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I have 3 nodemcu's(ESP32) each connected with a sensor, from which I get data and send it to aws IoT-core. How can I send all 3 sensor's data by a single MQTT subscription? The below code is for a single MQTT using one nodemcu.

#include "SPIFFS.h"
#include <WiFiClientSecure.h>
#include <Wire.h>
#include <PubSubClient.h>

float V_0 = 5.0; // supply voltage to the pressure sensor
float rho = 1.204; // density of air 

// parameters for averaging and offset
int offset = 0;
int offset_size = 10;
int veloc_mean_size = 20;
int zero_span = 2;

const char* ssid = ""; //Provide your SSID
const char* password = "";          // Provide Password
const char* mqtt_server = "a3mtl8k0aiowyx-ats.iot.us-east-2.amazonaws.com"; // Replace with your MQTT END point
const int mqtt_port = 8883;

String Read_rootca;
String Read_cert;
String Read_privatekey;
//=============================================================================================================================
#define BUFFER_LEN  256
long lastMsg = 0;
char msg[BUFFER_LEN];
int value = 0;
byte mac[6];
char mac_Id[18];
int count = 1;
int id=1;
//=============================================================================================================================

WiFiClientSecure espClient;
PubSubClient client(espClient);

void setup_wifi() {
  delay(10);
  Serial.println();
  Serial.print("Connecting to ");
  Serial.println(ssid);

  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }

  randomSeed(micros());

  Serial.println("");
  Serial.println("WiFi connected");
  Serial.println("IP address: ");
  Serial.println(WiFi.localIP());
}

void callback(char* topic, byte* payload, unsigned int length) {
  Serial.print("Message arrived [");
  Serial.print(topic);
  Serial.print("] ");
  for (int i = 0; i < length; i++) {
    Serial.print((char)payload[i]);
  }
  Serial.println();
}
void reconnect() {
  // Loop until we're reconnected
  while (!client.connected()) {
    Serial.print("Attempting MQTT connection...");
    // Create a random client ID
    String clientId = "ESP32-";
    clientId += String(random(0xffff), HEX);
    // Attempt to connect
    if (client.connect(clientId.c_str())) {
      Serial.println("connected");
      // Once connected, publish an announcement...
      //client.publish("ei_out", "connection success");
      // ... and resubscribe
      client.subscribe("ei_in");
    } else {
      Serial.print("failed, rc=");
      Serial.print(client.state());
      Serial.println(" try again in 5 seconds");
      // Wait 5 seconds before retrying
      delay(5000);
    }
  }
}

void setup() {
  Serial.begin(115200);
    for (int ii=0;ii<offset_size;ii++){
    offset += analogRead(34)-(1023/2);
  }
  offset /= offset_size;
  Serial.setDebugOutput(true);
  // initialize digital pin LED_BUILTIN as an output.
  pinMode(2, OUTPUT);
  setup_wifi();
  delay(1000);
  //=============================================================
  if (!SPIFFS.begin(true)) {
    Serial.println("An Error has occurred while mounting SPIFFS");
    return;
  }
  //=======================================
  //Root CA File Reading.
  File file2 = SPIFFS.open("/AmazonRootCA1.pem", "r");
  if (!file2) {
    Serial.println("Failed to open file for reading");
    return;
  }
  Serial.println("Root CA File Content:");
  while (file2.available()) {
    Read_rootca = file2.readString();
    Serial.println(Read_rootca);
  }
  //=============================================
  // Cert file reading
  File file4 = SPIFFS.open("/9bcef01134-certificate.pem.crt", "r");
  if (!file4) {
    Serial.println("Failed to open file for reading");
    return;
  }
  Serial.println("Cert File Content:");
  while (file4.available()) {
    Read_cert = file4.readString();
    Serial.println(Read_cert);
  }
  //=================================================
  //Privatekey file reading
  File file6 = SPIFFS.open("/9bcef01134-private.pem.key", "r");
  if (!file6) {
    Serial.println("Failed to open file for reading");
    return;
  }
  Serial.println("privateKey File Content:");
  while (file6.available()) {
    Read_privatekey = file6.readString();
    Serial.println(Read_privatekey);
  }
  //=====================================================
  char* pRead_rootca;
  pRead_rootca = (char *)malloc(sizeof(char) * (Read_rootca.length() + 1));
  strcpy(pRead_rootca, Read_rootca.c_str());

  char* pRead_cert;
  pRead_cert = (char *)malloc(sizeof(char) * (Read_cert.length() + 1));
  strcpy(pRead_cert, Read_cert.c_str());

  char* pRead_privatekey;
  pRead_privatekey = (char *)malloc(sizeof(char) * (Read_privatekey.length() + 1));
  strcpy(pRead_privatekey, Read_privatekey.c_str());

