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Getting Connected: Authentication Flow

Let’s follow a user named Alex as they open the MentraOS mobile app and connect their smart glasses to the cloud. This journey shows how authentication works and how WebSocket connections are established.

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Step 1: Mobile App Login

When Alex opens the MentraOS app on their phone:
  1. User Authentication
    • Alex logs in with their email/password
    • The app authenticates with our auth service
    • App receives a coreToken (JWT) that identifies Alex
  2. What’s in the coreToken? 
    {
  "sub": "[email protected]",  // User's email
  "exp": 1234567890,          // Expiration time
  "iat": 1234567800           // Issued at time
}

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Step 2: Glasses Connect via Phone

Once authenticated, the mobile app:
  1. Establishes Bluetooth connection with Alex’s smart glasses
  2. Opens WebSocket to cloud at wss://cloud.mentraos.com/glasses-ws
  3. Sends the coreToken in the Authorization header: 
    Authorization: Bearer eyJhbGciOiJIUzI1NiIs...

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Step 3: WebSocket Handshake

Now the magic happens in our cloud:

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1. Connection Arrives at WebSocketService

// In WebSocketService.ts
app.ws('/glasses-ws', (ws, req) => {
  // Extract and verify the JWT token
  const token = req.headers.authorization?.split(' ')[1];
  const decoded = jwt.verify(token, JWT_SECRET);
  const userEmail = decoded.sub;
  
  // Hand off to GlassesWebSocketService
  glassesWebSocketService.handleConnection(ws, userEmail);
});

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2. GlassesWebSocketService Takes Over

// In GlassesWebSocketService.ts
handleConnection(ws: WebSocket, userEmail: string) {
  // Send connection acknowledgment
  ws.send(JSON.stringify({
    type: 'CONNECTION_ACK',
    sessionId: generateSessionId()
  }));
  
  // Create or retrieve UserSession
  let session = SessionStorage.getSession(userEmail);
  if (!session) {
    session = new UserSession(userEmail, ws);
    SessionStorage.addSession(session);
  } else {
    // Reconnection - update WebSocket
    session.updateWebSocket(ws);
  }
}

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3. The First Message Exchange

Mobile app must send CONNECTION_INIT within 30 seconds: 
// From mobile app (defined in glasses-to-cloud.ts:15-19)
{
  type: "CONNECTION_INIT",
  userId?: "[email protected]",  // Optional user ID
  coreToken?: "eyJhbGci..."     // Optional auth token
}
Cloud responds with CONNECTION_ACK: 
// From cloud (defined in cloud-to-glasses.ts:19-23)
// Sent from websocket-glasses.service.ts:558-566
{
  type: "CONNECTION_ACK",
  sessionId: "alex-123456",      // Unique session ID
  userSession: {                 // Partial<UserSession> from transformUserSessionForClient
    userId: "[email protected]",
    startTime: "2024-01-01T00:00:00Z",
    activeAppSessions: ["com.app1", "com.app2"],
    loadingApps: [],
    appSubscriptions: {
      "com.app1": ["audio", "transcription"],
      "com.app2": ["notification"]
    },
    requiresAudio: true,
    minimumTranscriptionLanguages: ["en-US"],
    isTranscribing: true
  },
  timestamp: "2024-01-01T00:00:00Z"
}

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Step 4: Heartbeat Keeps Connection Alive

Every 10 seconds, the cloud sends a ping to check if the connection is alive: 
// In UserSession.ts
private startHeartbeat() {
  this.heartbeatInterval = setInterval(() => {
    if (this.websocket.readyState === WebSocket.OPEN) {
      this.websocket.ping();
    }
  }, 10000);
}
If the mobile app doesn’t respond to pings, the connection is considered dead and cleaned up.

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Step 5: Handling Reconnections

What happens when Alex’s phone loses connection?
  1. Grace Period (30 seconds)
    • UserSession remains in memory
    • Apps stay in “resurrecting” state
    • No data is lost
  2. Reconnection Within Grace Period
    • Same coreToken is used
    • UserSession is retrieved from SessionStorage
    • WebSocket is updated with new connection
    • Apps are notified of reconnection
    • Everything continues seamlessly
  3. Reconnection After Grace Period
    • New UserSession is created
    • Apps must be restarted
    • Previous state is lost

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Security Considerations

  1. JWT Validation
    • All tokens are verified for signature
    • Expired tokens are rejected
    • Invalid tokens result in CONNECTION_ERROR
  2. User Isolation
    • Each UserSession is keyed by email
    • No cross-user data access
    • Sessions are completely isolated
  3. Connection Limits
    • One active glasses connection per user
    • New connections replace old ones
    • Prevents connection flooding

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Common Connection Errors

// Connection rejected - invalid token
{
  type: "CONNECTION_ERROR",
  error: "Invalid authentication token"
}

// Connection rejected - expired token
{
  type: "CONNECTION_ERROR", 
  error: "Token expired"
}

// Connection timeout - no CONNECTION_INIT
{
  type: "CONNECTION_ERROR",
  error: "Connection initialization timeout"
}

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What’s Next?

Now that Alex is connected, the cloud creates a UserSession for them. This session is their personal command center in the cloud. Let’s explore UserSessions next!