KoalaSync Architecture

This document describes the communication flows and internal logic of the KoalaSync system.

1. Extension Startup & Connection

Initialization: On startup, background.js reads settings (Server URL, Username, Last Room) from chrome.storage.sync.
WebSocket Handshake:
1. Background creates a new WebSocket to /socket.io/?EIO=4&transport=websocket&version=1.0.0.
2. Server performs security checks:
  - IP Rate Limit: Checks if the IP has exceeded connection limits.
  - Protocol Version: Client must match the server's protocol (currently 1.0.0).
3. Server responds with Engine.IO handshake (0) and the client joins the namespace (40).
Room Join: Background emits JOIN_ROOM containing roomId, password, peerId, and username.
Deduplication: If a user joins with a peerId that already has an active socket, the server kills the old socket to prevent "Ghost Peers".

When a user interacts with a video:

Detection: content.js listens to native events (play, pause, seeked) on the <video> element.
Prevention of Loops: Uses lastTargetState to distinguish between user actions and programmatic actions triggered by the extension.
Reporting: content.js sends a CONTENT_EVENT to background.js.
Relay: The Server forwards the event to all other peers in the room.
Execution: Remote peers receive the command and call video.play(), video.pause(), or video.currentTime = targetTime.

Ensures all peers are frame-perfect and buffered before resuming:

Prepare: Initiator sends FORCE_SYNC_PREPARE with the target timestamp.
Buffer: Peers seek and pause. Once buffered (readyState >= 3), they send a FORCE_SYNC_ACK.
Execute: Once the Initiator collects ACKs (or after a 5s timeout), they send FORCE_SYNC_EXECUTE.
Resume: All peers call play() simultaneously.

To maintain a clean room state and eliminate "Ghost Peers":

Session Heartbeat (Background): Every 30 seconds, background.js sends an "I'm alive" signal to the server. This keeps you in the room even if no video is playing.
Video Heartbeat (Content): Every 15 seconds, content.js sends current playback metadata (time, title, state) if a video is found.
Server Pruning: The server runs a "Reaper" every 2 minutes. If a peer has sent zero activity (no events and no heartbeats) for 5 minutes, they are forcefully disconnected.
Immediate Cleanup: Rooms are deleted instantly when the last peer leaves or disconnects.

Service Worker Lifecycle: Uses chrome.alarms to prevent the Manifest V3 service worker from suspending while in an active room.
Rate Limiting: Server-side per-socket and per-IP rate limits to prevent sync-spamming or DoS.
Noise Filtering: Uses a curated blacklist of domains (Search Engines, Social Media) to declutter the "Target Tab" selector in the popup.
Diagnostics: A "Dev" tab provides real-time access to the underlying <video> state (readyState, paused, currentTime) for easier troubleshooting.

To maintain a "Single Source of Truth" across the server and extension without using a bundler:

Relay Server & Extension Modules: background.js and popup.js import constants directly from shared/constants.js.
Content Scripts: To ensure zero-latency execution, content.js uses a manual mirror of EVENTS.
Synchronization: The ./scripts/sync-constants.sh script ensures that the shared/ folder within the extension/ directory is kept up-to-date with the root shared/ source.
Verification: Any protocol change requires a manual verification sweep across all three constant locations (Shared, Server, and Content Script Mirror).