  Serial.println("================================================================================================");
  Serial.println("Certificates that passing to espClient Method");
  Serial.println();
  Serial.println("Root CA:");
  Serial.write(pRead_rootca);
  Serial.println("================================================================================================");
  Serial.println();
  Serial.println("Cert:");
  Serial.write(pRead_cert);
  Serial.println("================================================================================================");
  Serial.println();
  Serial.println("privateKey:");
  Serial.write(pRead_privatekey);
  Serial.println("================================================================================================");

  espClient.setCACert(pRead_rootca);
  espClient.setCertificate(pRead_cert);
  espClient.setPrivateKey(pRead_privatekey);

  client.setServer(mqtt_server, mqtt_port);
  client.setCallback(callback);

  //====================================================================================================================
  WiFi.macAddress(mac);
  snprintf(mac_Id, sizeof(mac_Id), "%02x:%02x:%02x:%02x:%02x:%02x",
       mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);
  Serial.print(mac_Id);
  //=====================================================================================================================
}


void loop() {
    float adc_avg = 0;
    float veloc = 0.0;
  
// average a few ADC readings for stability
  for (int ii=0;ii<veloc_mean_size;ii++){
    adc_avg+= analogRead(34)-offset;
  }
  adc_avg/=veloc_mean_size;
  
  // make sure if the ADC reads below 512, then we equate it to a negative velocity
  if (adc_avg>512-zero_span and adc_avg<512+zero_span){
  } else{
    if (adc_avg<512){
      veloc = -sqrt((-10000.0*((adc_avg/1023.0)-0.5))/rho);
    } else{
      veloc = sqrt((10000.0*((adc_avg/1023.0)-0.5))/rho);
    }
  }
//  Serial.println(veloc); // print velocity
//  delay(10); // delay for stability
  //===============================================

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

  long now = millis();
  if (now - lastMsg > 5000) {
    lastMsg = now;
    //=============================================================================================
    //String macIdStr = mac_Id;
    String sensor_id=String(id);
    String Velocity = String(veloc);
    snprintf (msg, BUFFER_LEN, "{\"sensor_id\" : \"%s\", \"Velocity\" : \"%s\"}", sensor_id.c_str(), Velocity.c_str());
    //"{\"mac_Id\" : \"%s\", \"Velocity\" : %s, \"sensor_id\" : \"%s\"}"
    Serial.print("Publish message: ");
    Serial.print(count);
    Serial.println(msg);
    client.publish("ei_out", msg);
    count = count + 1;
    //================================================================================================
  }
  digitalWrite(2, HIGH);   // turn the LED on (HIGH is the voltage level)
  delay(1000);                       // wait for a second
  digitalWrite(2, LOW);    // turn the LED off by making the voltage LOW
  delay(1000);                       // wait for a second
}

ei_out is the topic I have subscribed in iot-core, I wish to have one more ei_out_1 from another nodemcu

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  • ei_out is the topic I have subscribed in iot-core, I wish to have one more ei_out_1 from another nodemcu -- So... have another called ei_out_1...? I don't understand what you're asking. If you want to send to a different topic then send to a different topic.
    – Majenko
    Commented May 1, 2021 at 12:52
  • How can I send all 3 sensor's data by a single MQTT subscription? .... same way that you would place three words on one piece of paper? .... BTW, data is sent by publishing, subscription is for receiving data
    – jsotola
    Commented May 1, 2021 at 15:45
  • You're not subscribing to ei_out but only to a topic called ei_in (defined in reconnect()). What you want is to publish. At the moment, you're using only ei_out to publish (client.publish("ei_out") in loop()). You can freely create additional topics (with different names than "ei_out") to publish data from other sensors.
    – StarCat
    Commented May 1, 2021 at 17:26
  • Publishing has to have an explicit topic. Subscribing has two wild cards # which means anything below this level in the hierarchy and + which means anything at the next level of the hierarchy. The division between hierarchical levels is the / character. So if sensor one publishes on sensor/1/out/temp1 and sensor two publishes on sensor/2/out/temp1 you can subscribe to sensor/+/out/temp1 to read from both sensors. You can subscribe on sensor/1/out/+ to read anything sent by sensor one. You can subscribe to sensor/# to read any output from any sensor.
    – Dougie
    Commented May 2, 2021 at 13:52
  • change ei_out and ei_out_1 to ei/out and ei/out/1 to get the benefits of the hierarchical topic structure. The only hard part of this stuff is designing a sensible and coherent hierarchy for your topics.
    – Dougie
    Commented May 2, 2021 at 13:55

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