission is required and availability is constrained. |
minInterval is not a schedule guarantee. MDN and Chrome. |
| Workbox Background Sync | Queueing failed requests and replaying them with native sync where available. | Queues thrown fetch failures; without native sync, attempts replay when the service worker starts. | The fallback is less dependable, and HTTP error responses need explicit handling if they should be retried. Workbox documentation. |
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