Inconsistent Vercel Serverless Functions Behavior for CuRL and ESP32

When I run a CuRL POST request to my Vercel app, I get a 200 response code. When I do the same thing with HTTPClient on my ESP32, I get a 308 Permanent Redirect, with the redirect URL being the same URL I requested, causing an infinite loop.

Curl request: curl -X POST https://soil-moisture-monitor-phi.vercel.app/api/updateMeasurement -H "Content-Type: application/json" -d '{"measurement": 123.45}' ESP32 code:

1#include <WiFi.h>2#include <HTTPClient.h>3#include <LiquidCrystal_I2C.h>456#define SENSOR_INPUT 36//157#define LIGHT_PORT 1289LiquidCrystal_I2C lcd(0x27,16,2);10const String ssid = "REDACTED";11const String password = "REDACTED";12const String serverName = "https://soil-moisture-monitor-phi.vercel.app/api/updateMeasurement/";13int prevHumidity = 0;1415void setup() {16  Serial.begin(9600);17  pinMode(LIGHT_PORT, OUTPUT);18  lcd.init();19  lcd.clear();         20  lcd.backlight(); 21  WiFi.begin(ssid, password);22  while(WiFi.status() != WL_CONNECTED){23    Serial.print("Connecting...");24    delay(100);25  }26  Serial.println("Connected!");27  // delay(1000);28  pinMode(LIGHT_PORT, OUTPUT);29  if(WiFi.status() == WL_CONNECTED){30    WiFiClient client;31    HTTPClient http;32  33    // Your Domain name with URL path or IP address with path34    http.begin(client, serverName);35    const char* headerNames[] = { "Location", "Last-Modified" };36    http.collectHeaders(headerNames, sizeof(headerNames)/sizeof(headerNames[0]));37    // Data to send with HTTP POST38    String payload = "{\"measurement\": "+String(12345) + "}";39    // Authorization40    // Send HTTP POST request41    http.addHeader("Content-Type", "application/json");42    http.setFollowRedirects(HTTPC_FORCE_FOLLOW_REDIRECTS);43    int httpResponseCode = http.POST(payload);44    Serial.print("HTTP Response code: ");45    Serial.println(httpResponseCode);46    if (httpResponseCode == 308) {47      String location = http.header("Location");  // Get redirect target48      http.end();4950      Serial.println("Redirected to: " + location);5152      // Try the new location53      http.begin(client, location);54      http.addHeader("Content-Type", "application/json");55      httpResponseCode = http.POST(payload);56      Serial.print("New code: ");57      Serial.println(httpResponseCode);58    }59    // Free resources60    http.end();61  }62  else {63    Serial.println("WiFi Disconnected");64  }65}6667void loop() {68  // put your main code here, to run repeatedly:69  int sensorOutput = analogRead(SENSOR_INPUT);70  int humidityPercentage = map(sensorOutput, 0, 4095, 100, 0);71  if (humidityPercentage < 30) {72    digitalWrite(LIGHT_PORT, HIGH);73  } else {74    digitalWrite(LIGHT_PORT, LOW);75  }76  prevHumidity = humidityPercentage;77  lcd.setCursor(1,0);78  lcd.print("Soil Humidity:");79  lcd.setCursor(6,1);80  lcd.print(String(humidityPercentage)+"%");81  // When the probe is not in soil it reports moisture as 0 or 1%82  // if (humidityPercentage != prevHumidity && humidityPercentage > 1){ 83  //   if(WiFi.status() == WL_CONNECTED){84  //     WiFiClient client;85  //     HTTPClient http;86    87  //     // Your Domain name with URL path or IP address with path88  //     http.begin(client, serverName);8990  //     // Data to send with HTTP POST91  //     String httpRequestData = "";92  //     // Authorization93  //     // Send HTTP POST request94  //     http.addHeader("Content-Type", "application/json");95  //     int httpResponseCode = http.POST("{\"measurement\": "+String(humidityPercentage) + "}");96  //     Serial.print("HTTP Response code: ");97  //     Serial.println(httpResponseCode);98  //     // Free resources99  //     http.end();100  //   }101  //   else {102  //     Serial.println("WiFi Disconnected");103  //   }104  // }105  if (humidityPercentage < 30) {106    digitalWrite(LIGHT_PORT, HIGH);107  } else {108    digitalWrite(LIGHT_PORT, LOW);109  }110  prevHumidity = humidityPercentage;111}112

My webapp code is here: http://github.com/sr5434/soil-moisture-monitor

Vercel Staff

Hi , welcome to the Vercel Community!

