esp8266 sending synchronized https requests (using WiFiClientSecure) without delay - c

Question

i am trying to send a synchronized request to https server with esp8266 and i am using httpbin.org for testing purpose and i want to synchronize the requests. i mean how to not sending request until the previous is recieved and without using delay?
by comparing code (below) and result (below) you can see that httpsClient.readString() returns empty result from time to time. why? how to explain that? and how to fix that or get around it?
code and result are bellow. please help.
#include <ESP8266WiFi.h>
#include <WiFiClientSecure.h>
//i cut wifi ssid and pswrd declaration to shorten the question
const char *host = "httpbin.org"; //Domain to Server
String path = "/post"; //Path of Server
const int httpsPort = 443; //HTTPS PORT (default: 443)
WiFiClientSecure httpsClient;
String response;
void setup() {
//i cut wifi set up declaration to shorten the question
httpsClient.setInsecure();
if(httpsClient.connect(host, httpsPort)){
Serial.println("Connected to "+String(host));
}else
Serial.println("error connecting");
}
void loop() {
httpsClient.print(String("POST ") + "/response-headers" + " HTTP/1.1\r\n" +
"Host: " + "httpbin.com\r\n"+
"Content-Type: application/x-www-form-urlencoded\r\n" +
"Content-Length: 13\r\n\r\n" +
"say=Hi&to=Sam\r\n");
while(httpsClient.available()) {
response = httpsClient.readStringUntil('\n');
Serial.println(response);
}
Serial.println("---------------------------------------------------------");
}
Result:
---------------------------------------------------------
HTTP/1.1 200 OK
Date: Wed, 26 May 2021 16:20:01 GMT
Content-Type: application/json
Content-Length: 68
Connection: keep-alive
Server: gunicorn/19.9.0
Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true
{
"Content-Length": "68",
"Content-Type": "application/json"
}
---------------------------------------------------------
---------------------------------------------------------
HTTP/1.1 200 OK
Date: Wed, 26 May 2021 16:20:01 GMT
Content-Type: application/json
Content-Length: 68
Connection: keep-alive
Server: gunicorn/19.9.0
Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true
{
"Content-Length": "68",
"Content-Type": "application/json"
}
HTTP/1.1 200 OK
Date: Wed, 26 May 2021 16:20:01 GMT
Content-Type: application/json
Content-Length: 68
Connection: keep-alive
Server: gunicorn/19.9.0
Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true
{
"Content-Length": "68",
"Content-Type": "application/json"
}
---------------------------------------------------------
HTTP/1.1 200 OK
Date: Wed, 26 May 2021 16:20:02 GMT
Content-Type: application/json
Content-Length: 68
Connection: keep-alive
Server: gunicorn/19.9.0
Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true
{
"Content-Length": "68",
"Content-Type": "application/json"
}
---------------------------------------------------------
---------------------------------------------------------
---------------------------------------------------------
---------------------------------------------------------
---------------------------------------------------------
---------------------------------------------------------
HTTP/1.1 200 OK
Date: Wed, 26 May 2021 16:26:19 GMT
Content-Type: application/json
Content-Length: 68
Connection: keep-alive
Server: gunicorn/19.9.0
Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true
{
"Content-Length": "68",
"Content-Type": "application/json"
}
and so on.....