watchparty/docs/architecture.md

Architecture

Goals

• Watch the same title, in sync, with other users of the same Emby server.
• Use Emby as the media source (catalog + streaming) without requiring changes to Emby itself or its official clients.
• Access gated to people with an Emby account on this server — no anonymous use, no separate identity system.

Non-goals

• Chat/reactions — deliberately out of scope.
• OIDC/SSO — an OIDC provider (LemonLDAP::NG) is available in principle, but the explicit choice was Emby-only auth: simpler, no extra infra dependency, and everyone already has an Emby account.
• Full library navigation (Shows → Seasons → Episodes) in v1 — search-based catalog only, to keep the first version achievable. Revisit if browsing turns out to matter more than searching in practice.

Why this replaced an Emby plugin

The first attempt (still in src/WatchParty/, kept for reference) was a real Emby Server plugin: sync via ISessionManager's existing remote-control commands, a settings page via IHasWebPages, and — after a fair amount of reverse-engineering — a real user-facing page integrated into Emby's own web client menu (PluginPageInfo.EnableInUserMenu, undocumented but confirmed working; required a data-controller AMD module to satisfy Emby's modern ViewManager, found by reading the target server's actual viewmanager.js).

That plugin builds and installs correctly. What it hit a hard wall on: a button inside the video player itself, or on an item's detail page. An Emby developer confirmed on their own forum that plugins have no API for this ("It's currently not possible for a plugin to do this"), and nothing found in research since suggests that's changed. The only way to get a button into Emby's own player would be patching Emby's shipped web client files directly inside the Docker container — outside the plugin entirely, guaranteed to break on every Emby image update, and requiring manual reapplication each time. That trade-off wasn't worth it.

Building a standalone app sidesteps the question entirely: it's our own player, so there's no "how do I add a button to someone else's UI" problem in the first place, and sync can be tighter than nudging Emby's remote-control commands ever allowed (see below).

Full trace of the plugin investigation (real errors, real fixes, sources consulted) is in the git history of this file and of src/WatchParty/ — worth reading if a future Emby SDK update ever makes in-player integration possible and this gets revisited.

Components

server/   Node.js + Express + Socket.IO. Owns party/room state and the
          realtime sync channel. Never touches a media byte - see "Media
          never flows through our backend" below.

client/   Static frontend, plain JS + ES modules, no build step (same bias
          as the earlier plugin's control panel). Talks to two different
          things and must not confuse them:
            - Emby directly (embyApi.js): catalog, images, streaming.
            - Our own backend (party.js): create/join/leave a party, and
              the Socket.IO connection for realtime sync.

Auth and Emby access: the server proxies everything

Revised after the first version: the browser used to call Emby directly (catalog, images, streaming) and only used server/ for party coordination. That hit real CORS problems in practice (Emby/its reverse proxy don't answer cross-origin preflights the way that needed), and separately wasn't the wanted shape anyway — the browser holding a real Emby access token at all was more exposure than necessary. Current design:

• The browser sends a username/password once, to POST /api/auth/login on our own backend — never to Emby directly.
• Our backend (server/src/embyAuth.js) calls Emby's POST /Users/AuthenticateByName itself, using EMBY_SERVER_URL from its own environment (not something the user ever types — see .env.example). On success it mints its own opaque session id, stores {embyToken, userId, userName} server-side (in-memory, like party state), and hands the browser only that session id, as an httpOnly cookie (wp_session). The browser never sees or holds Emby's real access token.
• Every other request — catalog search, item details, images, HLS streaming, party actions — goes to our own origin only (/api/...), authenticated by that same cookie. server/src/embyProxy.js is what actually forwards these to Emby server-side and streams the response back, attaching the session's real Emby token itself.
• The /api/catalog/stream/:itemId/master.m3u8 and .../raw routes proxy HLS specifically: the master playlist is fetched from Emby server-side, and every URI line in it is rewritten to point back through .../raw (whether Emby wrote it as relative or absolute) — otherwise hls.js would follow the playlist's own URLs straight to Emby the moment it tried to fetch a segment, defeating the whole point. See rewritePlaylist in embyProxy.js.
• Images (<img src>) and video (hls.js/<video>) still can't carry a bearer header, but that's fine now: they're same-origin requests to our own /api/... paths, so the session cookie rides along automatically — no token in any URL anywhere, unlike the first version's ?api_key=....

This also means the client no longer needs to ask for a server URL at all — one less field, one less way to get it wrong. EMBY_SERVER_URL lives only in the backend's environment.

Control model: host drives, everyone else follows

Now that we own the player, the classic watch-party model (Teleparty, Amazon Watch Party, ...) fits better than the old plugin's "everyone keeps their own controls, shared reference position" — that design was specifically a concession to nudging pre-existing, independently-running Emby app sessions we didn't control. Here, there's only one player implementation, so:

• The party creator is the host. Their player has normal controls.
• Everyone else's <video> has controls hidden; a small notice says the host is in charge.
• Host → server → everyone else, over Socket.IO (server/src/socketHandler.js):
  • Event-driven: every play/pause/seeked on the host's video sends {type:"playstate", isPaused, positionTicks} immediately.
  • Heartbeat: also sent every 3s regardless, so latecomers/drift get corrected even without a discrete host action.
  • Followers apply pause/unpause immediately, and only seek if drift exceeds 1.5s (DRIFT_TOLERANCE_SECONDS in client/js/views/player.js) — avoids fighting small timing noise with constant re-seeks.
• If the host leaves, the server promotes another member (same fallback logic the old plugin used for its "reference" member).

