GET /api/photos sent only X-Next-Cursor — no ETag — so every gallery
re-mount rebuilt up to 500 PhotoDtos, serde-serialized the whole vector,
and shipped the full body even when nothing changed.
The handler now emits a lightweight content-derived ETag
(hash of before + limit + max(modified_at) + row count) and honours
If-None-Match, with Cache-Control: private, no-cache so the SPA's default
fetch cache mode always revalidates. An unchanged "navigate away and back"
becomes an empty 304 instead of a full rebuild + reserialize + transfer.
The DB query still runs (the cheap part); the win is skipping the DTO
build, serialization, and body bytes.
Proven end-to-end (throwaway Postgres + server, 7 images):
1st GET (no If-None-Match) -> 200 4586 bytes + ETag
2nd GET (If-None-Match matches) -> 304 0 bytes
3rd GET (If-None-Match stale) -> 200 4586 bytes (correctly invalidated)
~655 B/photo, so a full 500-row first page saves ~320 KB + a 500-DTO
build/serialize per unchanged revalidation. Unlike a cold load this is the
common gallery-navigation path, so it hits real user-facing latency.
Regression test: tests/api/photos_etag.hurl (added to the api-test suite).
Methodology in benches/PHOTOS-ETAG.md.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
DioCrafts
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