Chrome - Images disappearing from cache storage after few hours - google-chrome

Question

I have a website that is used as a kiosk app. When online, I preload data and images from a wordpress API, and store the images in the cache storage.
A service worker intercepts http gets to these images and serves the cache data instead. Like this, the app can run offline (API calls included).
But after few hours running offline (generally around 6h) some images disappear from the cache storage. And it's always the same ones.
But not all.
Any idea where should I check to see what's going wrong ?

Answer 1

There can be two possible reasons for such clearing of cache, either your storage gets full(chrome localStorage), so it gets clear OR check for the expiring length of the data send from the server, check for headers that might give you insights of time it takes to expire.
And for checking if the data gets evicted from the storage, try testing your website in Safari or Edge browser where such eviction does not occur.

Answer 2

How did you configured Cache-Control? The ones which are constantly deleted might configured as max-age.
It's still recommended that you configure the Cache-Control headers on
your web server, even when you know that you're using the Cache
Storage API. You can usually get away with setting Cache-Control:
no-cache for unversioned URLs, and/or Cache-Control: max-age=31536000
for URLs that contain versioning information, like hashes.
as stated at https://web.dev/service-workers-cache-storage
Also, you should have a check storage amount of cache storage. (https://developers.google.com/web/tools/chrome-devtools/storage/cache) Even they claim that either all caches are deleted or none of them, since currently it is experimental technology, it is normal to be suspicious about this point.
You are also responsible for periodically purging cache entries. Each
browser has a hard limit on the amount of cache storage that a given
origin can use. Cache quota usage estimates are available via the
StorageEstimate API. The browser does its best to manage disk space,
but it may delete the Cache storage for an origin. The browser will
generally delete all of the data for an origin or none of the data for
an origin. Make sure to version caches by name and use the caches only
from the version of the script that they can safely operate on. See
Deleting old caches for more information.
as stated at https://developer.mozilla.org/en-US/docs/Web/API/Cache

Answer 3

Service Workers use the Storage API to cache assets.
There are fixed quotas available and different from browser to browser.
You can get some more hints from your app using the following call and analysing the results:
navigator.storageQuota.queryInfo("temporary")
.then(function(info) {
console.log(info.quota);
// It gives us the quota in bytes
console.log(info.usage);
// It gives us the used data in bytes
});
UPDATE 1
Do you have anywhere a cleanup method or logic that removes old cache entries and eventually is triggered somehow unexpected? Something like the code below.
caches.keys().then(function(keyList) {
return Promise.all(keyList.map(function(key) {
if (key !== cacheName) {
return caches.delete(key);
}
}));
})
A couple of more questions:
Do you have any business logic in the SW dealing with caching the images?
Could you get any info by using the storageQuota methods to see the available quota and usage? This might be very useful to understand if the images are evicted because of reaching the limit. (even by setting a box as persistence, you are not 100% sure the assets are retained)
From "Building PWAs" (o'Reilly - Tal Alter):
Each browser behaves differently when it comes to managing CacheStorage, allocating space in the cache for each site, and clearing out old cache entries.
In addition to a storage limit per site (also known as an origin), most browsers will also set a size limit for their entire cache. When the cache passes this limit, the browser will delete the caches of the site accessed the longest time ago (also known as the least recently used).
Browsers will never delete only part of your site’s cache. Either your site’s entire cache will be deleted or none of it will be. This ensures your site never has an unpredictable partial cache.
Considering the last words "Browsers will never delete only part of your site’s cache." might me think that maybe it is not a problem with the cache limit reached, as otherwise all images wiould be deleted in the cache.
If interested, I wrote an article about this topic here.
You could mark your data storage unit (aka "box") as "persistent", making the user agent retain it as long as possible (more details in the Storage API link above).