sync_replay_project/docs/HOW-IT-WORKS.md

How it works

A five-minute tour. Details: ARCHITECTURE.md, wire format: 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.