# Architecture Plain overview first: [HOW-IT-WORKS.md](HOW-IT-WORKS.md). Wire format: [PROTOCOL.md](PROTOCOL.md). Plans: [ROADMAP.md](ROADMAP.md). ## Packages | Path | What | Rule | |---|---|---| | `packages/core` | protocol messages, sync engine (`SyncEngine`), constants, config, Range parsing, logging, Jellyfin client | pure Dart, no Flutter | | `packages/server` | Relic HTTP + WebSocket server (`SyncServer`, `ServerHub`, `Session`, media endpoint, Jellyfin relay), the `serve()` entrypoint, `bin/server.dart` (dedicated server) | pure Dart, no Flutter | | `apps/desktop` | Flutter app: UI, `SyncClient`, `SessionCoordinator`, `PlaybackController` (media_kit/mpv), `ServerIsolate`, updater | ships with `watch-together-server` | | `tool` | harnesses: `range_logger`, `shaper`, `fake_viewer`, `e2e_check` | dev only | | `deploy` | systemd unit for the dedicated server | | Guard: `packages/core/test/flutter_free_test.dart` fails if anything under `core/lib` imports Flutter, `dart:ui` or `signals_flutter`; CI runs analyze + tests for core and server on every push. Server has no such test but the same rule; CI checks it with `dart compile exe` of `bin/server.dart`. The client WebSocket (`web_socket_channel`) lives in `apps/desktop`; Relic has no client. ## Server lifecycle One entrypoint: `serve(config, events:)` in `packages/server`. `config` is an `AppConfig.toMap()` (primitives only, so it crosses isolates). It starts a `SyncServer` (bound to all IPv4 interfaces) and reports on the `events` port: `ready` (with the port and a `stop` SendPort), `error` (start failed), and every log record as `log`. Sending `{'t':'stop'}` to the `stop` port closes all sockets and returns, and the isolate exits. - **Hosting from the app**: `SessionState.host()` spawns an isolate running `serve` (`ServerIsolate`), waits for `ready`, then joins it over `127.0.0.1` like any other client. Server logs go to the app's log with a `[server]` prefix. The isolate runs with `errorsAreFatal: false`: an uncaught async error is logged and the server keeps going. If the isolate dies anyway, the session shows "server crashed: ..." and leaves; Home shows it on the Host card (during setup the host attempt fails with it). Stop (leave, close, cancel during setup, a failed own join, being kicked) sends `stop`, waits up to 3 s for exit, then kills the isolate. - **Dedicated server**: `bin/server.dart` compiled with `dart compile exe` and shipped next to the app as `watch-together-server`. It parses flags (`--port`, `--secret`, `--name`, `--media`, `--advertise-host`, `--jellyfin `, `--config`; default port 8080) into the same map and calls `serve`; SIGINT and SIGTERM stop it. A Flutter binary cannot run without a display, which is why the server is its own build. `SyncServer` runs a 1 s heartbeat: stale sweep, closed-socket sweep, silent socket drop, state + feed broadcast. There is one session, id `default`. Passwords: the default secret is empty, which means an open server (a warning is logged). Otherwise one shared secret, compared at hello. Host: when hosting, the app generates a random `hostKey` per hosting, puts it in the server config and sends it in its own `hello`. The member whose key matches is the host and cannot be kicked. The address a connection comes from plays no part (so a reverse proxy or tunnel on the same machine does not make everyone the host). The dedicated server has no key, so it has no host. Kick = revoke token, send `kicked`, close; the person may rejoin at once with a new token. ## Sync model Loosely modeled on Syncplay, with a different readiness design. - **Transport** is `playing`, `held` or `finished`. The server owns the clock: `positionAt(now) = position + (playing ? now - serverTime : 0)`. Reports from clients never move it. Only seek, swap, gate transitions and the end of media write `position`/`serverTime` and bump `eventId`. - **Clock offset**: `serverTime` is on the server's clock. Each client sends a `ping` with its clock every second and estimates the offset NTP style (`ClockSync`: `offset = s - (t0 + t3) / 2` of the sample with the smallest round trip among the last 16). All extrapolation uses `SyncClient.serverNow()` (local clock plus offset), so a client whose clock is off still lands on the server position. - **Reports**: clients send `position`, `buffered`, `eventId` every 1 s, plus the media `duration` and its `mediaId` once the player knows it. The server stores `buffered` and liveness, and takes the first reported duration for the current media (swap resets it); a report with `eventId` older than current has its position ignored. - **Ready gate** (HELD to PLAYING): the in-set is non-empty, every in-set member is ready, fresh and has `buffered >= need`, and quorum holds: in-set