Party lifecycle

In-memory only on the backend (server/src/partyManager.js) — a restart clears every party, which is fine: a party is meant to last one watch session, not persist for weeks. removeIdleParties sweeps anything with no activity for 4 hours.

Realtime transport: Socket.IO, and two real memory leaks

The realtime layer was originally a raw WebSocket (the ws package). Two things prompted a rework — a requirement (use Socket.IO instead) and a specific worry (memory leaks) — and auditing for the second turned up two real, concrete leaks, independent of which transport library is used:

1. Server-side: dead connections never got cleaned up. A raw ws server has no idea a client is gone unless it receives a proper close frame. A laptop put to sleep, a network drop, a killed tab that never gets to send one — none of those fire ws's 'close' event, so the PartyMember's socket reference (and everything it closes over) would sit in memory forever, and the party would never get cleaned up by anything (removeIdleParties only looks at lastActivityUtc, which a phantom connection doesn't affect). ws's own docs recommend a manual ping/pong heartbeat to detect this - Socket.IO has one built in (pingInterval/pingTimeout), which is a real, not just cosmetic, reason to prefer it here, not only the requested API ergonomics.
2. Client-side: leaving the player any way other than its own "Quitter" button leaked everything. router.js only ever did currentRoot.innerHTML = '' between routes — that destroys DOM nodes, but nothing told the outgoing view's JavaScript to stop. Clicking a sidebar link or the browser's back button while watching left the player's 3-second heartbeat setInterval, its realtime connection, and its hls.js instance all running against a detached <video> element indefinitely - for as long as the tab stayed open. Fixed by having route() handlers optionally return a cleanup function; router.js calls whatever the previous view returned right before rendering the next one, regardless of how navigation happened. player.js returns one that clears the heartbeat, destroys hls, disconnects the socket, and removes the one document-level click listener it adds (named, not inline-anonymous, specifically so it's removable at all - an anonymous listener added fresh on every visit would itself have been a slow leak even with everything else fixed).

Both are now covered by the same class of test: drive the router through several "join a party, then navigate away without clicking Quitter" cycles and check nothing accumulates. Verified directly (see session history) by instrumenting document.addEventListener/removeEventListener and setInterval/clearInterval around three such cycles against a really-running local server (Socket.IO connection included, not mocked) - adds and removes matched exactly (3/3 listeners, 2/2 intervals across two cycles), nothing left pending. The server-side heartbeat fix is Socket.IO's own well-established mechanism, not custom code, so it wasn't separately instrumented the same way.

server/src/socketHandler.js mirrors the old wsHandler.js protocol closely (see its own top comment for the exact event list) - the one naming difference is the error event, sent as "errorMessage" rather than "error", since Socket.IO reserves the latter for its own internal error handling.

Live lobby, host handoff, no join codes

Three related changes, all in service of "the home page should reflect reality without a refresh":

• The home page's party list is a live subscription, not a fetch. client/js/party.js's connectLobby joins a "lobby" Socket.IO room (subscribeLobby → server/src/socketHandler.js) and gets a fresh snapshot on subscribe, then again every time broadcastLobby(io) runs - which is now wired into every place party membership changes: creating a party, joining one (both in index.js's REST handlers), and leaving (applyLeave, used by both the socket leave/disconnect handlers and the REST leave endpoint, so this - and the host-handoff notification below
  • only has to be right in one place). home.js returns this subscription as part of its router cleanup function, same as the player does for its own socket - an open lobby subscription left running after navigating away would have been exactly the same class of leak just fixed above.
• Host and elapsed time are shown per party, per the request. Host name and member count come from the same snapshot. Elapsed time doesn't: it ticks client-side from a createdUtc timestamp the server includes once (home.js's setInterval re-renders every .elapsed label every second) - pushing a recomputed duration from the server every second for every viewer would be pure waste when the client can derive it from one number. Verified live (see session history): faking a parties push with a createdUtc two minutes in the past and waiting rendered "2:06", not a static value.
• No join codes. PartyManager no longer generates or tracks one - see its class comment. The home page's live list is now the only way to find a party; a code that has to be copied and typed out of band doesn't add anything on top of a list that's already always current. POST /api/parties/:partyId/join replaces the old :joinCode route.
• Host handoff is live, not just correct in the data model. PartyManager.leave already promoted another member to host when the host left and destroyed the party once empty - that part predates this change. What was missing: a player already open as a follower had no way to find out it became host mid-session. applyLeave now emits a personal "hostChanged" (to that one socket, not the room) when this happens; player.js's becomeHost() - the same function init() calls when you already are the host - unlocks the controls live, no reload needed. becomeHost is written to tolerate being called more than once safely: addEventListener with the same function reference is a no-op if already attached, and the heartbeat setInterval is explicitly guarded (if (!heartbeat)) since setInterval has no such built-in idempotence - calling it twice would leak the first interval's id, the exact class of bug the rest of this pass was about fixing, not reintroducing.

One incidental bug found and fixed while touching this code: the home page's "Rejoindre" button on a live party card used to navigate() straight to the player without ever calling the join endpoint first - it worked well enough to load the video and read basic party info (GET /api/parties/:id never checked membership), but the realtime "join" would be rejected server-side ("Tu n'es pas membre de cette party.") since socketHandler.js's join handler does check membership - meaning sync silently never worked for anyone who joined this way. Fixed by awaiting joinParty() before navigating, same as the library grid's tiles already did correctly.

