Firebase Realtime Database with ESP32 Using the REST API and Auth

#include <WiFi.h>
#include <WiFiClientSecure.h>
#include <HTTPClient.h>
#include <ArduinoJson.h>
#include <DHT.h>

const char* WIFI_SSID = "YourWiFi";
const char* WIFI_PASS = "YourWiFiPassword";
const char* API_KEY   = "YOUR_WEB_API_KEY";
const char* DB_URL    = "https://PROJECT-default-rtdb.firebaseio.com";
const char* EMAIL     = "node1@yourproject.dev";
const char* PASSWORD  = "device-password";
const int RELAY_PIN = 26;

DHT dht(4, DHT22);
String idToken, uid;
uint32_t tokenTime = 0, lastSend = 0, lastPoll = 0;

// Generic HTTPS request; returns status code and fills 'reply'
int request(const char* method, const String& url, const String& body, String& reply) {
  WiFiClientSecure client;
  client.setInsecure();               // demo only: no certificate check
  HTTPClient http;
  if (!http.begin(client, url)) return -1;
  http.addHeader("Content-Type", "application/json");
  int code = http.sendRequest(method, body);
  if (code > 0) reply = http.getString();
  http.end();
  return code;
}

bool signIn() {
  String url = "https://identitytoolkit.googleapis.com/v1/accounts:signInWithPassword?key=" + String(API_KEY);
  JsonDocument req;
  req["email"] = EMAIL;
  req["password"] = PASSWORD;
  req["returnSecureToken"] = true;
  String body, reply;
  serializeJson(req, body);
  if (request("POST", url, body, reply) != 200) {
    Serial.println("Sign-in failed: " + reply);
    return false;
  }
  JsonDocument res;
  deserializeJson(res, reply);
  idToken = res["idToken"].as<String>();
  uid     = res["localId"].as<String>();
  tokenTime = millis();
  Serial.println("Signed in, uid = " + uid);
  return true;
}

String path(const char* child) {      // https://.../devices/<uid>/<child>.json?auth=...
  return String(DB_URL) + "/devices/" + uid + "/" + child + ".json?auth=" + idToken;
}

void sendReading() {
  float t = dht.readTemperature(), h = dht.readHumidity();
  if (isnan(t) || isnan(h)) return;
  JsonDocument doc;
  doc["temp_c"] = t;
  doc["hum_pct"] = h;
  doc["ts"][".sv"] = "timestamp";     // server fills in the time
  String body, reply;
  serializeJson(doc, body);
  int c1 = request("PUT", path("latest"), body, reply);   // overwrite
  int c2 = request("POST", path("history"), body, reply); // append
  Serial.printf("latest %d, history %d", c1, c2);
  Serial.println();
}

void pollRelay() {
  String reply;
  if (request("GET", path("relay"), "", reply) == 200) {
    reply.trim();                     // body is true, false or null
    digitalWrite(RELAY_PIN, reply == "true" ? HIGH : LOW);
  }
}

void setup() {
  Serial.begin(115200);
  pinMode(RELAY_PIN, OUTPUT);
  dht.begin();
  WiFi.begin(WIFI_SSID, WIFI_PASS);
  while (WiFi.status() != WL_CONNECTED) delay(500);
  while (!signIn()) delay(10000);
}

void loop() {
  if (WiFi.status() != WL_CONNECTED) { WiFi.reconnect(); delay(2000); return; }
  if (millis() - tokenTime > 50UL * 60 * 1000) signIn();  // refresh before 1 h expiry
  if (millis() - lastSend > 60000) { lastSend = millis(); sendReading(); }
  if (millis() - lastPoll > 3000)  { lastPoll = millis(); pollRelay(); }
}