MQTT

#include <PubSubClient.h>
 
const String Device_ID = "11"; //ID urządzenia [String]
const String myHostname = "ssh_device_" + Device_ID;
 
String Mqtt_Server = "adres_serwera"; //MQTT broker address
int Mqtt_Port = numer_portu; //MQTT broker port
String Mqtt_User = "nazwa_uzytkownika"; //MQTT broker username
String Mqtt_Password = "haslo"; //MQTT broker user password
// const int Mqtt_Buffer_Size = 512; //powiększenie rozmiaru bufora publikowania
 
PubSubClient mqttClient(wifiClient);
 
void callback(char*, byte*, unsigned int);
void connectMqtt();

void setup()
{
  connectWiFi();
  connectMqtt();
}

void loop()
{
  if (WiFi.status() != WL_CONNECTED) { connectWiFi(); }
  if (!mqttClient.connected()) { connectMqtt(); }
  mqttClient.loop();
  vTaskDelay(pdMS_TO_TICKS(10));
}

void connectMqtt()
{
  digitalWrite(BUILT_LED, LOW);
  int hostnameLenght = myHostname.length() + 1;
  char myMqttName[hostnameLenght];
  myHostname.toCharArray(myMqttName, hostnameLenght);
 
  mqttClient.setServer(Mqtt_Server.c_str(), Mqtt_Port);
  mqttClient.setCallback(callback);
  mqttClient.setBufferSize(Mqtt_Buffer_Size);
 
  while (!mqttClient.connected() and apiGetSuccess) {
    vTaskDelay(pdMS_TO_TICKS(500));
    digitalWrite(BUILT_LED, HIGH);
    Serial.println("Connecting to MQTT...");
 
    //willMessage:
    String json = "{\"device\":{\"id\":" + Device_ID 
    + ",\"name\":\"" + myHostname 
    + "\",\"type\":\"" + ESP.getChipModel() 
    + "\",\"online\":false}}";
 
    if (mqttClient.connect(myMqttName, Mqtt_User.c_str(), Mqtt_Password.c_str(), String("ssh/devices/status/" + Device_ID).c_str(), 1, true, json.c_str()))
    {
      Serial.println("Connected to MQTT");
      mqttClient.subscribe("adres_kanalu"); //subskrypcja
    } 
    else 
    {
      Serial.print("MQTT Client Failed with state ");
      Serial.println(mqttClient.state());
 
      if (WiFi.status() != WL_CONNECTED) 
      {
        Serial.println("WiFi disconnected!");
        break;
      }
      vTaskDelay(pdMS_TO_TICKS(500));
      digitalWrite(BUILT_LED, LOW);
    }
  }
  digitalWrite(BUILT_LED, HIGH);
}

void callback(char* topic, byte* payload, unsigned int length)
{
  char *token;
  const char *delimiter ="/";
  String splitTopic[4] = {};
  token = strtok(topic, delimiter);
 
  int i = 0;
  while (token != NULL) 
  {
    splitTopic[i] = token;
    i++;
    token=strtok(NULL, delimiter);
  }
 
  if (splitTopic[0] == "lvl_1" and splitTopic[1] == "lvl_2" and splitTopic[2] == "lvl_3") //kanał odebranej wiadomości
  {
    ...    
  }
}

Proste

mqttClient.publish("topic", "message");

Sprawdzające rozmiar wiadomości

String topic = "topic");
int pub = mqttClient.publish(topic.c_str(), "message", true);
if (!pub)
{
  Serial.println("MQTT publish fail!");
  Serial.println("json size: " + String(json.length()) + " bajts");
  Serial.println("message buffor: " + String(Mqtt_Buffer_Size));
}