Player: no page scroll

.playerWrap was width:100%; height:100vh sitting inside .mainContent, which has its own padding - the two stacked (100vh plus padding) made the whole shell taller than the viewport, so the page grew a scrollbar even though the video itself was already full-height. Changed to position:fixed; inset:0 so it's sized against the real viewport regardless of any ancestor's padding, and (like most players' theater mode) covers the sidebar while watching - confirmed via computed style (position: fixed, all four insets 0px) rather than just asserted.

Presence overlay and "request pause"

Requested as a pair: a semi-transparent overlay showing who's in the party, and a way for a non-host to ask the host to pause without a chat feature (deliberately out of scope — see Non-goals). Revised once after the first pass, based on concrete feedback: the overlay moved from bottom-left to the top of the player, and the "someone requested a pause" indicator became a specific orange border + pause-icon badge rather than a generic pulse animation.

• Layout: the player's top gradient bar is now .topOverlay, a column of two stacked rows — .actionBar (title/members-text/next/settings/leave, what used to be the whole bar) and, directly below it, #presenceOverlay. Stacking them in normal flow inside one column means the avatar row always lands right after the action bar regardless of how tall that bar renders (it wraps to two lines on narrow screens) — deliberately not a second absolutely-positioned block at a guessed pixel offset, which would either overlap the action bar when it wraps or leave an inconsistent gap when it doesn't.
• Presence: client/js/views/player.js's renderMembers() — already responsible for the text member list — now also rebuilds #presenceOverlay, one circle per member. Each circle shows the first two characters of userName, uppercased (initialsFor), on a background color picked deterministically from a small fixed palette by hashing the name (colorForName) — same person, same color, every render, with no need for the server to assign or track one. opacity: .75 on .avatar is the "semi transparence" itself, applied to the whole circle (initials included) rather than just its background, since a chat-bubble-style translucent chip reads clearly enough at that opacity and avoids converting the hash color to an rgba string just to keep text fully opaque.
• Which circle is "yours" is decided client-side by comparing member.userName === auth.userName (the same auth singleton embyApi.js already populates at login/session-restore) rather than by userId — consistent with the rest of the client already treating userId as a server/protocol-internal detail (see hostChanged's empty payload) and not something to key UI logic on directly.
• Request pause: your own circle — only when you're not the host, the host already has direct controls — gets .avatarClickable and a click handler that calls the party connection's requestPause() (client/js/party.js → "requestPause" socket event). The server (server/src/socketHandler.js) broadcasts "pauseRequested" {userId, userName} to everyone in the party, not just the host, so the whole group sees why playback is about to pause, and throttles it server-side per member (5s, member.lastPauseRequestUtc) independently of the client's own identical cooldown (PAUSE_REQUEST_COOLDOWN_MS in player.js) — the server-side one is the real guard (anyone poking the socket directly bypasses a client-only check); the client one just avoids a pointless click doing nothing.
• Receiving "pauseRequested" shows a short banner (#pauseRequestBanner, "<name> demande une pause") and flags the matching person's circle (.avatarPauseRequested: an orange #ff9800 border plus a small pause-icon badge in its corner — .avatarPauseBadge, a plain inline two-bar SVG rather than a Unicode glyph, since pause/play characters default to emoji presentation on several platforms and look muddy at 10px) — both matched by the event's userId against each circle's data-user-id, both clearing automatically after PAUSE_REQUEST_HIGHLIGHT_MS (4s), for every member, including the requester and the host — deliberately not host-only, so a request landing while the host isn't looking at the tab is still visible to everyone else, and the requester gets confirmation their own click registered. This replaced an earlier plain box-shadow pulse animation and a separate immediate-local-feedback ring on just the requester's own circle (.avatarSent) — collapsed into one unambiguous visual language driven purely by the server broadcast (which the requester also receives, so there's no need for a separate client-only echo).
• .followerNotice picked up a margin: 0 it was missing: it's a <p>, and for an absolutely positioned element bottom places the margin edge, so its default ~1em user-agent margin was quietly rendering it 13px higher than its declared bottom: 24px ever suggested. Harmless on its own until .pauseRequestBanner needed to sit a specific gap above it — found by measuring both rects and seeing the gap read as 2px instead of the ~15px the two bottom values alone implied.
• Nothing about this feature actually pauses anything automatically — "request" is literal, the host still has to act on it. Adding an actual forced-pause would be a real design decision (can any member override the host's playback state? that cuts against "host drives, everyone follows") and wasn't asked for.

Verified end to end with mocked members/pauseRequested Socket.IO events against the real running local server (see session history): the presence row renders directly under the action bar; initials/colors render correctly for a two-member party; the host's circle never gets .avatarClickable (neither when viewed by themselves nor by others); a pauseRequested event gives the right circle rgb(255, 152, 0) as its computed border color and display: flex on its badge (confirmed via computed styles, not just class names) while leaving every other circle untouched; rapid repeat clicks on your own circle within the cooldown collapse to exactly one requestPause emission (three synchronous clicks in one script → the emitted-event count increased by exactly one — an earlier two-separate-clicks check had looked like a cooldown failure at first, but that was real wall-clock time between two separate tool calls exceeding the 5s window, not a code bug); and the banner/.followerNotice gap measured 15px after the margin fix, versus 2px before it. The host-only controls (becomeHost, settings menu, next-episode button) were re-checked after this change and still work.