size >= 2, unless at most one member is fresh. `need` is 4 s (`minBufferSecs`), or near the end what is left minus 2 s (`endSlackSecs`, floor 0), since mpv cannot buffer past the end. So a lone viewer plays alone; with a second viewer present one Space waits for the other. Joining members start with `inSet=false, ready=false` and never block. Disconnect removes from the set; swap clears everyone. - **Pause**: while PLAYING, any in-set member who becomes not ready, under `need` buffered or stale flips the server to HELD (feed action `pause` by `server`). Recovery resumes by itself when the gate passes. - **End**: while PLAYING with a known duration, reaching it sets FINISHED (feed `finished` by `server`, position = duration). Clients pause where they are; FINISHED never auto-resumes. A seek turns it into HELD at the new position (resuming at once if the gate passes); a swap resets to HELD. - **Seek**: allowed from any member in any transport; the server writes the position, bumps `eventId` and records who sent it (`seekBy`, `seekSeq` in `state` for that event); flags are untouched. Clients diff each `state` broadcast by `eventId` (`StateDiff`): new event means seek to the computed server position, transport change means play/pause. Between events: drift over 5 s for more than 2 s seeks, ahead by more than 1.5 s sets rate 0.95 until level, otherwise nothing. - **User seeks**: the player is a `SyncPlayer` whose public `seek` is the user's: media_kit's seek bar (one `seek` per pointer move while dragging), the arrow keys and `seekTo` all become explicit events. The player moves at once (scrub preview); `UserSeekGate` sends one `seek` intent with a fresh `seq` after 300 ms without another request (`seekDebounceMs`). While the user is seeking, and until the echo of the last sent seq arrives (3 s at most), server seeks never touch the player; the echo of our own seek is not applied either (the player is already there). A server seek that has not landed after 1.5 s is re-applied once (`_checkSeekLanded`), to the server position recomputed now, and only while its event is still current and the user is not seeking. - **Liveness**: WS ping every 5 s; silent over 15 s means stale (out of the gate, `stale` feed); over 30 s the socket is closed and the member removed. A stale member that was ready goes back into the set with its next frame. Closed sockets are polled with `isClosed` every second: Relic's server-side `onDone` does not fire for dead peers, so presence never relies on it. - **Resume**: a reconnecting client sends its previous token as `hello.resume`. If the server still has that member (an unclean drop it has not noticed), the new socket takes it over: same token, member id, ready flag and media URL (the player keeps streaming), no join/leave feed, and the dead socket is closed without removing the member. An unknown or kicked token joins as a new member. - **Client policies**: no state broadcast for 5 s means connection lost, clear the server state and reconnect every 1.5 s until cancelled (an epoch counter makes a cancelled attempt stale) or kicked. After a reconnect the ready flag is restored from the server's state. Cancel during setup aborts the join. Closing the window leaves gracefully (2 s timeout). - **Idle** (`IdleTracker`): measured from the later of the last input and the moment playback stopped. While HELD and self ready: warning at 8 min, auto-unready at 10 min, and never less than 2 min after the warning. - **Deviations from Syncplay**: clock never follows the slowest watcher; no pause-on-leave; anyone can force ready/unready (all or one); quorum gate; the clock is server time (clients correct for their offset to it). Additive wire changes only; bump `proto` and PROTOCOL.md in the same commit. ## Media Three sources, same sync. - **Link**: the media field holds an http(s) URL. It travels as `media.url` in `swap` and `state`, and as `mediaUrl` in `welcome`. Each app plays it directly; the server is only the clock. - **Host local file**: `SyncServer.setLocalFile` / `mediaPath` makes the file the session media. `GET /m///` serves it with a Range table, gated by the viewer's token. The server builds the URL per viewer (`http://:/m/default//`), so tokens never reach other viewers. `` is the authority the client dialled; `advertiseHost` (default empty) only overrides it when set explicitly. When a `state` broadcast has no `media.url`, the client builds the URL itself from the host it connected to. Bytes stream file to socket and are counted as streamed (`bytesServed`); nothing is stored. The host plays through loopback like everyone else. - **Jellyfin item**: the media is a ref (`{serverId, itemId, ...