syncplay

syncplay is a small Odin program that relays play, pause, and seek actions between a few trusted mpv users. One executable provides both the TCP relay server and the macOS client.

Requirements

• Odin 2026-07 or newer
• macOS for client mode
• macOS or Linux for server mode
• mpv installed on every client
• The same media file (including the same cut) on every client

Build and test

odin check .
odin test .
odin build . -out:syncplay

Run

Start one relay server:

./syncplay server --host 0.0.0.0 --port 9000

Then each viewer starts a client with their local copy of the movie:

./syncplay client \
  --host server.example.test \
  --port 9000 \
  /path/to/movie.mkv

The macOS defaults can be overridden:

./syncplay client \
  --host server.example.test \
  --port 9000 \
  --mpv /custom/path/to/mpv \
  --socket /tmp/custom-mpv-socket \
  /path/to/movie.mkv

The client starts at 00:00:00 in the paused state. Closing mpv closes the client. Interrupting the client terminates the mpv process it launched. If the server connection is lost after startup, mpv continues normally and the client logs that synchronization has stopped.

Protocol

The trusted-network protocol is newline-delimited JSON over TCP:

{"version":1,"type":"welcome","client_id":1}
{"version":1,"type":"pause","paused":true}
{"version":1,"type":"pause","paused":false}
{"version":1,"type":"seek","position":3723.125}

Messages are limited to 4096 bytes. The server validates and re-encodes every message, preserves processing order, and broadcasts it to every connection except its sender. It assigns each connection a numeric client ID and adds the triggering ID to relayed events. Server and client terminals log play, pause, and seek actions with that ID; the triggering client is marked with (you) in its own terminal.

Remote seeks are applied using an mpv IPC transaction: seek event delivery is disabled for this IPC connection, an exact absolute seek is sent, and the client waits for mpv's playback-restart completion event before restoring seek event delivery. A command reply can arrive before the seek effects, so it is not sufficient as the end of the suppression window. Event delivery is still restored if the operation fails. This prevents a remote seek from being mistaken for a new local seek and sent back indefinitely.

Prototype limitations

• One global session; no rooms or late-join state snapshot
• No authentication, encryption, or Internet-facing security
• No automatic reconnection
• No media identity checking
• No clock synchronization or drift correction
• mpv can report internally generated operations as seek events
• A local seek made during the brief remote-seek suppression transaction may not be relayed because mpv events do not identify their origin