I have wrote the code for server.js and main.dart.

Functionality which I am trying to achieve is very simple but somehow I cannot make it work.

server receives an incoming request, app fetches that request from server and ring/vibrate the device.

**main.dart code
**

void main() {
  runApp(MyApp());
}

String receiverDeviceId =
    ''; // Define and declare the receiverDeviceId variable

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      themeMode: ThemeMode.dark,
      home: MyHomePage(),
    );
  }
}

Future<String> fetchReceiverDeviceId(String serverUrl) async {
    final response =
        await http.get(Uri.parse('$serverUrl/getReceiverDeviceId'));

    if (response.statusCode == 200) {
      final result = jsonDecode(response.body);
      return result['receiverDeviceId'];
    } else {
      throw Exception(
          'Failed to fetch receiverDeviceId from the server. Status code: ${response.statusCode}');
    }
  }

void sendSlap(String serverUrl, String senderDeviceId,
      Function(String) onServerResponse) async {
    try {
      String receiverDeviceId = await fetchReceiverDeviceId(serverUrl);

      final response = await http.post(
        Uri.parse('$serverUrl/notifySlap'),
        headers: {'Content-Type': 'application/json'},
        body: jsonEncode(
            {'deviceId': senderDeviceId, 'receiverDeviceId': receiverDeviceId}),
      );

      if (response.statusCode == 200) {
        final result = jsonDecode(response.body);

        if (result['success']) {
          print(
              'Slap notification sent from $senderDeviceId to $receiverDeviceId');
          showSnackbar(
              'Slap notification sent from $senderDeviceId to $receiverDeviceId');

          // Notify the app that the server received the slap request
          onServerResponse('Server received slap notification');
        } else {
          print('Error sending slap notification: ${result['message']}');
          showSnackbar('Error sending slap notification: ${result['message']}');
        }
      } else {
        print('Slap notification request failed. Please try again.');
        showSnackbar('Slap notification request failed. Please try again.');
      }
    } catch (e) {
      print('Error sending slap notification: $e');
      showSnackbar('Error sending slap notification. Please try again.');
    }
  }

  void receiveSlap(String senderDeviceId, String receiverDeviceId) {
    // Logic to handle receiving a slap notification
    print(
        'Slap notification received from $senderDeviceId on $receiverDeviceId');
    showSnackbar(
        'Slap notification received from $senderDeviceId on $receiverDeviceId');

    // Vibrate the device
    vibrateDevice();

    // Play the notification sound
    playNotificationSound();
  }

  void playNotificationSound() async {
    AudioCache player = AudioCache();
    await player.load('noti/notification_sound.mp3');
  }

  void vibrateDevice() {
    Vibration.vibrate(duration: 500); // Vibrate the device for 500 milliseconds
  }

ElevatedButton(
                    onPressed: () async {
                      print('Slap button pressed'); // Add this debug statement
                      final response = await http
                          .get(Uri.parse('$serverUrl/getReceiverDeviceId'));
                      if (response.statusCode == 200) {
                        receiverDeviceId =
                            jsonDecode(response.body)['receiverDeviceId'];
                        print(
                            'Receiver Device ID: $receiverDeviceId'); // Add this debug statement
                        sendSlap(serverUrl, deviceId!, (String serverResponse) {
                          print(
                              serverResponse); // This will notify the app that the server has received the slap request
                        });
                      } else {
                        print(
                            'Failed to fetch receiverDeviceId from the server. Status code: ${response.statusCode}'); // Add this debug statement
                      }
                    },
                    child: Text('Slap!'),
                  ),

**server.js code
**

fastify.post('/notifySlap', (request, reply) => {
  const deviceId = request.body.deviceId;
  const receiverDeviceId = request.body.receiverDeviceId;

  const connectedDevicesToNotify = connectedDevices.filter(id => id !== deviceId);

  if (connectedDevicesToNotify.length > 0) {
    console.log(`Slap notification sent to devices: ${connectedDevicesToNotify.join(', ')}`);
    sendNotificationCommandToDevices(connectedDevicesToNotify); // Send command to devices to play sound or vibrate
  }

  if (deviceId === receiverDeviceId) {
    console.log(`Slap notification sent to device: ${deviceId}`);
    // Add logic here to handle the slap received by the same device
    // For example, you can play a sound or vibrate the device
  }

  reply.send({ success: true });
});

fastify.get('/getReceiverDeviceId', (request, reply) => {
  const receiverDeviceId = generateReceiverDeviceId();
  reply.send({ receiverDeviceId });
});

function sendNotificationCommandToDevices(devices) {
  devices.forEach(deviceId => {
    if (connectedDevices.includes(deviceId)) {
      console.log(`Sending slap notification to device: ${deviceId}`);
      if (isVibratingDevice(deviceId)) {
        console.log(`Vibrating device: ${deviceId}`);
        // Add code here to trigger device vibration
      } else {
        console.log(`Sending sound notification to device: ${deviceId}`);
        // Add code here to play a sound notification
      }
    } else {
      console.log(`Device ${deviceId} is not connected.`);
    }
  });
}

function isVibratingDevice(deviceId) {
  // Add logic here to determine if the device should vibrate upon receiving a slap notification
  // For example, you can check a database or a list of vibrating devices
  return true; // Return true if the device should vibrate, false otherwise
}

// Add a new endpoint to fetch server logs
fastify.get('/serverLogs', (request, reply) => {
  // Implement logic to fetch and return server logs here
  // You can return an array of log messages from the server
  const serverLogs = ['I/flutter (3471): Server received slap notification', 'Vibrating device: '];
  reply.send(serverLogs);
});

I have tried to fetch the server log and make the app ring/vibrate by matching the server logs, but nothing happens.