}`, wire in [PROTOCOL.md](PROTOCOL.md) "Jellyfin media"); the server never stores or sends a URL or token for it. Each app resolves it with its own account and direct-plays the static stream; guests use the relay (see "Jellyfin relay"). Range table (`resolveMediaRequest`, tested byte-exact): | Request | Response | |---|---| | no Range | 200, `Content-Length` = size | | `bytes=a-b` | 206, `Content-Range: bytes a-b/size` | | `bytes=a-` | 206 to EOF | | `bytes=-n` | 206 last n bytes | | unsatisfiable | 416, `Content-Range: bytes */size` | | host has no file | 503 + `Retry-After: 2` | | bad / unknown token, kicked, unknown session | 401, 403, 410 | Always `Accept-Ranges: bytes`; `Content-Type` from the extension (`contentTypeForFile`). Relic hands the Range over as a typed `RangeHeader`; the endpoint takes the first range and keeps the unit, so non-`bytes` units still get 416. What mpv does against it (recorded with `tool/range_logger.dart`): mp4 opens once with no Range then streams `bytes=0-` to the end; mkv opens `bytes=0-` twice (probe, playback), reads a small tail at `size-n` (cues, expressed open-ended), then every seek is a fresh `bytes=N-` connection. A server outage of a few seconds is ridden out from mpv's cache with no error. Longer outages exhaust the cache and rely on reconnect (not chaos-tested yet). mpv settings (`PlaybackController`): `Player` with `bufferSize` 256 MiB (media_kit's `bufferSize` sets both `demuxer-max-bytes` and `demuxer-max-back-bytes`), `libass` on; pinned via runtime property: `demuxer-readahead-secs=150`, `cache-secs=150` (about two minutes of read-ahead: rides out short drops, intro skips are instant), `demuxer-max-bytes=268435456` (150 s up to ~14 Mbit/s; a 1.4 GB / 24 min file is ~8), `demuxer-max-back-bytes=67108864`, `stream-lavf-o=reconnect_streamed=1,reconnect_on_http_error=1,reconnect_delay_max=4` (comma form; this mpv build rejects colon-joined). The cache is in RAM: media_kit sets `cache-on-disk` without a `cache-dir`, and mpv falls back. mpv ignores invalid values silently, so every pin is read back and a mismatch is logged and kept in `pinMismatches`. Measured against the `/m` endpoint (`readahead_e2e_test`): a paused player buffers ~147 s ahead (10 s before). media_kit has no public `setProperty`/`getProperty`; the code calls them through a guarded dynamic call on `player.platform`. Hardware decoding comes from Settings (auto, off, custom) and is passed to the `VideoController`; env `SYNC_RELAY_HWDEC` (e.g. `no`) overrides it. It is read per player, so a change applies on the next join or Reload video. Shaders (per viewer, local only, no wire change): mpv `glsl-shaders`, set through the same dynamic `setProperty` and read back (`PlaybackController.applyShaders`). The value is the platform path list (`:` joined, `;` on Windows). One saved choice in `AppSettings` (`shaderPreset`, `shaderQuality`, `customShaders`): applied on every player creation (join, Reload video) and live when changed from the session (the control bar's shader menu, Ctrl+0..6, both also save it). The Anime4K v4.0.1 files live in `apps/desktop/assets/shaders/anime4k/` (only the 14 the presets use, plus its MIT LICENSE); mpv needs real paths, so `extractAnime4k` copies them once per version to `/shaders/anime4k-/` (a `.complete` marker is written last). `anime4kFiles` maps each mode to the file list of Anime4K's mpv templates: HQ = the high-end GPU set (VL + M networks), Fast = the low-end set (M + S). A missing file, a path containing the list separator, a read-back mismatch, or an mpv `libmpv_render` error while shaders are on turns shaders off (saved) with a toast. Render mode: media_kit's Linux/Windows video output creates mpv's render context (OpenGL, else its software renderer) when the `VideoController` is created, and nothing in its API says which. The player therefore requests mpv logs at `v`: OpenGL init logs `GL_VERSION` under `libmpv_render` (`RenderMode.gpu`; "suspected software renderer" means llvmpipe, `softwareGl`); none within 1.5 s of the controller being ready means the software path (`software`, where shaders do nothing; the shader toast, the menu and Settings say so). After startup the `v` level adds no steady traffic (measured: 0 messages over 5 s of playback). The hardware path is fragile: media_kit borrows whatever EGL context GDK left current on the main thread (upstream media-kit#1404), so an X11 session or a Flutter/GDK change can silently fall back to software. Player lifecycle: the `Player` and its `VideoController` live for one session: created on join, disposed on leave, replaced by `recreate()` for "Reload video" (button or Ctrl+R; volume kept). `openUrl` opens with `play: false`; play/pause/seek/rate come only from `StateDiff` actions, the play gate (`localPlayAllowed`: self in-set and ready) or the