Hi.
I’m not very familiar with Python code to control the RPi.
But I found this tutorial that shows how to subscribe and publish with the Pi: https://www.digikey.com/en/maker/blogs/2019/how-to-use-mqtt-with-the-raspberry-pi
You need to take a look at the sections that refer to publish and then insert it on your callback.
Regards,
Sara

Thanks Sara.   Then to add a subscription to the acknowledgment in the esp32, how would that be written?  I am not totally familiar working with c++ and pointers.  Presently the code shows

(sorry I cannot figure out how to copy/paste the code here with the appropriate indents) –

// MQTT
const char* mqtt_server = “x.x.x.x”; // IP of the MQTT broker
const char* battlevel_topic = “home/battlevel”;
const char* moisture_topic = “home/moisture”;
const char* temperature_topic = “home/temperature”;
const char* ID_topic = “home/ID”;
const char* mqtt_username = “xx”; // MQTT username
const char* mqtt_password = “xx”; // MQTT password
const char* clientID = “esp32_client1”; // MQTT client ID

void loop() {
while (millis() < wake){
Serial.print(“Time(s) = “);
Serial.println(millis()/1000);

displayTime(); // display the real-time clock data on the Serial Monitor,

connect_MQTT();
Serial.setTimeout(5000);
getReadings();

Serial.print(“soilMoisture: “);
Serial.println(soilMoisture);
Serial.print(“Moisture: “);
Serial.print(h);
Serial.println(” %”);
Serial.print(“Temperature: “);
Serial.print(t);
Serial.println(” *F”);

// MQTT can only transmit strings
String hs=”Hum: “+String((float)h)+” % “;
String ts=”Temp: “+String((float)t)+” F “;

// PUBLISH to the MQTT Broker (topic = Temperature, defined at the beginning)
if (client.publish(temperature_topic, String((float)t).c_str())) {
Serial.println(“Temperature sent!”);
}

// Again, client.publish will return a boolean value depending on whether it succeded or not.
// If the message failed to send, we will try again, as the connection may have broken.
else {
Serial.println(“Temperature failed to send. Reconnecting to MQTT Broker and trying again”);
client.connect(clientID, mqtt_username, mqtt_password);
delay(10); // This delay ensures that client.publish doesn’t clash with the client.connect call
client.publish(temperature_topic, String(t).c_str());
}

Hi.
To receive the message sent by the Raspberry Pi, the ESP32 needs to subscribe to the same topic that the Rpi is publishing in.
What specific project are you following?
Regards,
Sara

I was trying to combine Project 4 modified without LoRa and the chapter on MQTT so I can transmit data from the weather station directly to another device.  But I forgot the MQTT code above I borrowed from somewhere else, so apologies for that.

Take a look at the MQTT section again and check the parts of the code that are responsible for subscribing and the parts of the code that are responsible for publishing.
Isolate those sections and try to apply them to your project.
Without further information, it is very difficult to understand what you’re trying to achieve and what issues you’re facing.