How to fix issue with post requests from arduino, only being received 2-3 times before failing - c++

Question

EDIT5:
I eventually fixed this issue by more or less throwing away half my code. Rather than sending data to a ruby server using HTTP, I'm now using MQTT to a broker to a NodeJS server. The NodeJS part here isn't important but to anyone else having this issue I STRONGLY recommend sending all IoT data using MQTT, and that's what solved my issue.
I'm currently trying to send data collected from sensors on an Arduino WiFi rev2, to my rails server hosted on Heroku. I do this by sending my data in a JSON format. My problem is that while my methods seem to work initially, with the first few POST requests being received and processed fine, after 2-3 requests the arduino hangs, and I receive status code: -2. I'm using the ArduinoHttpClient library.
I've tried using a local server, which has the same problem, aswell as sending the POST request via both curl and postman. Both curl and postman seem to work as expected, so I imagine the issue is with the arduino code although I can't be sure.
client.beginRequest();
client.post("/input");
client.sendHeader("Content-Type", "application/json");
client.sendHeader("Content-Length", postData.length());
client.beginBody();
client.println(postData);
client.endRequest();
LED(0,128,0);
Serial.println("Gone");
int statusCode = client.responseStatusCode();
String response = client.responseBody();
Serial.print("Status code: ");
Serial.println(statusCode);
Serial.print("Response: ");
Serial.println(response);
When this code fails, the arduino will hang for about 20-40 seconds and I will receive 'status code -3' from the Serial. However I have also received status code -2 and -4 in the past.
When it does succeed I receive the following: "Status code: 204" which is what I would expect.
EDIT:
I've since tried posting to requestcatcher.com, and the problem persisted. I'm therefore fairly confident this is an arduino problem, I also received the following output:
POST /input HTTP/1.1
Host: arduino.requestcatcher.com
Connection: close
Connection: close
Content-Length: 88
Content-Type: application/json
User-Agent: Arduino/2.2.0
{"inputs":[{"input_id":"1","value":1.778002}{"input_id":"2","value":18.037}],"id":"13"}
EDIT 2:
I accidentally discovered that the POST requests go through fine if the "Content-Length:" Header is omitted. Obviously no JSON actually gets sent so this does not fix my issue but it is likely that this header or the JSON itself is the issue.
EDIT 3:
Regardless of server I receive either status code -4, or -3, even on request catcher.
EDIT 4:
After various adjustments, code now looks as below. This seems to have helped a little and it fails less often but still does fail. I'm beginning to wonder if this is a problem with ArduinoHttpClient.
String postData = "";
serializeJson(doc, postData);
serializeJson(doc, Serial);
Serial.println(postData)
client.post("/input", "application/json", postData.c_str());
LED(0,128,0);
Serial.println("Gone");
int statusCode = client.responseStatusCode();
Serial.print("Status code: ");
Serial.println(statusCode);
client.stop();
doc.clear();
lastCycle = millis();

Answer 1

Try replacing
client.beginRequest();
client.post("/input");
client.sendHeader("Content-Type", "application/json");
client.sendHeader("Content-Length", postData.length());
client.beginBody();
client.println(postData);
client.endRequest();
with just
String contentType = "application/json";
client.post("/input", contentType, postData);
or
client.post("/input", "application/json", postData.c_str());
You don't need to explicitly specify the request headers - or call beginRequest(), etc. - when using the post() method(s) in that library.