Player track selection: forcing our pick onto Emby's own negotiation

First real-world bug report once someone actually watched something end to end: picking a language or subtitle track in the settings menu had no effect on playback. Streaming itself working at all (this is the first confirmation of that - see below) narrowed it down: PlaybackInfo and its TranscodingUrl are reachable and produce a playable HLS stream, but the specific track requested wasn't the one that came out the other end.

Likely cause, reasoned from how Emby/Jellyfin's own web client actually negotiates this (not verified against this exact server - no way to, from here): that client sends AudioStreamIndex/SubtitleStreamIndex as part of a POST /Items/{id}/PlaybackInfo body alongside a DeviceProfile, not as bare query-string params on a GET - which is all this app has ever sent. Emby's TranscodingUrl reflects whatever PlaybackInfo actually negotiated, so if the plain GET doesn't apply those params, the returned URL just encodes Emby's own default track choice regardless of what we asked for - and the old code used that URL as-is. Separately, even a correctly-selected subtitle track has a second way to silently not show up: Emby can deliver text subtitles either burned into the video (SubtitleMethod=Encode) or as their own selectable HLS rendition (SubtitleMethod=Hls) - this player's hls.js integration never wires up subtitleTrack/subtitleTracks to pick a rendition, so a Hls-method subtitle would be present in the manifest but never actually rendered.

Fixed defensively rather than by trying to exactly replicate Emby's own client (server/src/index.js's /master.m3u8 route + its new applyPlaybackOverrides helper): whichever URL we end up with - Emby's own TranscodingUrl, or the hand-built fallback (which, before this fix, dropped the audio/subtitle/quality query params entirely - a second, independent way the same symptom could happen if a title ever took that path) - our own AudioStreamIndex/SubtitleStreamIndex/ MaxStreamingBitrate are forced onto its query string afterward, overwriting whatever Emby put there. Picking a subtitle also forces SubtitleMethod=Encode; picking "Aucun" removes both SubtitleStreamIndex and SubtitleMethod rather than just not adding one, so it also cancels a subtitle Emby's own negotiation had defaulted to. This means the negotiation call itself no longer has to be trusted at all - it only needs to return a playable MediaSource, since every parameter this app's UI actually exposes gets forced afterward regardless.

Verified as far as possible without real Emby access: applyPlaybackOverrides is pure URLSearchParams manipulation, unit-tested in isolation (adding a subtitle Emby didn't pick, removing one Emby had defaulted to, re-sending an unchanged selection across an audio-only change, quality override, and the no-selection-yet initial-load case) - all five cases produce the expected final query string. Separately confirmed client-side, by mocking Hls to capture what URL loadSource was actually called with: the initial load already sends the default-selected track's index, and each dropdown change re-sends a full reload with exactly the accumulated params expected (switching audio → AudioStreamIndex changes; adding a subtitle → both params present; back to "Aucun" → SubtitleStreamIndex dropped again). What's not verified is the actual Emby round trip - whether applyPlaybackOverrides's forced params, once they reach the real server, produce a transcode that really carries the requested track. That needs a real retest.

Round 2, after a retest: subtitles specifically still didn't show. Found a real gap in the fix above: applyPlaybackOverrides was only ever applied to the final stream URL, not to the PlaybackInfo negotiation call itself - so SubtitleMethod=Encode never reached Emby until after it had already decided, during negotiation, whether this playback session would be a full re-encode or a plain remux. Burning a subtitle into the video is only possible with a real re-encode - appending SubtitleMethod=Encode to a URL for a session Emby had already set up as a remux plausibly does nothing, regardless of how correct the URL looks. Fixed by applying applyPlaybackOverrides to the negotiation's own params too (server/src/index.js), so Emby is asked to encode before it decides how the session will actually be handled, not just told afterward. Unit-tested (see session history): the negotiation call now measurably carries SubtitleMethod=Encode whenever a subtitle is requested, same as the final URL always did.

Also added server/src/logger.js's streamLog ([watchparty:stream], alongside the party-lifecycle partyLog added separately - see "Server-side logging" below) specifically because two rounds of blind, reasoned-but- unverified fixes for this one bug is the point where guessing stops being efficient: one master.m3u8 request now logs its itemId and the resolved audio/subtitle/subtitleMethod/quality values (never the full URL or params blob - Emby's own TranscodingUrl can carry an api_key, confirmed kept out of the log line by a dedicated test case), so the next retest can confirm from docker logs/journalctl alone whether a requested override actually reached Emby, without needing a devtools session on whatever machine runs the server. If subtitles still don't show once this log line confirms subtitleMethod=Encode really was sent, the fault has been narrowed to Emby's own handling of that parameter rather than anything before it in this app.

Assumptions to verify

Confirmed against the real server (not a guess — actually exercised, see session history): POST /Users/AuthenticateByName really is reachable from Node's server-side fetch with no CORS issue at all (server-to-server has no CORS concept), returns a real 401 for bad credentials, and the whole login round-trip (browser → our backend → real Emby → real rejection → our error message) renders correctly end to end. The --theme-primary-color-* etc. custom properties client/css/app.css is built from were read directly off this exact server's live dark theme, not guessed.

