FileContentCache (moka, 512 MiB) was keyed by the file UUID, so content that
the CDC store already deduplicates to ONE blob on disk was cached once PER
FILE in RAM: N files sharing a blob held N copies, all counting against the
512 MiB cap. With effective dedup the cache filled with duplicates and
thrashed.
Key it by the blob hash instead (already on FileDto::content_hash):
- The in-RAM cache now benefits from dedup like the disk does — each distinct
blob is cached once and shared across every file/user that references it,
so a download by user A warms the cache for user B's identical content.
- Content is immutable by hash, so entries never go stale; the existing
invalidate(file_id) calls become harmless no-ops (a UUID never matches a
hash key) and can be removed in a later cleanup.
- ETag is now the immutable content hash (strong validator).
- Guarded: a hash-less stub DTO disables caching for that request rather than
colliding every such file on the empty key.
Response Content-Type still comes from the DTO, not the cache, so keying does
not affect the served MIME (verified).
Benchmark (real moka, exact 512 MiB/weight config, 400 files x 4 MiB = 1600 MiB
working set, 4000 uniform-random accesses):
dedup 1x : file_id 35.8% hit / 2568 reads vs hash 35.6% / 2577 (no dedup -> no change; control)
dedup 5x : file_id 35.8% hit / 2570 reads vs hash 98.0% / 80 (32x fewer disk reads, RAM 512->320 MiB)
dedup 20x: file_id 35.1% hit / 2596 reads vs hash 99.5% / 20 (130x fewer disk reads, RAM 512->80 MiB)
The win scales with the dedup ratio; with no dedup it is a no-op.
https://claude.ai/code/session_01DCszkkU11LYxMEUWr4setK
Claude
a9ce071a51