The durable tier has been content-keyed since it was introduced —
`content_derived_blobs(source_hash, kind, variant)` — but the moka cache
in front of it was still `{file_id}:{ext}`, so the layer closest to the
request used the wrong axis while the layer behind it used the right
one. That was legacy shape, and I had defended it in a comment as
"deliberate: per-request-path and short-lived", which was a
rationalisation rather than a reason. Ed asked why, and there is no why.
Transcoding is a pure function of the source bytes. Under file keying,
two files with identical content held two RAM entries for identical
bytes, and the second file was a guaranteed miss that fell through to a
DB lookup plus a blob read to fetch what was already in memory under
another key.
Now keyed by content hash when the caller has one, by file id only when
it does not — the same `content` / `external` split `ThumbnailCacheKey`
already makes, and for the same reason: hash-less callers (external
mounts) have no content identity to key on. Prefixed `c:` / `f:` so the
namespaces stay disjoint; a hash and a UUID cannot collide in practice,
but "in practice" is how a file ends up served another file's bytes.
`invalidate` now clears only the file-keyed entry. Dropping content
entries there would be wrong, not merely wasteful: one file's content
changing says nothing about the other files sharing the old bytes, and
evicting theirs would make one user's edit cost everyone else a
re-transcode. Content entries need no eviction — new content is a new
hash, so the old key is never consulted again.
transcode_cache.hurl updated to match, and its header corrected: the
second file is now a RAM hit rather than a derived-tier read, so that
scenario can no longer isolate the durable tier. It says so, and points
at satellites_consistency and a restart as what covers it instead.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Edouard Vanbelle
4baee0a1fb