OpenWatchParty: synchronize playback despite latency

Mon, 19 Oct 2026 00:00:00 +0000

In my first OpenWatchParty article, I introduced the Jellyfin plugin and its Rust session server. The harder question remained: how do you make the same scene play at the same time on several machines when their clocks, networks, and players behave differently?

Establish a shared clock

The server cannot assume that two computers display the same time. The client therefore sends a timestamped ping. When the response arrives, it measures the round-trip time and estimates the offset between its clock and the server’s. Several measurements are smoothed so that a latency spike does not cause a sudden jump.

OpenWatchParty: Synchronized Watch Parties for Jellyfin

Mon, 19 Jan 2026 00:00:00 +0000

What is OpenWatchParty?

OpenWatchParty is a Jellyfin plugin that enables synchronized media playback across multiple clients. Watch movies and shows with friends, wherever they are.

Unlike Jellyfin’s built-in SyncPlay, OpenWatchParty uses a dedicated external session server, providing more reliable synchronization and better latency handling.

Architecture

The system has three components:

Component	Technology	Role
Jellyfin Plugin	C#	Delivers client JavaScript and configuration UI
Session Server	Rust	Manages rooms and relays synchronization messages over WebSocket
Web Client	JavaScript	Embedded in the Jellyfin UI; coordinates playback

flowchart LR
 A[Client A] <-->|WebSocket| S[Session Server<br/>Rust]
 B[Client B] <-->|WebSocket| S
 C[Client C] <-->|WebSocket| S

Features

Real-time Synchronization: Playback stays in sync across all participants
Room-based Management: Create and join watch party rooms
Host Control: The party creator controls playback (play, pause, seek)
Latency Monitoring: RTT (Round-Trip Time) measurement and display
Drift Correction: Automatic playback rate adjustment to maintain sync
HLS Support: Works with transcoded content via HTTP Live Streaming
JWT Authentication: Secure user identity verification

Installation

1. Deploy the Session Server

docker run -d \
 -p 3000:3000 \
 -e ALLOWED_ORIGINS=https://your-jellyfin-domain.com \
 ghcr.io/mhbxyz/openwatchparty-session-server:latest

2. Add the Plugin Repository

In the Jellyfin dashboard:

Synchronization on mhbxyz.me