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sync_replay_project/docs/HOW-IT-WORKS.md
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foxyzandClaude Opus 5.5 ae4edc29e8 Jellyfin end to end: per-viewer playback, progress, episodes, swap browser
- Host/swap send refs only; pasted Jellyfin links become refs; the host
  hands the relay its own app-obtained credential.
- Each viewer resolves with their own account (token in a header) or
  via the relay, else a clear blocked card with Link account.
- Per-account progress reporting (Playing/Progress/Stopped), 401 marks
  the account expired.
- Previous/next episode in the controls (< > / Shift+P/N), floating
  Next episode pill near the end, auto-advance countdown (one
  designated swapper), track carry-over per series.
- Swap panel uses the shared MediaPicker; browser location remembered
  per server and opens on the current series.
- Fix: states are applied one at a time (a stale pause could land
  after a newer play).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
2026-10-07 02:38:24 +03:00

85 lines
4.1 KiB
Markdown

# How it works
A five-minute tour. Details: [ARCHITECTURE.md](ARCHITECTURE.md), wire format:
[PROTOCOL.md](PROTOCOL.md).
## The pieces
```
your PC friend's PC
+-------------------+ +-------------------+
| app (mpv inside) | | app (mpv inside) |
+---------+---------+ +---------+---------+
| sync (WebSocket) | sync (WebSocket)
v v
+----------------------------------------+
| server (who is ready, where are we) |
+----------------------------------------+
video bytes come from somewhere else:
- a link (any http/https URL): each app fetches it directly
- a Jellyfin item: each app plays it with its own Jellyfin account,
friends without one through the server (/j/... endpoint)
- a file on the host's disk: the server streams it (/m/... endpoint)
```
- **App**: the desktop window. It draws the video with mpv, shows who is
here, and sends what you do (ready, seek, chat).
- **Server**: keeps one shared clock and the list of people. It never sees
video frames, only "position 123 s, playing". There is exactly one server
and one watch session on it.
- **Jellyfin** (or any web host): where the video lives. For a Jellyfin
item the server only knows which item it is; each app signs in with its
own account and reports what it watched. For friends without an account
the server fetches the item with the host's sign-in and passes it on.
## Ways to run
- **Host from the app**: the app you click Host in also runs the server.
Simplest. The server lives as long as that app.
- **Dedicated server**: `watch-together-server`, a separate program in the
release tarball, for an always-on box. See the README.
Friends join either way with the Join card on Home.
## One session, start to finish
1. **Connect.** The app opens a WebSocket to the server and says hello
(version, password, name, avatar). Wrong password or version: refused with
a reason. Otherwise the server answers with a welcome and the current state.
2. **Catch up.** The app opens the current video, jumps to where the server
says we are, and buffers. A new arrival is not ready yet and never holds
anyone back.
3. **Ready gate.** Press Space to say you are ready. The server starts
playback only when everyone who is ready has at least 4 seconds buffered
(or what is left, near the end). At the end everyone stops and the server
stays finished until someone seeks back or changes the video.
If two or more people are ready, one person alone cannot start it for the
others. A lone viewer can play alone.
4. **Play.** The server owns the clock. Each app compares its mpv position
to the clock: a small lead slows mpv a little (0.95x), a big gap (over 5 s)
jumps. If someone's buffer runs dry the server pauses everyone and resumes
when they have caught up.
5. **Seek.** Anyone can seek, anytime. The server moves the clock and
everybody follows. Seeking does not change anyone's ready flag.
6. **Swap.** Anyone can change the video by pasting another link. Everybody
stops at 0, ready flags clear, and the gate runs again.
7. **Leave.** Closing or leaving drops you from the server; the others keep
going. If you vanish without saying goodbye, you are marked stale after 15 s
(stops blocking) and removed after 30 s. The app reconnects by itself and
takes over your old place (same member, ready flag and video).
There are no roles: anyone can seek, swap, force-ready others and kick
anyone except the host.
## Where video comes from
- **Link**: you paste an http(s) URL. Every app plays it directly with mpv.
A pasted Jellyfin link becomes the Jellyfin item below when possible.
- **Jellyfin item**: picked in the app's library browser. Shared as "this
item on this Jellyfin server", never as a link with someone's sign-in.
- **Local file (host only)**: you pick a file on your disk. The server
streams it to everyone, host included, over HTTP with range requests, each
with a per-person token. Nothing is stored or converted.
- No transcoding, ever. Quality is whatever the file is.