Safari Service Workers and the Cache API: Can Offline Caches Survive Between Sporadic Visits?

Answer

Yes – cached assets will still be available on the next visit as long as Safari has not evicted them due to storage pressure or its inactivity‑based cleanup. Service Worker termination itself does not delete Cache API entries.

Confirmed facts

1. Safari may idle‑terminate a Service Worker after ~5 minutes, but termination is a runtime lifecycle event that does not affect data stored in the Cache API.
2. Cache API storage shares the site’s overall quota and is subject to Safari’s best‑effort eviction policy; observations indicate that entries can be removed after roughly 7 days of site inactivity when space is needed.
3. Safari does not currently support the navigator.storage.persist() permission, so there is no way to request persistent storage for Cache API data.
4. Without Periodic Background Sync, cache updates occur only when the user opens the site in the foreground.

Likely explanation for sporadic visitors

If a user returns within the inactivity window (≈ 7 days) and the device has not reached its storage quota, the cached app shell and assets will be served from the Cache API on the next load, resulting in faster startup and fewer network requests. If the return occurs after that window or under heavy storage pressure, Safari may have purged some or all entries, causing a network fallback.

Steps to verify and rely on this behavior

1. Register a Service Worker that precaches the app shell and static assets during the install event.
2. In the fetch handler, respond with caches.match(request) before falling back to the network.
3. On each page load, log the age of a known cached asset (e.g., a timestamped version file) to detect whether it survived.
4. Monitor the “Cache Storage” panel in Safari’s Web Inspector to see if entries persist between sessions.
5. If you need to know the exact eviction threshold for your audience, collect data on how many days of inactivity pass before the cached timestamp disappears on representative iOS and macOS devices.

Missing diagnostic detail

To refine the recommendation, please confirm the observed period of inactivity after which Safari evicts Cache API entries on your target devices (e.g., 5 days, 7 days, 10 days). This detail changes whether you can rely on a specific cache‑retention window or need additional strategies such as periodic re‑caching on user‑initiated visits.