user's own seeks (see Sync model). A server seek within 0.5 s of the current position is skipped. `leave()` stops the player (otherwise audio keeps playing). On media change (`mediaId` bump) the coordinator opens the new URL and `StateDiff` seeks to 0, holds and resets rate. Frozen picture: `FrameWatchdog` samples about once a second. It hashes a small capture of the compositor's centre (`ShownFrameProbe`) and suspects a freeze when position advances while that hash stays constant for 3 s. Two differing mpv `screenshot-raw` frames show the decoder is still producing pictures, which confirms the freeze (a still scene is dismissed). A confirmed freeze reloads the video with a toast. It fires once until the picture moves again for 10 s, so it cannot loop. Seeks, pause, buffering and a hidden window disarm it. Never unmount GPU-textured widgets (avatar images) on a transport-gated branch: teardown races mpv's frame upload and the window goes black. Keep them mounted and skip painting (`Visibility` with `maintainState`). ## Jellyfin client `packages/core/lib/src/jellyfin/`, pure Dart over `dart:io` `HttpClient` behind a `JellyfinHttp` interface (tests inject a fake). Plans: [ROADMAP.md](ROADMAP.md) "Epic: Jellyfin". - `JellyfinClient`: one server + one user. Auth (Quick Connect, password, `/Users/Me` check, logout), browsing (views, Resume, NextUp, Latest, paged `/Items`, search, seasons, episodes), image URLs (tag + maxWidth, no token needed), PlaybackInfo, stream URLs, Playing / Progress / Stopped reports, played / unplayed / resume position. - `normalizeServerUrl`, `parseJellyfinLink`: pasted addresses and item links -> `(baseUrl, itemId)`; accounts are keyed by `ServerId` (`/System/Info/Public`). - `episodeAfter`: next episode is the successor in `/Shows/{id}/Episodes` (crosses seasons, skips other copies of the same episode); NextUp means first unwatched, not next-after-X. - `matchTrack` / `matchTracks`: carry audio/subtitle choice to the next episode (language required, then title words, forced/HI flags, codec, channels). Gotchas (Jellyfin 10.11): - Every request sends `Authorization: MediaBrowser Client, Device, DeviceId, Version[, Token]` (values percent-encoded). Unauthenticated calls missing a field get 400 with no body. - A POST must carry `Content-Length` even when empty: behind nginx a body-less POST without it gets 400 (this was the Quick Connect Initiate failure). - Jellyfin overwrites a token's stored device name and app version with the header's. Only use tokens this app obtained itself; a relay using a viewer's token must send that viewer's client fields (the server relay sends exactly its credential's fields). - Static streams (`/Videos/{id}/stream?static=true`) serve the original file with byte ranges. The share URL has no credentials; each side adds its own `api_key` (`authorizeUrl`). - Ticks: 10,000,000 per second. ## Jellyfin relay For viewers without an account on the item's Jellyfin (`packages/server/lib/src/jellyfin_relay.dart`). The server may hold one `JellyfinCredential` (config key `jellyfin`: `baseUrl`, `serverId`, `token`, `client`, `device`, `deviceId`, `version`; primitives, so it crosses the isolate boundary in the `serve()` map). The hosting app fills it with its own app-obtained token and that token's device fields; the dedicated server reads it from `--jellyfin ` (YAML/JSON with those keys, keeps the token off the command line) or a `jellyfin:` map in `--config`. An empty `serverId` is looked up at start (`/System/Info/Public`); if that fails the relay stays off. The token is never logged (`toString` and config parse errors leave it out). - Available (`state.relay`) only while the current media is a ref whose `serverId` equals the credential's. Only that item is relayed, only to a valid, unkicked viewer token: no open proxy. - Each request opens `/Videos/{id}/stream?static=true&mediaSourceId=` with `Authorization: MediaBrowser` built from exactly the configured fields (Jellyfin rewrites a token's device fields from the header), forwards `Range` untouched and `Accept-Encoding: identity`, and returns Jellyfin's status and range headers. Bytes stream through (no buffering) and count toward `bytesServed` as sent. A viewer disconnect cancels the upstream response, which closes that connection (tested: `HttpClientRequest.abort` does nothing once the response has arrived; cancelling the response subscription does). - No progress reporting in the server: watched state is per client. ## Desktop UI One `HomeScreen` (`screens/home/`) with a single top bar (`top_bar.dart`): profile chip, Join and Host tabs, the update indicator, Settings / Integrations / About buttons, and the window drag area and buttons. Only the selected view is mounted; form state