Communicate between 2 NodeMCUs using MQTT

receiver-device.ino

#include <ESP8266WiFi.h>
#include <PubSubClient.h>
 
const char* ssid = "xxx";
const char* password = "xxx";

const char* mqttServer = "m24.cloudmqtt.com";
const int mqttPort = 15031;
const char* mqttUser = "xxx";
const char* mqttPassword = "xxx";
 
int ledPin = 13; // GPIO13
bool isOn = false;

WiFiClient espClient;
PubSubClient client(espClient);
 
void setup() {
  Serial.begin(9600);
  delay(10);
 
  pinMode(ledPin, OUTPUT);
  digitalWrite(ledPin, LOW);
 
  // Connect to WiFi network
  Serial.println();
  Serial.println();
  Serial.print("Connecting to ");
  Serial.println(ssid);
 
  WiFi.begin(ssid, password);
 
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println("");
  Serial.println("WiFi connected");

  client.setServer(mqttServer, mqttPort);
  client.setCallback(callback);

  while (!client.connected()) {
    Serial.println("Connecting to MQTT...");
 
    if (client.connect("ESP8266ClientB", mqttUser, mqttPassword )) {
      Serial.println("connected");
    } else {
      Serial.print("failed with state ");
      Serial.print(client.state());
      delay(2000);
    }
  }

  client.subscribe("esp/blinker");
}

void callback(char* topic, byte* payload, unsigned int length) {
  Serial.println(topic);
  
  if(strcmp(topic, "esp/blinker") == 0) {
    // blink message
    char message[50];
    sprintf(message, "esp/blinker sent a message: %s", (char *)payload);
    Serial.println(message);
  }
}
 
void loop() {
  client.loop();
}

sender-device.ino

#include <ESP8266WiFi.h>
#include <PubSubClient.h>
 
const char* ssid = "xxx";
const char* password = "xxx";

const char* mqttServer = "m24.cloudmqtt.com";
const int mqttPort = 15031;
const char* mqttUser = "xxx";
const char* mqttPassword = "xxx";
 
int ledPin = 13; // GPIO13
bool isOn = false;

WiFiClient espClient;
PubSubClient client(espClient);
 
void setup() {
  Serial.begin(9600);
  delay(10);
 
  pinMode(ledPin, OUTPUT);
  digitalWrite(ledPin, LOW);
 
  // Connect to WiFi network
  Serial.println();
  Serial.println();
  Serial.print("Connecting to ");
  Serial.println(ssid);
 
  WiFi.begin(ssid, password);
 
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println("");
  Serial.println("WiFi connected");

  client.setServer(mqttServer, mqttPort);

  while (!client.connected()) {
    Serial.println("Connecting to MQTT...");
 
    if (client.connect("ESP8266ClientA", mqttUser, mqttPassword )) {
      Serial.println("connected");
    } else {
      Serial.print("failed with state ");
      Serial.print(client.state());
      delay(2000);
    }
  }
 
  client.publish("esp/test", "Hello from ESP32");
}
 
void loop() {
  client.publish("esp/blinker", isOn ? "On" : "Off");
  isOn = !isOn;
  delay(1000);
}