Reply a message after receiving data on subscriber in Google PubSub Pull - google-cloud-platform

Question

Is it possible to reply a message once you received a data from Publisher.
It must be a direct reply, once the Publisher published a message.
I'm using Google PubSub Service.
https://cloud.google.com/pubsub/docs/pull
Publisher/Sender (PHP):
$sendToOps =[];
$sendToOps['MESSAGE'] = "my message";
$topicName = env('GOOGLE_CLOUD_TO_OPS_TOPIC');
$pubSub = new PubSubClient();
$topic = $pubSub->topic($topicName);
$ret = $topic->publish([
'attributes'=>$sendToOps
]);
//**********The word "Apple" must output here**********
echo $ret;
//*****************************************************
Subscriber/Receiver (Javascript):
'use strict';
//Get .env File Data
require('dotenv').config({path: '/usr/share/nginx/html/myProject/.env'});
var request = require('request');
var port = process.env.PORT_GATEWAY;
var host = process.env.IP_PUSH;
var test = process.env.TEST_FLAG;
var pubsubSubscription = process.env.GOOGLE_CLOUD_TO_OPS_SUBSCRIPTION;
const keyFilePath= 'public/key/key.json';
// Imports the Google Cloud client library
const {PubSub} = require('#google-cloud/pubsub');
// Creates a client; cache this for further use
const pubSubClient = new PubSub({
keyFilename: keyFilePath
});
function listenForMessages() {
// References an existing subscription
const subscription = pubSubClient.subscription(pubsubSubscription);
// Create an event handler to handle messages
const messageHandler = message => {
console.log(message.attributes);
//*****************************************************
//I want to reply to Sender with the word "Apple" here
//*****************************************************
message.ack()
};
subscription.on('message', messageHandler);
}
listenForMessages();

Answer 1

Is it possible to reply a message once you received a data from
Publisher.
Depends on what you mean by "reply". The publisher of a message posts a message on a Pub/Sub Topic. Subscribers receive messages from a Pub/Sub Subscription. There is no two-way communications channel here. There is no Pub/Sub reply back method.
A subscriber could publish a message to a different topic that the publisher reads as a subscriber. Both sides would be publisher and a subscriber but on different topics.
Once a message is received, a subscriber could directly call an API on the publisher.
However, the intent of Publish/Subscribe is to decouple senders from receivers and not to lock them together.