PubSubClient is a lightweight MQTT client library for Arduino and ESP8266. It allows Arduino devices to publish messages to and subscribe to topics on an MQTT broker, enabling IoT communication and device management.

Pros

• Simple and easy to use for beginners
• Low memory footprint, suitable for resource-constrained devices
• Supports QoS 0 and 1 for message delivery
• Compatible with a wide range of Arduino boards and ESP8266

Cons

• Limited support for advanced MQTT features
• No built-in SSL/TLS support (requires external libraries)
• May not be suitable for complex IoT applications requiring extensive functionality
• Limited documentation for advanced use cases

Code Examples

1. Connecting to an MQTT broker:

#include <ESP8266WiFi.h>
#include <PubSubClient.h>

const char* ssid = "YourWiFiSSID";
const char* password = "YourWiFiPassword";
const char* mqtt_server = "broker.mqtt.com";

WiFiClient espClient;
PubSubClient client(espClient);

void setup() {
  WiFi.begin(ssid, password);
  client.setServer(mqtt_server, 1883);
  client.connect("ArduinoClient");
}

2. Publishing a message:

void loop() {
  if (client.connected()) {
    client.publish("sensor/temperature", "25.5");
  }
  delay(5000);
}

3. Subscribing to a topic and handling incoming messages:

void callback(char* topic, byte* payload, unsigned int length) {
  String message = "";
  for (int i = 0; i < length; i++) {
    message += (char)payload[i];
  }
  Serial.println("Message received: " + message);
}

void setup() {
  // ... (previous setup code)
  client.setCallback(callback);
  client.subscribe("control/led");
}

Getting Started

1. Install the PubSubClient library in Arduino IDE:

   • Go to Sketch > Include Library > Manage Libraries
   • Search for "PubSubClient" and install it

2. Include the library and set up the client:

#include <ESP8266WiFi.h>
#include <PubSubClient.h>

const char* ssid = "YourWiFiSSID";
const char* password = "YourWiFiPassword";
const char* mqtt_server = "broker.mqtt.com";

WiFiClient espClient;
PubSubClient client(espClient);

void setup() {
  Serial.begin(115200);
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  client.setServer(mqtt_server, 1883);
}

void loop() {
  if (!client.connected()) {
    reconnect();
  }
  client.loop();
}

void reconnect() {
  while (!client.connected()) {
    if (client.connect("ArduinoClient")) {
      client.subscribe("inTopic");
    } else {
      delay(5000);
    }
  }
}