What is the difference between MQTT and WebSocket?

Overview

MQTT and WebSocket are both used for real-time communication, but they serve different purposes. MQTT is a lightweight messaging protocol designed for IoT, while WebSocket is a full-duplex communication protocol for real-time web applications.

MQTT (Message Queuing Telemetry Transport)

MQTT is a publish-subscribe messaging protocol built for constrained devices and low-bandwidth networks. It requires a broker (server) that routes messages between publishers and subscribers.

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// Flutter MQTT using mqtt_client package
import 'package:mqtt_client/mqtt_client.dart';
import 'package:mqtt_client/mqtt_server_client.dart';

final client = MqttServerClient('broker.hivemq.com', 'flutter_client');
client.port = 1883;
client.keepAlivePeriod = 60;

await client.connect();

// Subscribe to a topic
client.subscribe('sensors/temperature', MqttQos.atLeastOnce);

// Publish a message
final builder = MqttClientPayloadBuilder();
builder.addString('{"temp": 25.5}');
client.publishMessage('sensors/temperature', MqttQos.atLeastOnce, builder.payload!);

// Listen for messages
client.updates!.listen((List<MqttReceivedMessage<MqttMessage>> messages) {
  final message = messages[0].payload as MqttPublishMessage;
  final payload = MqttPublishPayload.bytesToStringAsString(message.payload.message);
  print('Received: $payload');
});

WebSocket

WebSocket provides a persistent, full-duplex TCP connection between client and server. Communication is bidirectional — both sides can send messages anytime without a broker.

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// Flutter WebSocket using web_socket_channel package
import 'package:web_socket_channel/web_socket_channel.dart';

final channel = WebSocketChannel.connect(
  Uri.parse('wss://echo.websocket.events'),
);

// Send a message
channel.sink.add('Hello Server!');

// Listen for messages
channel.stream.listen(
  (message) => print('Received: $message'),
  onError: (error) => print('Error: $error'),
  onDone: () => print('Connection closed'),
);

// Close connection
channel.sink.close();

Key Differences

MQTT QoS Levels

MQTT Over WebSocket

MQTT can run over WebSocket to work through web browsers and firewalls:

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// MQTT over WebSocket in Flutter
final client = MqttServerClient.withPort(
  'ws://broker.hivemq.com',
  'flutter_ws_client',
  8000,
);
client.useWebSocket = true;
await client.connect();

When to Use Which

• Use MQTT for IoT projects, sensor networks, home automation, or any scenario with constrained devices and unreliable networks
• Use WebSocket for real-time web apps like chat, live notifications, collaborative editing, or gaming
• Use MQTT over WebSocket when you need MQTT's pub-sub model through web browsers or corporate firewalls

Key Insight: MQTT and WebSocket are complementary, not competing. MQTT is a messaging protocol (defines how messages are routed), while WebSocket is a transport protocol (defines how data is transmitted). You can use both in the same Flutter project.