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Server Security (S-1 through S-8): - S-1: Type-check and clamp peerId, protocolVersion, password - S-2: Validate numeric/boolean/enum fields in relay peerData - S-3: Construct explicit relay payload (stop spreading raw data) - S-4: Type-check targetId and actionTimestamp in EVENT_ACK - S-5: Restrict room IDs to [a-zA-Z0-9-] only - S-7: Add eventCounts periodic cleanup alongside connectionCounts - S-8: Guard version parsing against NaN bypass Documentation (P-1, R-1 through R-6): - P-1: Fix PRIVACY.md typo, document all in-memory data maps - R-1/R-5: Fix stale sync-constants.bat references in shared/ - R-2: Fix stale lastTargetState ref in ARCHITECTURE.md - R-3: Extension README title reflects cross-browser support - R-6: Document content injection markers in scripts/README.md
68 lines
5.6 KiB
Markdown
68 lines
5.6 KiB
Markdown
# KoalaSync Architecture
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This document describes the communication flows and internal logic of the KoalaSync system.
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## 1. Extension Startup & Connection
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- **Initialization**: On startup, `background.js` reads settings (Server URL, Username, Last Room) from `chrome.storage.sync`.
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- **WebSocket Handshake**:
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1. Background creates a `new WebSocket` to `/socket.io/?EIO=4&transport=websocket&version=1.0.0`.
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2. Server performs security checks:
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- **IP Rate Limit**: Checks if the IP has exceeded connection limits.
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- **Protocol Version**: Client must match the server's protocol (currently `1.0.0`).
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3. Server responds with Engine.IO handshake (`0`) and the client joins the namespace (`40`).
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- **Room Join**: Background emits `JOIN_ROOM` containing `roomId`, `password`, `peerId`, and `username`.
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- **Deduplication**: If a user joins with a `peerId` that already has an active socket, the server kills the old socket to prevent "Ghost Peers".
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## 2. Media Event Synchronization
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When a user interacts with a video:
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1. **Detection**: `content.js` listens to native events (`play`, `pause`, `seeked`) on the `<video>` element.
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2. **Prevention of Loops**: Uses an `expectedEvents` Set to distinguish between user actions and programmatic actions. Expected events are consumed on match and expire via timeout.
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3. **Reporting**: `content.js` sends a `CONTENT_EVENT` to `background.js`.
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4. **Relay**: The Server forwards the event to all other peers in the room.
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5. **Execution**: Remote peers receive the command and call `video.play()`, `video.pause()`, or `video.currentTime = targetTime`.
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## 3. Two-Phase Force Sync
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Ensures all peers are frame-perfect and buffered before resuming:
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1. **Prepare**: Initiator sends `FORCE_SYNC_PREPARE` with the target timestamp.
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2. **Buffer**: Peers seek and pause. Once buffered (`readyState >= 3`), they send a `FORCE_SYNC_ACK`. (Note: `content.js` limits polling to 8000ms).
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3. **Execute**: Once the Initiator collects ACKs (or after an 8.5s timeout), they send `FORCE_SYNC_EXECUTE`.
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> [!IMPORTANT]
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> **Network Transit Buffer Rule**: The orchestrator (`background.js`) must always use a timeout at least 500ms longer than the worker (`content.js`) to account for IPC and network transit time. Never align them exactly 1:1, as this will introduce a race condition on slow connections.
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4. **Resume**: All peers call `play()` simultaneously.
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## 4. Episode Auto-Sync
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Maintains continuous synchronized viewing when watching series:
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1. **Detection**: `content.js` monitors the Media Session API for title changes.
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2. **Lobby Creation**: When a new title is detected, the peer initiates an `EPISODE_LOBBY` and broadcasts the new title.
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3. **Wait State**: All peers freeze their video until they have also loaded the exact same title.
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4. **Mid-Lobby Joins**: If a new user joins the room during an active lobby, the lobby initiator broadcasts the active lobby state so the newcomer can sync up.
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5. **Resume**: Once all peers report `EPISODE_READY`, the lobby is resolved and playback resumes perfectly.
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## 5. Peer Lifecycle & Dual Heartbeat
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To maintain a clean room state and eliminate "Ghost Peers":
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- **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.
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- **Video Heartbeat (Content)**: Every 15 seconds, `content.js` sends current playback metadata (time, title, state) if a video is found.
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- **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.
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- **Immediate Cleanup**: Rooms are deleted instantly when the last peer leaves or disconnects.
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> [!CAUTION]
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> **Identity Rule**: Differentiate between `peerId` and `socket.id`. Use `socket.id` exclusively for ephemeral transport routing on the server. Use `peerId` exclusively for identity, state management, and room tracking across the stack.
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## 6. Broadcast Protocol & Routing
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KoalaSync uses a megaphone routing approach to minimize server logic:
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- **`emit()` Broadcast Behavior**: Any `emit()` from the extension client is unconditionally broadcast to **all other peers in the room**. It is not a direct message.
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- **Storm Prevention**: When dispatching state updates in response to a new user joining (e.g., an active lobby state), ensure ONLY the initiator (or a designated leader) calls `emit()` to prevent $O(N)$ broadcast storms.
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## 7. Security & Stability
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- **Service Worker Lifecycle**: Uses `chrome.alarms` to prevent the Manifest V3 service worker from suspending while in an active room.
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- **Rate Limiting**: Server-side per-socket and per-IP rate limits to prevent sync-spamming or DoS.
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- **Noise Filtering**: Uses a curated blacklist of domains (Search Engines, Social Media) to declutter the "Target Tab" selector in the popup.
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- **Diagnostics**: A "Dev" tab provides real-time access to the underlying `<video>` state (`readyState`, `paused`, `currentTime`) for easier troubleshooting.
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## 8. Constant Synchronization & Consistency
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To maintain a "Single Source of Truth" across the server and extension without using a bundler:
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- **Relay Server & Extension Modules**: `background.js` and `popup.js` import constants directly from `shared/constants.js`.
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- **Content Scripts**: To ensure zero-latency execution, `content.js` uses a synchronized copy of `EVENTS` and constants.
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- **Automation**: The `node scripts/build-extension.js` script automatically injects these constants into `content.js` during the build process, eliminating the risk of manual mirror mismatch.
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- **Verification**: Any protocol change is automatically propagated across the stack by running the build script.
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