Login and basic usage are now confirmed for real — updated once actual bug reports started coming from real usage rather than mocked testing (overlay/pause-request feedback, the series-detail request, and the track-selection bug above, which is only reachable once a stream is already playing). That means the following are confirmed working end to end against this real server, not just reasoned about, even though none were checked in isolation: response field names (AccessToken, User.Id/Name, item Id/Name/Overview/ProductionYear/Type, Views' CollectionType, Shows/{id}/Seasons/Episodes, Items/Latest's plain-array shape, PlaybackInfo's MediaStreams), the X-Emby-Authorization/X-Emby-Token header format, rewritePlaylist against a real Emby-generated playlist, and PlaybackInfo → TranscodingUrl → HLS producing a real watchable stream. None of this was independently exercised - it's inferred from "the features built on top of it work in practice, and nobody's reported the classes of failure that would follow from any of it being wrong" - worth keeping in mind if something in this area breaks in a way that doesn't fit whatever specific bug is being chased at the time.

Genuinely not yet confirmed:

• /Users/{userId} for token validation was dropped — the proxy redesign no longer needs it at all (sessions are validated once at login and cached server-side, not re-checked against Emby per-request).
• Player track selection (AudioStreamIndex/SubtitleStreamIndex actually taking effect) — see the section just above; a fix is in place, reasoned from how Emby's API is known to work, but not yet re-tested for real.
• Whether TranscodingUrl is reliably present for every source type (a direct-play-eligible source might not transcode at all, which hls.js on the client wouldn't handle — see Known limitations), and whether it's always HLS.
• The Emby web client deep-link shape (/web/index.html#!/item?id=X&serverId=Y) used for "Voir sur Emby" — matches the long-standing convention, not confirmed against this exact build's routing (nobody's reported clicking it yet either way).

UI: Emby-like shell, per-type browsing, episode queue

Reworked to look and navigate more like Emby itself, and to treat each collection type (movies / TV / music) as its own experience rather than one generic grid:

• client/js/shell.js renders the persistent chrome once per login: a sidebar (one plain-text link per library from GET /api/catalog/libraries — no icons, no emoji prefix, per an explicit earlier request) + a #mainContent element the router (router.js) swaps on every navigation. GET /api/catalog/libraries itself only ever returns libraries whose CollectionType is movies/tvshows/music/musicvideos (VISIBLE_COLLECTION_TYPES in server/src/index.js) — Emby's /Views can return all sorts of things this app has no UI for at all (boxset collections, playlists, live TV, photos, books, a mixed-content folder with no CollectionType), and filtering once at the source means the sidebar, the home page's per-library carousels, and direct navigation to /library/:id are all restricted the same way without repeating the check in each place — a stale link to a since-hidden library's id just renders library.js's existing "Bibliothèque introuvable" state, since it looks the id up in this same filtered list. router.js was made idempotent (start(root) can be called again safely) specifically so login → shell boot can happen more than once in a page's lifetime without piling up duplicate hashchange listeners — the same class of bug the earlier Emby plugin work ran into with its page lifecycle, worth remembering.

• views/home.js (route /home): a horizontal carousel of every active party server-wide (the live lobby subscription, not a fetch — see "Live lobby, host handoff, no join codes" above) plus one more carousel per library of its recently-added items (GET /api/catalog/libraries/:id/recent, Emby's own /Users/{id}/Items/Latest) — skipping music (see below).

• views/library.js (route /library/:id): dispatches purely on the library's collectionType. movies/tvshows/musicvideos all get a tile grid (GET /api/catalog/libraries/:id/items, Recursive=false — Movies and music videos as themselves, TV as Series tiles); music gets a tile grid too, just of artists (GET /api/catalog/libraries/:id/artists) — see "Music: participative playlist" below for the rest of that hierarchy.

• Tiles (client/js/tiles.js, shared by the home carousels and the library grid so they can't drift apart): poster + name, hover-revealed overlay with "Lancer une watch party" (only for directly-playable types — WATCH_PARTY_TYPES in server/src/index.js, which had to grow a MusicVideo entry once musicvideos became a visible library type, otherwise every tile in it would render with no way to start a party at all; a Series tile gets "Voir les épisodes" instead, linking to /series/:id) and "Voir sur Emby" (opens Emby's own web client via a embyUrl the backend builds from EMBY_SERVER_URL + the session's serverId). :hover/:focus-within reveal it on mouse/keyboard; @media (hover: none) keeps it always visible on touch devices, which have no hover state at all.

  Bug: "Voir les épisodes" used to render as a bare, default-blue link, much less visible than the other two buttons next to it. The global button {} rule (app.css) that gives "Lancer une watch party" its solid green look is tag-scoped, so it never applied to "Voir les épisodes" — the one .tileBtn that's an <a> (a plain hash link) rather than a <button>. "Voir sur Emby" avoided the same fate only because it additionally carries .tileBtnLink, which explicitly sets a background/ color for exactly this reason. Fixed by moving the solid-button look onto .tileBtn itself, tag-agnostic, with .tileBtnLink now overriding just the two properties that make it read as the secondary action. This also happens to be the semantically correct fix, not just a visual patch: for a Series tile "Voir les épisodes" is the primary action (a Series is never itself directly playable, so the two buttons never compete for primacy on the same tile) - it should look like one.

• views/series.js (route /series/:id): a detail header — poster, title + year, overview, a "Voir sur Emby" link — above season tabs (GET /api/catalog/series/:id/seasons, Emby's /Shows/{id}/Seasons) and an episode list per season (GET /api/catalog/series/:id/episodes, Emby's /Shows/{id}/Episodes?SeasonId=...), each episode startable as its own party. The header is just toItemDto's existing fields (GET /api/catalog/item/:id, the same DTO shape as everything else) — the season/episode browsing itself predates this pass and was already wired end to end (library grid → Series tile → this route → "Lancer une watch party" per episode); what this pass added was making the page actually look like a detail page instead of a bare <h1> above the tabs. .linklike's CSS selector was widened from button.linklike to .linklike so the new "Voir sur Emby" <a> picks it up too — there's no global a reset in app.css, so an unclassed anchor would otherwise fall back to the browser's default blue/underline against this dark theme.