Sorry that you're facing this issue. In your code, I see you have a trailing slash / in the URL. Can you try removing that and then checking if the issue exists?

Removing the trailing slash does not change anything. I noticed that in the logs on my ESP32 it said that it received an HTTP 1.0 response but in CuRL it said it was running HTTP 2.0. Could that be the reason why I am getting redirected?

Vercel Staff

Hi , all traffic from Vercel is served by h2. Are you using some VPN or firewall software on your computer or the local network? I've noticed in the past that such programs can affect the outcome in some rare cases.

No, I’m not using a vpn or firewall.

Vercel Staff

I see. Can you share the response you get in your ESP32 code? I mean the response headers and status code. That'll help us narrow down.

Can you also share the updated copy of the ESP32 code?

I am getting a 308 response code. Below are the response headers: Header Name: Location Header Value: https://soil-moisture-monitor-phi.vercel.app/api/updateMeasurement Header Name: Last-Modified Header Value:

Below is my updated code:

1#include <WiFi.h>2#include <HTTPClient.h>3#include <LiquidCrystal_I2C.h>4#include <esp_log.h>567#define SENSOR_INPUT 36//158#define LIGHT_PORT 12910LiquidCrystal_I2C lcd(0x27,16,2);11const String ssid = "REDACTED";12const String password = "REDACTED";13const String serverName = "https://soil-moisture-monitor-phi.vercel.app/api/updateMeasurement";14int prevHumidity = 0;1516void setup() {17  Serial.begin(9600);18  esp_log_level_set("*", ESP_LOG_VERBOSE);19  pinMode(LIGHT_PORT, OUTPUT);20  lcd.init();21  lcd.clear();         22  lcd.backlight(); 23  WiFi.begin(ssid, password);24  while(WiFi.status() != WL_CONNECTED){25    Serial.print("Connecting...");26    delay(100);27  }28  Serial.println("Connected!");29  // delay(1000);30  pinMode(LIGHT_PORT, OUTPUT);31  if(WiFi.status() == WL_CONNECTED){32    WiFiClient client;33    HTTPClient http;34  35    // Your Domain name with URL path or IP address with path36    http.begin(client, serverName);37    const char* headerNames[] = { "Location", "Last-Modified" };38    http.collectHeaders(headerNames, sizeof(headerNames)/sizeof(headerNames[0]));39    // Data to send with HTTP POST40    String payload = "{\"measurement\": "+String(12345) + "}";41    // Authorization42    // Send HTTP POST request43    http.addHeader("Content-Type", "application/json");44    http.setFollowRedirects(HTTPC_FORCE_FOLLOW_REDIRECTS);45    int httpResponseCode = http.POST(payload);46    Serial.print("HTTP Response code: ");47    Serial.println(httpResponseCode);48    int headerCount = http.headers();49    Serial.printf("Header count: %d50", headerCount);5152    for (int i = 0; i < headerCount; i++) {53      Serial.printf("Header Name: %s54", http.headerName(i).c_str());55      Serial.printf("Header Value: %s56", http.header(i).c_str());57    }58    if (httpResponseCode == 308) {59      String location = http.header("Location");  // Get redirect target60      http.end();6162      Serial.println("Redirected to: " + location);6364      // Try the new location65      http.begin(client, location);66      const char* headerNames[] = { "Location", "Last-Modified" };67      http.collectHeaders(headerNames, sizeof(headerNames)/sizeof(headerNames[0]));68      http.addHeader("Content-Type", "application/json");69      httpResponseCode = http.POST(payload);70      Serial.print("New code: ");71      Serial.println(httpResponseCode);72      int headerCount = http.headers();73      Serial.printf("Header count: %d74", headerCount);7576      for (int i = 0; i < headerCount; i++) {77        Serial.printf("Header Name: %s78", http.headerName(i).c_str());79        Serial.printf("Header Value: %s80", http.header(i).c_str());81      }82    }83    // Free resources84    http.end();85  }86  else {87    Serial.println("WiFi Disconnected");88  }89}9091void loop() {}```

Vercel Staff

Hi , thanks for sharing additional details. I think the issue/limitation lies into the HTTP Client we're using here because Vercel responses are be same as reflected by cURL.

Are there alternative HTTP client that you can use for your program? Maybe just for testing purpose to see what could be the actual issue.

