Background sync: retrying failed requests

Limited availabilityNot supported in Firefox (Desktop), Firefox (Android), Safari (macOS), Safari (iOS)WICG draft

In one line: Background sync lets a web app’s service worker defer an action — such as sending a queued message — until the browser reports that the device has connectivity again, instead of failing it outright when the network is unavailable.

Background sync: queue, register, replay on the sync event, and the three ways the promise can settle.

What it does

When a fetch fails because the device is offline, a page can register a sync request instead of giving up. The browser holds onto that registration and, once it decides the device has connectivity, fires a sync event in the service worker so the app can retry the deferred work — even if the page that originally registered it has since closed.

The registration and event API

The API is reached through ServiceWorkerRegistration.sync, which returns a SyncManager:

async function queueMessage() {
  try {
    await sendMessage();
  } catch {
    const registration = await navigator.serviceWorker.ready;
    if ('sync' in registration) {
      await registration.sync.register('send-queued-message');
    }
  }
}

• SyncManager.register(tag) registers a one-off sync request under the given tag, returning a promise that resolves once registration completes.
• SyncManager.getTags() resolves with the list of tags currently registered.

In the service worker, the app listens for the sync event and passes the retry work’s promise to waitUntil(), which extends the event’s lifetime. The user agent can still impose its own execution and lifetime limits on that extended time, so waitUntil() does not guarantee the work finishes — see “What goes wrong” below:

self.addEventListener('sync', (event) => {
  if (event.tag === 'send-queued-message') {
    event.waitUntil(sendQueuedMessage());
  }
});

Availability across browsers

Background sync is a Chromium (Blink) feature — Chrome, Edge, and Samsung Internet implement it, while WebKit (Safari) and Gecko (Firefox) do not.

See the compatibility page for more detail.

What goes wrong

• Per the spec, while the registering page or worker is still running, the event fires as soon as connectivity becomes available — not necessarily immediately. If that context is no longer running, the user agent should instead run the event at its “soonest convenience,” not on any guaranteed schedule.
• The spec allows a user agent to impose its own execution and lifetime limits on a SyncEvent — calling event.waitUntil() does not guarantee the browser will let arbitrarily long work finish before terminating the worker.
• Background sync is unsupported in WebKit (Safari) and Gecko (Firefox) as of this writing — always design the retry as an enhancement, not the only path to success.
• Per the spec, a failed sync event may be retried according to user-agent-defined heuristics unless it was the final chance, at which point no further attempts are made — retries are not guaranteed to eventually succeed, and this is not a periodic schedule.

How to detect it at runtime

Feature-detect SyncManager before registering a tag, and fall back to retrying immediately (or on the next page load) when it’s unavailable:

function supportsBackgroundSync(registration) {
  return 'sync' in registration;
}

async function deferOrRetry(registration, tag, retryNow) {
  if (!supportsBackgroundSync(registration)) {
    return retryNow(); // no Background Sync: retry immediately instead
  }
  return registration.sync.register(tag);
}

Where to go next

• Background sync: browser support — browser support matrix
• Periodic background sync — a related, separate API

Specifications

Specification	Status
Background Sync	WICG draft

• Legend
• Yes
• Partial
• Flag
• No
• Unknown

Browser / Platform	Support	Versions	Confidence	Source	Notes
Chrome (Desktop)	Yes	49	high	source	—
Chrome (Android)	Yes	49	high	source	1
Edge (Desktop)	Yes	79	high	source	2
Firefox (Desktop)	No	—	high	source	3
Firefox (Android)	No	—	high	source	45
Safari (macOS)	No	—	high	source	6
Safari (iOS)	No	—	high	source	78
Samsung Internet	Yes	5.0	high	source	9
WebView (Android)	No	—	high	source	10

1. Derived by browser-compat-data mirroring from Chrome.
2. Derived by browser-compat-data mirroring from Chrome.
3. No Firefox support is recorded in browser-compat-data.
4. No Firefox for Android support is recorded in browser-compat-data.
5. Derived by browser-compat-data mirroring from Firefox.
6. Implementation tracking: https://webkit.org/b/182565.
7. Implementation tracking: https://webkit.org/b/182565.
8. Derived by browser-compat-data mirroring from Safari.
9. Derived by browser-compat-data mirroring from Chrome Android.
10. Implementation tracking: https://crbug.com/40449796.

Source data: /compatibility/background-sync.json · Global usage: 72 % (StatCounter 2026-05)

Source: spec · MDN · Last verified 2026-10-03 · Confidence: high (computed from sources)