Files
sync_replay_project/docs/HOW-IT-WORKS.md
T
foxyzandClaude Opus 5.5 95cd668cfb One app menu style for every picker; links go Through me by default
- AppSelect / AppMenuButton (widgets/app_menu.dart): glass menu anchored
  to its button, check on the current item, keyboard and Esc; replaces
  the Material dropdowns in Settings, player settings, Jellyfin and the
  member menu. Material menus elsewhere get the same surface by theme.
- Link page: Through me is the main choice (you can't know which sites
  need cookies); Everyone / Server fetches it sit under "Other ways".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
2026-10-11 05:58:18 +03:00

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5.3 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): the member who put it up passes it on
through the relay (or each app, or the server, fetches it)
- a Jellyfin item: each app plays it with its own Jellyfin account,
friends without one through the server
- a file on the host's disk: the server streams it (/m/... endpoint)
- the relay (/r/...): a member's computer passes a file, a link or a
Jellyfin item on to the server, which keeps a few minutes in memory
and streams it to everyone who needs it
```
- **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 member who put it up passes it on through the server (their sign-in
stays on their computer).
## 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 Join a friend in the strip at the top of the
window; guests need no Jellyfin or account.
- **Watch solo**: put something up without joining or hosting and the
app hosts a private server reachable only from this computer, so
watching alone works like a session (progress, next episode). Invite
friends turns it into a normal hosted server, video and position kept.
## 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. Arriving while the video is stopped, you are
waited for; arriving while it plays, you never hold 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.
Everyone who was there before it started must be ready, so 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 and the gate runs again for everyone there; ready flags stay
as they were (a setting unreadies you instead).
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, nudge
anyone, and sit anyone out (the host too). Someone sitting out stays
connected and in the chat, but their app loads nothing and nobody waits
for them until they join back.
## Where video comes from
- **Link**: you paste an http(s) URL. By default your computer opens it and
passes it on (it plays for everyone whenever it plays for you, cookies
included); under "Other ways" every app plays it directly, or the server
fetches it once. A web page (YouTube, X, ...) goes through yt-dlp, which
the app and the server download and update by themselves.
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.