Shrinking the covers changed nothing on the device, twice over, because two layers were
independently serving the old bytes and each had the same wrong premise written into its
comment: that a cover is "content-addressed by album id". The id addresses which album
the art belongs to. The bytes behind the URL change whenever the art is reprocessed.
The backend sent `public, max-age=604800`, so every browser that had loaded the page in
the previous week kept decoding the 3000px original out of its own disk cache without
asking. It now sends `no-cache`, which does not mean "do not store" but "revalidate
before reusing" - the file stays cached and the usual answer is a 304.
The service worker cached covers cache-first on a URL that never changes, which means a
client could serve a stale cover for ever; bumping the cache name fixed today's covers
and would have had to be done again for the next batch. It now does
stale-while-revalidate: the cached copy is returned immediately, so a grid of album art
still never waits on the network, and a fresh copy is fetched behind the page for next
time. Neither layer needs a version bump when art is reprocessed again.
The revalidation is off the paint path entirely, which is what makes `no-cache`
affordable here: the service worker answers first, the network call happens behind it.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
A DevTools trace recorded on musicdolphin put the whole question beyond argument:
ImageDecodeTask 29,939 ms x20 (~1.5 s each)
Decode LazyPixelRef 3,924 ms x20
Decode Image 3,530 ms x22
Paint 99.7 ms
Layout 77.3 ms
all app JavaScript <400 ms across 22 seconds
Image decoding is not the largest cost on that page, it is very nearly the only one.
The covers should already have been small - the backend downscales to 640 px now - but
the page was still decoding them at 1920, 1600, 1400 px, with deliveryType
"cache-storage" on every one. The service worker caches cover art cache-first, keyed on
URL, on the premise that it is "immutable per album id". The id did not change; the
bytes did. So every client that had ever loaded a cover kept serving the 3000 px
original from its own disk and never asked the backend for the new one.
Renaming the cache is the retirement mechanism the worker already has - `activate`
deletes every cache that is not one of the two current names - so COVERS becomes v2,
with a note saying that changing how covers are produced means bumping it.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
