39 lines
2.0 KiB
Markdown
39 lines
2.0 KiB
Markdown
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# Photos timeline — ETag / 304 conditional revalidation
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`GET /api/photos` returned only `X-Next-Cursor` — no ETag. So every time the
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gallery re-mounts (navigate away and back), the server rebuilds up to 500
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`PhotoDto`s (each a `FileDto::from` with several allocations), serde-serializes
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the whole vector, and ships the full JSON body — even when nothing changed.
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The handler now emits a lightweight, content-derived ETag and honours
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`If-None-Match`, with `Cache-Control: private, no-cache` so the browser always
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revalidates. An unchanged gallery re-mount becomes an empty **304** instead of a
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full rebuild + reserialize + re-transfer.
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- **ETag** = hash of `(before, limit, max(modified_at), row count)` over the page
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— page identity plus a freshness signal, mirroring the file-list endpoint.
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Any upload/edit/delete that changes the page bumps `max(modified_at)` or the
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count, so the ETag changes and the client gets the fresh body.
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- The browser revalidates because the SPA's `apiFetch` uses the default fetch
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cache mode and `no-cache` forces a conditional request.
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- The DB query still runs (it's the cheap part); the win is skipping the DTO
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build + serialization + body bytes.
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## Reproduce / proof
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`tests/api/photos_etag.hurl` (run by the api-test suite) asserts: first GET → 200
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+ ETag + `Cache-Control: no-cache`; conditional GET with the matching ETag →
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empty 304; a stale `If-None-Match` → full 200. End-to-end measurement against a
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throwaway Postgres + server (7 uploaded images):
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```
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1st GET (no If-None-Match) : 200 4586 bytes + ETag
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2nd GET (If-None-Match matches) : 304 0 bytes ← the win
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3rd GET (If-None-Match stale) : 200 4586 bytes ← correctly invalidated
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```
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~655 B/photo on the wire. At a full 500-row first page that's **~320 KB +
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500-DTO build/serialize saved per unchanged "navigate away and back"**, plus the
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response bytes. Steady-state gallery navigation is the common case, so this hits
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real user-facing latency (unlike a one-time cold load).
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