Episode queue / "next episode"

Starting a party from an episode (POST /api/parties) now has the backend fetch the rest of that series (/Shows/{seriesId}/Episodes, no season filter) and store everything from the clicked episode onward as the party's queue (server/src/partyManager.js). The host gets an "Épisode suivant" button once hasNext is true; clicking it (or the video's ended event firing) sends a "next" Socket.IO event (server/src/socketHandler.js), host-only, which advances queueIndex, resets every member's known position, and broadcasts {type:"itemChanged", itemId, itemName, ...} — every client (including the host) reloads the player against the new itemId on that message. A movie or a standalone-started episode just gets a one-item queue, so this is a strict superset of the old single-item behavior, not a separate code path.

Player: language, subtitles, quality

GET /api/catalog/item/:id/playback-options reads PlaybackInfo's MediaSources[0].MediaStreams and returns the Audio/Subtitle entries as {index, label} pairs for two <select>s in the player's host-only controls. Picking a value reloads the stream (/api/catalog/stream/:id/master.m3u8?AudioStreamIndex=&SubtitleStreamIndex=&MaxStreamingBitrate=) at the same position (hls.js destroyed and rebuilt, currentTime/paused state preserved across the reload) rather than restarting from zero. "Quality" is a fixed set of MaxStreamingBitrate presets (Auto/1080p/720p/ 480p) forwarded to PlaybackInfo — the same simplification Emby's/ Jellyfin's own web clients use; it's a transcode ceiling, not true client-side adaptive bitrate switching. Subtitles picked this way get burned into the transcoded video (Emby's doing, via SubtitleStreamIndex) rather than rendered as a toggleable <track> — simpler to implement correctly than sidecar WebVTT, at the cost of a re-transcode to turn them on/off.

Confirmed vs. not, for all of the above

Driving an actual browser against this (see session history) confirms the rendering and interaction logic: sidebar highlighting, home carousels (parties + one row per non-music library), library grid dispatch (including a tvshows library's Series tiles linking to /series/:id both via the poster and the "Voir les épisodes" button), and the series page's detail header + season-tab switching + episode list + starting a party from an episode all render and navigate correctly. That test mocked every /api/... response (canned JSON), since exercising it for real needs a logged-in session — so what's not confirmed yet is the shape of the real Emby responses this now depends on: Views items actually carrying a CollectionType field the way assumed, /Shows/{id}/Seasons and /Shows/{id}/Episodes, Items/Latest's plain-array response shape (handled defensively — see index.js — but not seen for real), and PlaybackInfo's MediaStreams shape for the track pickers. The queue/"next episode" WebSocket flow and the player's track/quality reload are new protocol surface with no test coverage at all yet (no way to exercise real HLS playback without a live session) — the first things to check once a real party actually plays something.

Non-goals

Chat/reactions and OIDC/SSO remain out of scope - see Goals/Non-goals at the top. Music was in this list for a while (see git history) - it's implemented now, see its own section below.

Server-side logging

Requested once the app was running for real on the user's own server: visibility into what's actually happening, from docker logs/journalctl, without needing to reproduce a bug through the UI first. server/src/logger.js is deliberately tiny - partyLog(message) ([watchparty:party]) and streamLog(message) ([watchparty:stream], added a bit later for the streaming route specifically - see "Player track selection" above), both plain console.log with an ISO timestamp, plus a who(userId, userName) formatter shared by every party-lifecycle call site - no logging library, consistent with this app's general bias against dependencies it doesn't need.

Covers the party's business lifecycle end to end, called from wherever the state actually changes so every caller is covered without repeating the check - server/src/partyManager.js's create/join/leave (which also covers a host handoff or the party being destroyed, both of which fall out of leave)/advanceQueue/removeIdleParties, plus server/src/socketHandler.js's playstate and requestPause handlers, since those two mutate member state directly rather than through a PartyManager method.

The one thing this needed real care over: the host's player re-sends its current play/pause state every 3 seconds regardless of whether anything changed (the heartbeat - see "Realtime transport" above), on top of firing once per actual play/pause/seeked event. Logging every "playstate" message as it arrives would mean one log line every 3 seconds for every active party, forever - noise that would drown out everything else within minutes on a server with even a couple of parties running. The handler instead compares the incoming isPaused against the member's previous value and only logs on an actual flip, silently updating position on every other message - so a party generates exactly one "playstate" line per real pause or resume, never one per heartbeat tick.

Verified directly against the real shipped modules (not reimplemented mocks) - see session history: a scripted create/join×2/advanceQueue (twice — once real, once a no-op past the end of the queue, correctly silent)/leave×3 (plain, host handoff, final-member destroy)/idle-sweep sequence against the real partyManager.js produced exactly the expected line for every step, in the expected order; a fake io/socket pair driving the real socketHandler.js through a start → 5 unchanged heartbeats → pause → 3 unchanged heartbeats → resume sequence logged exactly 3 "playstate" lines (not 9) at exactly the right points, and a throttled repeat "requestPause" produced no second "pauseRequested" line - all assertions passed, counted programmatically rather than eyeballed.

