jellytau/docs/specs/read-through-media-cache.md

# Spec: Two-path media — selectable playback bitrate, independent whole-file download

**Status:** Partially implemented. Landed: the cache/download unification
(DR-126, DR-127 — a cache entry *is* a `downloads` row with a shorter life, and
eviction only reclaims the temporary tier), local playback of downloaded media
(DR-128), and the one-path/one-row invariants that followed (DR-133 … DR-138).
DR-123 is in progress. Still open: the read-through capture itself — DR-122,
DR-124, DR-125.

**DR-121 has shipped and left this spec.** The player quality selector, the
per-playback bitrate ceiling, and the backend-owned stream decision it needed
were built as *backend-owned stream selection* (DR-225 … DR-228) and are
described in
[01-rust-backend.md](../architecture/01-rust-backend.md#stream-selection) and
[03-data-flow.md](../architecture/03-data-flow.md#video-stream-selection-flow).
The settings-level ceiling (DR-162) is the same section. What remains here is the
*capture* half only — this spec no longer specifies anything about choosing a
bitrate.

**Requirements:** UR-070, UR-071 → DR-122, DR-123, DR-124, DR-125; IR-032
**Related:** the locally-indexed search and downloaded-browse work, both
shipped — see
[03-data-flow.md](../architecture/03-data-flow.md) and
[06-downloads-and-offline.md](../architecture/06-downloads-and-offline.md)

## Summary

Two things that are today tangled become explicitly separate:

- **The playback path** streams at a bitrate the viewer can change from the
  player. It is ephemeral and its rendition is volatile.
- **The download path** fetches the whole file at one canonical quality, in the
  background, independently of whatever playback is doing.

Bytes fetched for playback are kept **only** when the playback rendition happens
to be the same artifact the download path would produce — i.e. direct play.
Otherwise playback bytes are discarded and the download path does its own fetch.

## Motivation

The appealing version of this — "stream and download at once, switch when enough
has arrived" — breaks the moment the viewer can change bitrate. A capture taken
while the rendition changes underneath it is a splice of two encodings: not a
playable file, and not something that can be honestly recorded as a download.
Once bitrate is selectable, one stream cannot serve both jobs.

Separating the paths also removes the thing that made the original idea
expensive: there is no mid-playback source swap to engineer, because the download
never has to take over the live session. It lands on disk and is used at the next
natural boundary — next episode, or next time the item is played.

What exists already and is *not* this: `SmartCache` predictively downloads *other*
items, `player_preload_upcoming` warms the next one, and
`refresh_queue_local_sources` swaps queue entries to local at boundaries. All of
it concerns items you are not currently playing.

## Layer assignment

| Logic / responsibility | Layer | Why it belongs there |
|------------------------|-------|----------------------|
| Available bitrate options for an item | **Rust** | Derived from Jellyfin's media sources and playback-info negotiation; changes with the API. |
| Mapping a chosen bitrate to transcode parameters | **Rust** | Domain vocabulary. `get_video_download_url` already owns the quality→params mapping; playback must reuse it, not restate it. |
| Deciding whether playback bytes are keepable (direct play vs transcode) | **Rust** | Depends on the negotiated session. |
| Canonical download quality | **Rust** | Policy over domain data. |
| Cache eviction, storage budget, sparse-range bookkeeping | **Rust** | Storage policy. |
| Promotion to a `downloads` row, and what invalidates a cache entry | **Rust** | Domain state. |
| Rendering the quality selector; remembering the last choice | **Frontend** | Presentation and a view preference. The *list* comes from Rust. |
| WiFi-only / opt-in toggles | **Frontend collects, Rust enforces** | The control is UI; the gate must hold even if the UI never calls. |

Borderline, recorded: the **default** playback bitrate could look like a user
preference (frontend). It goes to Rust because it must be reconcilable with what
the server can actually produce for a given media source — a preference the
backend has to validate is not a preference the frontend can own alone. The
frontend stores the user's *choice*; Rust decides what that choice resolves to.

## Design

### DR-121 — moved out (shipped)

Bitrate selection in the player shipped as DR-225 … DR-228; see
[01-rust-backend.md](../architecture/01-rust-backend.md#stream-selection).

The one constraint here that the capture work still has to respect: a quality
change re-negotiates **within HLS**. Returning a progressive `stream.mp4` for a
transcode means playback never starts, because the server encodes the whole file
before serving a byte (DR-140). That is why DR-122 below abandons a capture on a
quality change rather than trying to splice one.

### DR-122 — The playback path is ephemeral

Playback bytes are not persisted unless DR-124 says they are keepable. No partial
capture is ever retained across a quality change: on change, any in-flight capture
for that session is abandoned and its partial file deleted.

### DR-123 — The download path is independent

Downloading the whole file is a separate operation through the existing download
manager, at one canonical quality (default `original`, the direct static copy),
using `/Videos/{id}/stream.mp4` — progressive and Range-capable, which is what
the resumable download worker relies on. It is unaffected by what playback is
doing, and playback is unaffected by it.

Once complete it becomes an ordinary download row, so everything already built on
top of downloads — offline browsing, `refresh_queue_local_sources`, the Downloads
page — picks it up with no further work.

**Prerequisite:** downloaded *video* is currently never played locally.
`repository_get_video_stream_url` goes straight to the online repo and
[player/[id]/+page.svelte:316](../../src/routes/player/[id]/+page.svelte#L316)
calls it with no local check — so a completed video download is still streamed.
This must be fixed or the whole feature is invisible for video.