I found some relevant posts:

It seems that the tutorials you sent are out of date, as they require libraries that are no longer available.

Vercel Staff

Oh, I see. I'd suggest to look up for alternatives and share if the problem persists. Because this is hard to reproduce without the same codebase/hardware client.

This topic is closed. It no longer accepts new replies.

Curl request: curl -X POST https://soil-moisture-monitor-phi.vercel.app/api/updateMeasurement -H "Content-Type: application/json" -d '{"measurement": 123.45}' ESP32 code:

1#include <WiFi.h>2#include <HTTPClient.h>3#include <LiquidCrystal_I2C.h>456#define SENSOR_INPUT 36//157#define LIGHT_PORT 1289LiquidCrystal_I2C lcd(0x27,16,2);10const String ssid = "REDACTED";11const String password = "REDACTED";12const String serverName = "https://soil-moisture-monitor-phi.vercel.app/api/updateMeasurement/";13int prevHumidity = 0;1415void setup() {16  Serial.begin(9600);17  pinMode(LIGHT_PORT, OUTPUT);18  lcd.init();19  lcd.clear();         20  lcd.backlight(); 21  WiFi.begin(ssid, password);22  while(WiFi.status() != WL_CONNECTED){23    Serial.print("Connecting...");24    delay(100);25  }26  Serial.println("Connected!");27  // delay(1000);28  pinMode(LIGHT_PORT, OUTPUT);29  if(WiFi.status() == WL_CONNECTED){30    WiFiClient client;31    HTTPClient http;32  33    // Your Domain name with URL path or IP address with path34    http.begin(client, serverName);35    const char* headerNames[] = { "Location", "Last-Modified" };36    http.collectHeaders(headerNames, sizeof(headerNames)/sizeof(headerNames[0]));37    // Data to send with HTTP POST38    String payload = "{\"measurement\": "+String(12345) + "}";39    // Authorization40    // Send HTTP POST request41    http.addHeader("Content-Type", "application/json");42    http.setFollowRedirects(HTTPC_FORCE_FOLLOW_REDIRECTS);43    int httpResponseCode = http.POST(payload);44    Serial.print("HTTP Response code: ");45    Serial.println(httpResponseCode);46    if (httpResponseCode == 308) {47      String location = http.header("Location");  // Get redirect target48      http.end();4950      Serial.println("Redirected to: " + location);5152      // Try the new location53      http.begin(client, location);54      http.addHeader("Content-Type", "application/json");55      httpResponseCode = http.POST(payload);56      Serial.print("New code: ");57      Serial.println(httpResponseCode);58    }59    // Free resources60    http.end();61  }62  else {63    Serial.println("WiFi Disconnected");64  }65}6667void loop() {68  // put your main code here, to run repeatedly:69  int sensorOutput = analogRead(SENSOR_INPUT);70  int humidityPercentage = map(sensorOutput, 0, 4095, 100, 0);71  if (humidityPercentage < 30) {72    digitalWrite(LIGHT_PORT, HIGH);73  } else {74    digitalWrite(LIGHT_PORT, LOW);75  }76  prevHumidity = humidityPercentage;77  lcd.setCursor(1,0);78  lcd.print("Soil Humidity:");79  lcd.setCursor(6,1);80  lcd.print(String(humidityPercentage)+"%");81  // When the probe is not in soil it reports moisture as 0 or 1%82  // if (humidityPercentage != prevHumidity && humidityPercentage > 1){ 83  //   if(WiFi.status() == WL_CONNECTED){84  //     WiFiClient client;85  //     HTTPClient http;86    87  //     // Your Domain name with URL path or IP address with path88  //     http.begin(client, serverName);8990  //     // Data to send with HTTP POST91  //     String httpRequestData = "";92  //     // Authorization93  //     // Send HTTP POST request94  //     http.addHeader("Content-Type", "application/json");95  //     int httpResponseCode = http.POST("{\"measurement\": "+String(humidityPercentage) + "}");96  //     Serial.print("HTTP Response code: ");97  //     Serial.println(httpResponseCode);98  //     // Free resources99  //     http.end();100  //   }101  //   else {102  //     Serial.println("WiFi Disconnected");103  //   }104  // }105  if (humidityPercentage < 30) {106    digitalWrite(LIGHT_PORT, HIGH);107  } else {108    digitalWrite(LIGHT_PORT, LOW);109  }110  prevHumidity = humidityPercentage;111}112

My webapp code is here: http://github.com/sr5434/soil-moisture-monitor