Music: participative playlist

The last deferred piece from the original UI spec, built once asked for directly. Three things needed designing before any code: the permission model for the shared queue (asked, not assumed - see session history), how audio playback fits the existing party/socket infrastructure, and how "browse and add a track" works without leaving the player.

Chosen model: free add, vote-to-skip, no host for playback. Anyone in the party can add a track to the shared queue at any time. Anyone can pause/resume directly (reversible, low-stakes - unlike video's host-only "playstate"). Skipping the current track needs a strict majority vote of currently connected members - the one action asked to be gated, since unlike adding (purely additive) or pausing (trivially undone), skipping throws away where the party is now. This is a genuinely different shape of shared state than video's "one host drives, everyone follows" - not a size difference, a different model - which is exactly why it's a separate service (server/src/musicQueueService.js, MusicQueueService) rather than more branches inside partyManager.js/socketHandler.js's existing video-shaped logic.

Data model and server-authoritative timing

Party gained a mediaType: 'video' | 'music' field (PartyManager.create's new last argument, inferred server-side in POST /api/parties from the clicked item's Emby Type - 'Audio' means music, everything else already handled means video - the client never has to know or say which). A music party's party.music object ({queue, current, skipVotes, advanceTimer}) is entirely separate from the fixed pre-built party.queue/queueIndex video parties use - a music queue is live and mutable, not something computed once at creation.

No single client is "the position" for music the way a video host's heartbeat is. Instead, party.music.current.startedUtc (a wall-clock timestamp) plus the track's known duration is the only state that exists - getElapsedSeconds() computes position on demand (Date.now() - startedUtc, or a frozen pausedElapsedMs while paused), never ticked or stored. This is also what makes auto-advance possible without any client "reporting" a track ended (which, with several clients' players all finishing within milliseconds of each other and no single host, could race): _scheduleAdvance sets one setTimeout for exactly when the current track's duration runs out, computed from the same startedUtc. A track with no known duration just has nothing scheduled - it relies on a manual vote-skip instead, same as it would if the timer fired.

That timer is the one genuinely new leak surface this feature adds, given how much this project has already had to fix real memory leaks (see "Realtime transport" above) - _clearTimer runs before every reschedule and from a new destroy(party) hook wired into both of partyManager.js's existing party-destruction paths (leave's last-member case and removeIdleParties' sweep), so a torn-down party can never leave a timer firing against state that no longer exists in PartyManager's map.

MusicQueueService needs to broadcast from inside that timer's callback, which runs with no socket/request in scope at all - solved with attachIo(io, onTrackChanged), called once at server startup (index.js, right next to where attachSocketServer itself is wired up), the same "give the singleton what it needs once, up front" shape PartyManager's own singleton already has. onTrackChanged is wired to broadcastLobby(io) specifically so a party's home-page card (preview image/name) stays current even when the timer, not a person clicking anything, is what changed what's playing.

Catalog: Artists → Albums → Tracks

Three more dedicated Emby endpoints, same reasoning as Shows → Seasons → Episodes: guessing folder structure via generic recursive /Items calls is fragile, a real endpoint per level isn't. GET /Artists?ParentId=... for a library's artists, GET /Users/{id}/Items?ArtistIds=... &IncludeItemTypes=MusicAlbum for an artist's albums, GET /Users/{id}/Items?ParentId=<albumId>&IncludeItemTypes=Audio for an album's tracks. toItemDto grew durationTicks/indexNumber/ artistName/albumName - additive, undefined for every other item type, so nothing existing had to change shape. WATCH_PARTY_TYPES grew 'Audio' (a track can start a party) - MusicArtist/MusicAlbum stay un-playable, drill-down only, joining Series in tiles.js's DRILL_DOWN table (generalized from a single seriesHref special case to a small per-type lookup once there were three, not two, drill-down targets - /artist/:id → artist.js, /album/:id → album.js, each a smaller version of the library-grid/episode-list shape already used elsewhere).

Streaming: the same route, one hint

Audio goes through the exact same /api/catalog/stream/:itemId/master.m3u8 route as video, not a parallel endpoint - Emby's PlaybackInfo negotiation isn't video-specific, so the whole TranscodingUrl-first, hand-built-fallback dance (see "Player track selection" above) applies identically. The one real difference is the hand-built fallback's path prefix (/Videos/ vs /Audio/), which the response doesn't reliably reveal on its own and looking up would cost an extra Emby round trip for something the client already knows - so the client tags the request (streamUrl(itemId, {mediaType:'audio'}), audioStreamUrl() in embyApi.js) and the route reads it back (req.query.mediaType === 'audio'). The music player uses hls.js for the same reason the video player does: most browsers have no native .m3u8 support at all, and there's no reason to assume Emby's negotiation produces anything other than HLS just because the media is audio.

Player: not theater-mode, and why adding a track doesn't navigate away

renderPartyPlayer (route /party/:id, player.js) is now a small dispatcher: it fetches the party once (both the video and music renderers need mediaType before they can render anything at all) and delegates to renderVideoPlayer or the new renderMusicPlayer (views/musicPlayer.js). The dispatch itself can't be async despite needing to await that fetch - route()'s contract needs its cleanup function back synchronously (see router.js), and an async handler would hand back a Promise instead, which router.js's typeof cleanup === 'function' check silently skips - reintroducing the exact leak class this project already spent real effort fixing once. Solved with a mutable box: renderPartyPlayer returns a wrapper immediately, which calls whichever real cleanup function the fetch eventually produces - or nothing at all, correctly, if the party navigates away before that fetch even resolves (nothing was set up yet in that case - no socket, no timer, nothing to leak).