### DR-124 — Keep playback bytes only when they *are* the download

Capture is enabled only where the played bytes and the canonical download artifact
are the same thing — a **direct-play** session. Then:

| Path | Mechanism |
|---|---|
| Android / ExoPlayer | `SimpleCache` + `CacheDataSource`, keyed by item id **and** media-source id so renditions never collide. LRU evictor sharing the existing smart-cache budget — not a second budget over the same disk. |
| Linux audio / MPV | `stream-record`, set through the existing `set_property` plumbing. |
| Linux video (HLS transcode) | **Not captured.** Segments are not a file; assembling one needs ffmpeg, which is not a dependency and which CI is forbidden from installing at job time. The download path (DR-123) covers this case instead. |

Two abandonment rules, both of which must delete the partial rather than promote
it:

- **Seek during an mpv capture.** `stream-record` is documented as intended for
  linear streams; seeking breaks the recording. Straight-through listening
  captures, scrubbing does not.
- **Any quality change** (DR-122).

### DR-125 — Promotion, rendition, and invalidation

A capture is promoted to a `downloads` row (`status = 'completed'`) only when it
covers the whole resource. Partial captures stay cache and remain evictable.

A new `downloads.source_rendition` column records the negotiated
quality/container/codec of whatever produced the bytes; `NULL` for rows fetched by
the existing paths, which are always `original`. This is what makes an "upgrade to
original" action possible later, and what stops a 720p capture and a 4K download
being indistinguishable rows.

**Invalidation.** A quality change never touches a file that already exists —
neither a permanent download nor a completed temporary one. Both remain valid
copies of the rendition they hold, and deleting either would throw away bytes
already paid for.

What a quality change *does* invalidate is an **in-flight** capture or background
download of cached media: it is abandoned and restarted at the newly chosen
quality, because a capture spanning a rendition change is a splice of two
encodings rather than a playable file (DR-122).

So the rule is about *ongoing* work, not stored files. Nothing in this spec
deletes user data.

### Gating

Capture and background download obey the existing WiFi-only gate and storage
budget, and are off unless opted in. Enforcement is in Rust.

## Out of scope

- **Mid-playback switch onto a completing download.** Two independent paths make
  it unnecessary; the download is used from the next boundary.
- **Backfilling the unplayed remainder of a capture.** Watch 40 minutes and you
  have 40 minutes; completing it needs sparse-range bookkeeping and a resumable
  tail fetch. The DR-123 download path already produces a complete file, which is
  the reason this can wait.
- **Bundling ffmpeg** to make transcoded video capturable. Real option, large
  packaging decision, its own proposal.
- **Routing Linux video playback through `stream.mp4`.** Regresses a documented,
  hard-won fix.

## Acceptance criteria

- [ ] The player offers the qualities Rust reports, and changing one resumes at
      the same position with audio/subtitle selection preserved.
- [ ] A quality change abandons any in-flight capture and leaves no partial file.
- [ ] A quality change never deletes a `downloads` row.
- [ ] A completed background download of a video is *played from disk* on the next
      play (the DR-123 prerequisite).
- [ ] A direct-play session played start-to-finish leaves a complete local file
      with no second fetch; replaying it fetches no media bytes.
- [ ] Seeking during an mpv capture abandons it; no truncated file is promoted.
- [ ] A transcoded Linux video session is never captured, and never partially
      promoted.
- [ ] Promoted rows record their rendition; existing paths still record
      `NULL`/`original`.
- [ ] Gates hold with the setting off *and* with the frontend never sending it.
- [ ] Eviction cannot delete bytes backing a promoted download row.
- [ ] `bun run check`, `bun run test`, `cargo fmt`, `cargo clippy`,
      `bun run test:rust`, `bun run check:boundary` pass; `bindings.ts`
      regenerated if Rust types changed.

## Testing

Rust, table-driven and pure where possible: quality→params resolution shared with
the download path; keepability (direct play vs transcode vs gate off); promotion
(complete → promoted, partial → not, seek-abandoned → not, quality-changed → not);
invalidation (evicts cache, never a download row); rendition round-trip.

Android: instrumented — a played direct-play item yields cache entries, and a
replay issues no media network request.

Frontend: the quality list renders from backend data with no item-type or
codec taxonomy in `src/`; the selector's remembered choice is a view preference.

## TRACES

| Piece | Tag |
|---|---|
| Ephemeral playback / capture abandonment | `// TRACES: UR-070 \| DR-122` |
| Independent whole-file download + local video playback fix | `// TRACES: UR-071 \| DR-123, IR-032` |
| ExoPlayer cache / mpv stream-record / keepability | `// TRACES: UR-071 \| DR-124` |
| Promotion, `source_rendition`, invalidation | `// TRACES: UR-071 \| DR-125` |

## Notes for the implementer

- **A parallel Claude session is active in this repo.** `git diff` before
  "repairing" anything you did not write.
- Do not duplicate the quality→transcode-parameter table. Call the existing one.
- Reuse the smart-cache storage budget; two budgets over one disk is how devices
  fill up.
- The `downloads` FK to `items` is relaxed (migration 005) — exercise promotion
  for an item that was never cached.
- Build DR-123's local-playback fix first. Without it nothing in this spec is
  observable for video.