lives in `HomeShell`. Join (`join_tab.dart`): one compact address/port/password row, then up to 6 Recent server tiles in a grid that adds columns as the window widens. While the Join tab is visible each tile polls the server's `GET /health` every 10 s (`health_probe.dart`) for online, name, viewer count and a "needs update" badge when `proto` differs from ours. Host (`host_tab.dart`): the media picker (`media_picker.dart`: Jellyfin placeholder, Link, File; a reusable widget driven by a `MediaPickerController`) fills the main area; port, password and Host sit in a 340 px column on the right, below the picker under 860 px. Text fields have fixed or capped widths (720 px max) so they stay compact on ultrawide screens. Submit state (busy, cancel, per-tab errors, server crash) is `ConnectFlow`. Settings (profile, hardware decoding, updates, video shaders, shortcuts), Integrations (Jellyfin "coming soon") and About open in the same body. Settings persist in two Hive boxes: `connect_form` (last form and recents; saved even when a join fails) and `settings`. Link media names are derived from the URL path and never contain a query string. Window chrome is frameless (`window_manager`): every screen brings its own drag area and min/max/close buttons (Home's top bar; the session uses `widgets/window_chrome_bar.dart`; both use `window_buttons.dart`); the session drag layer is `window_drag_layer.dart`. ## Health reporting Every failure must show in the UI with a cause and a next step; nothing stalls silently. The app has a `HealthStatus` signal (severity + message) for connecting, reconnecting, wrong password, version mismatch, kicked, host offline, and starvation ("waiting for X - N.N s buffered"). Errors are logged with a stack trace at the boundary, then turned into a displayable state. The server's `GET /health` returns JSON (`ok`, `name`, `proto`, `viewers`, `bytesServed`, `uptimeSecs`); it is unauthenticated, so it carries nothing sensitive. Home's recent-server tiles poll it for live status. ## Updater `apps/desktop/lib/src/update/updater.dart` (no Flutter imports, unit tested). Reads a manifest (`version`, `protocol`, `url`, optional `sha256`, `notes`) from `updateManifestUrl`, compares versions, downloads the tarball, checks sha256 when present, then extracts into a staging dir inside the install folder (same filesystem, so moves are renames; the folder must be user-writable). Each top-level entry is moved aside and the new one moved in, with rollback on failure (writing the running executable fails with ETXTBSY; rename works). A packaged install is detected by `data/` and `lib/` next to the binary; anything else exits with "update manually". A manifest whose protocol is newer than the local one is a required update. Windows apply (`*.old` rename, swept on next start) exists but is untested. The version is stamped with `--dart-define=APP_VERSION` (dev runs report `0.0.0-dev` and skip checks). `watch_together --upgrade` (or the shipped `update.sh`) runs it from the CLI; `UpdateController` runs the same code for Home's update indicator, which opens the update card, and restarts the app ("Update and restart"). Settings has Check now and an auto-check toggle. The manifest is `updates/manifest.json` in this repo, written by `tool/release.sh`. ## Decisions - Desktop + mpv only, no web clients: exists because Jellyfin SyncPlay is bad. - No transcoding: bandwidth is plentiful and the Jellyfin box should not be loaded. - No roles, friends trust model: simplest thing that fits the use. - Blank password = open server: the owner chooses between open and private. - The host cannot be kicked: the server dies with the host anyway. - Server runs inside the app on an isolate, and ships as a separate compiled server: a Flutter binary cannot run headless, so a dedicated server needs its own build of `packages/server`. An isolate keeps the UI responsive and shares the process (sockets, file paths) without a second executable. - Host by key, not by address: loopback detection broke behind a same-machine reverse proxy or tunnel. - Server owns the clock; reports never move it: avoids ping-pong between clients. - Quorum gate: a lone viewer must be able to watch; two or more must start together. - Relic for the server, `web_socket_channel` for the client; packages are chosen on merit, not locked. - Plain `ws://`, no TLS: friends use LAN, VPN or a forwarded port; TLS (e.g. a reverse proxy) is a documentation item. - One session per server, id `default`: the map is keyed for later but unused. - No server storage: config and logs only; media never touches the server disk. - Avatars ride in the 1 Hz `state` broadcast (cap 256 KiB base64 per avatar): simple, costs host uplink; hash-based serving is on the roadmap.