Unlike the video player, the music player is not full-viewport/theater -mode (no .playerWrap-style position:fixed) - there's no video image to declutter around, so it renders as a normal page and the sidebar stays up. That's also what makes "add tracks while listening" simple: rather than navigating to a separate browsing page and losing the player's own state (and having to thread "which party am I adding to" through a chain of query params across three route levels - considered, rejected as needless complexity for what browsing-with-context would have bought), the player has its own small embedded Artist → Album → Track browser (musicPlayer.js's loadMusicLibraries/loadArtists/loadAlbums/ loadTracks), reusing the exact same catalog endpoints, rendered inline in a toggleable panel. Adding a track is REST (POST /api/parties/:partyId/tracks, addTrackToQueue() in party.js), not a socket event like voteSkip/musicPlaystate - deliberately, since it needs to work from this same panel regardless of realtime-connection state, and MusicQueueService.addTrack's own "queueUpdated" broadcast (via the attachIo wiring above) reaches everyone actually connected either way, so nothing about the realtime side needed a second code path.

A real bug found while testing, not just the mock's fault

applyMusicState (the "queueUpdated"/initial-render handler) used to update the queue list and vote button after the current-track branch, which calls loadTrack() - and loadTrack()'s hls.js setup can throw (a bad manifest, hls.js failing to load from the CDN, any of it). Because JavaScript doesn't roll back a function's earlier statements when a later one throws, but does abort everything after the throw in the same call, an exception there took the queue list and vote tally down with it even though they have nothing to do with whether one specific track's audio loaded - and worse, on the very first render, it escaped renderMusicPlayer entirely into the dispatcher's .catch(), which alerts and navigates back to /home, discarding the whole view. Fixed two ways: reordered applyMusicState so the queue/vote/icon updates - which can't fail - run before the current-track branch that can (mirroring player.js's onItemChanged, which already updates the title/next-button before calling loadVideo for exactly this reason), and wrapped loadTrack's hls.js setup in its own try/catch so a failure there logs and stops, rather than propagating out of the handler at all. Found by a test whose own mock happened to be missing Hls.Events - which pointed at a real fragility in the shipped code, not just an incomplete mock, so it's fixed in musicPlayer.js itself, not worked around in the test.

Verified

Server-side, directly against the real shipped modules, not reimplemented mocks (see session history): a scripted sequence against the real musicQueueService.js alone - init auto-starts the first track, a second add while one plays queues rather than jumping the queue, a two-vote majority skips and clears the tally, pause/resume freezes and correctly resumes elapsed time, a short real track auto-advances on its own via a genuine setTimeout after its duration, and destroy() clears a pending timer (18 checks, instrumented setTimeout/clearTimeout counting to catch anything left dangling - same technique used for the client-side router-cleanup leak earlier in this project). A second pass through the real socketHandler.js + partyManager.js together (fake io/sockets) confirmed voteSkip/musicPlaystate are no-ops on a video party, a non-host member can pause and resume a music party's track directly, a two-connected-member vote-skip needs exactly two votes, and the resulting "queueUpdated" really reaches the party's socket room (7 checks).

Client-side, against the real running local server with mocked fetch/Hls/io (see session history): the full library → artist → album browsing chain renders correct tiles and drill-down links at each level; starting a party from a track correctly creates one and navigates; the dispatcher correctly renders the music player (not the video one) for a mediaType:'music' party and still renders the video player when mediaType is absent (back-compat); the now-playing header, vote tally (computed against the live member list, matching musicQueueService.js's own majority math), and queue list all render correctly and update correctly on a "queueUpdated" push including a live track change (new audio source loaded); play/pause and vote-skip clicks emit the right socket events; the inline browse-and-add panel correctly walks artist → album → tracks and POSTs the exact right payload shape to /api/parties/:partyId/tracks; and the crash fixed above was confirmed fixed by re-running the same broken-Hls.Events scenario through the full dispatcher afterward - no alert, no navigation away, the queue/track info render correctly regardless.

Not verified - the usual limit of testing without real Emby access: whether /Artists/the album-tracks query really return what's assumed here, whether a music item's PlaybackInfo/TranscodingUrl behaves the same way a video's does (reasoned to, per "Streaming" above, not observed), and the real audio quality/gapless-ness of playback through a transcode. First things to check once this gets a real retest.

Known limitations

• Playback always goes through hls.js against whatever Emby's PlaybackInfo suggests. If that ever comes back as a direct-play source with no TranscodingUrl (no transcoding needed/possible), the client has no fallback path yet - it'll try to hand hls.js something that isn't HLS.
• One fixed item/episode-queue per video party (a music party's queue is the live, participative one - see "Music" above).
• A video party member needs a browser tab open and connected for sync to apply - there's no "catch up automatically when you come back" beyond whatever the next heartbeat corrects. Music has no equivalent heartbeat at all (a deliberate simplification, not an oversight - see "Music" above): position only resyncs on a discrete event (add/skip/pause/ resume), so two listeners' <audio> elements could drift apart slightly during a long stretch with no such event - accepted for v1 given music's more forgiving tolerance for exact sync compared to video+subtitles.
• No persistence: server restart or long idle time ends every party.
• No free seeking within a music track (only vote-skip to the next one)
  • not asked for, and would need its own protocol addition (MusicQueueService has no notion of "jump to position X" today, only play/pause and advance).