HTTP Server Experiment

1. HTTP Overview

HTTP is the "passport" for IoT devices to integrate into the Web ecosystem—mastering HTTP communication allows ESP32 to access any API on the internet like a browser, and also turn itself into a Web server for direct device control via webpages from phones or computers. Whether doing remote sensor data reporting, weather fetching, OTA firmware upgrades, or building smart home web control panels, HTTP is the most versatile choice. This tutorial covers the server RESTful API routine, teaching core HTTP communication skills on ESP32.

HTTP (HyperText Transfer Protocol) is the most widely used application layer protocol on the internet. It is based on TCP and uses a request-response model: client sends GET request, server returns status code + data.

This project contains both HTTP client and server routines: the client gets webpage data via GET request; the server provides RESTful API on port 80.

2. Core Concepts

2.1 HTTP Request-Response Model

┌─── HTTP Client (ESP32) ────────────┐    ┌─── HTTP Server ───────────────────┐
│                                   │    │                                  │
│  GET /index.html HTTP/1.1         │───→│  Parse request → route to handler │
│  Host: example.com                │    │  ↓                                │
│                                   │    │  Execute handler logic            │
│                                   │    │  ↓                                │
│  HTTP/1.1 200 OK                 │←───│  Return status code + Headers + Body│
│  Content-Type: text/html          │    │                                  │
│  Content-Length: 125             │    │                                  │
│                                   │    │                                  │
│  <html>...</html>                │    │                                  │
└───────────────────────────────────┘    └──────────────────────────────────┘

2.2 HTTP Methods

2.3 HTTP Status Codes

2.4 esp_http_client Event-Driven Model

ESP-IDF's HTTP client uses event callback mechanism. Key events:

3. Project Architecture and Flow

3.1 HTTP Server

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app_main()
  └─ xTaskCreate(vHttpServerTask, stack=8KB, prio=5)
      └─ vHttpServerTask():
          ├─ wifi_init()                      ← WiFi connect + wait for IP
          ├─ start_http_server()
          │   ├─ httpd_start(port 80)         ← Start HTTP server
          │   ├─ Register GET  "/"       → root_handler
          │   └─ Register GET  "/get"    → get_handler
          └─ while(1) vTaskDelay(1000ms)      ← Keep running

3.3 Server Routing Table

Routing determines "access which path → execute which function → return what content":

• GET / → call root_handler, return {"msg":"ESP32 HTTP Server is running!"}

• GET /get → call get_handler, return {"status":"ok","msg":"hello from esp32 http server"}

Both have Content-Type application/json.

4. Hardware Dependencies

• Hardware Connection: No external wiring, WiFi wireless communication

• Client required components: esp_http_client, esp_wifi, esp_netif, esp_event, nvs_flash

• Server required components: esp_http_server (instead of esp_http_client), others same as above

5. Source Code Analysis — HTTP Server

Full source: Source Code → ESP-IDF → 5.Network_Protocol → 6.HTTP → HTTP_Server

5.1 Route Handlers

The most core concept of HTTP server is routing—client (browser/Postman) accesses different paths, ESP32 returns different content. Each route is a function that writes response to req. httpd_resp_set_type() sets Content-Type (set "application/json" for returning JSON, "text/html" for webpages), httpd_resp_send() sends the content out.

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static esp_err_t root_handler(httpd_req_t *req)
{
    char resp[] = "{\"msg\":\"ESP32 HTTP Server is running!\"}";
    httpd_resp_set_type(req, "application/json");
    httpd_resp_send(req, resp, strlen(resp));
    ESP_LOGI(TAG, "Root / Request processed successfully");
    return ESP_OK;
}
static esp_err_t get_handler(httpd_req_t *req)
{
    char resp[] = "{\"status\":\"ok\", \"msg\":\"hello from esp32 http server\"}";
    httpd_resp_set_type(req, "application/json");
    httpd_resp_send(req, resp, strlen(resp));
    ESP_LOGI(TAG, "GET Request processed successfully");
    return ESP_OK;
}

5.2 start_http_server() — Start Server

start_http_server() starts an HTTP server, then hangs routes one by one using httpd_register_uri_handler(). Each httpd_uri_t specifies three things: path (.uri), HTTP method (.method), handler function (.handler). Client accessing http://ESP32_IP/ calls root_handler, accessing /get calls get_handler.

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static void start_http_server(void)
{
    httpd_config_t config = HTTPD_DEFAULT_CONFIG();
    httpd_handle_t server = NULL;

    if (httpd_start(&server, &config) != ESP_OK) {
        ESP_LOGE(TAG, "Failed to start HTTP server!");
        return;
    }
    ESP_LOGI(TAG, "HTTP server started, port: 80");

    httpd_uri_t root_uri = {
        .uri      = "/",
        .method   = HTTP_GET,
        .handler  = root_handler,
        .user_ctx = NULL,
    };
    httpd_register_uri_handler(server, &root_uri);

    httpd_uri_t get_uri = {
        .uri      = "/get",
        .method   = HTTP_GET,
        .handler  = get_handler,
        .user_ctx = NULL,
    };
    httpd_register_uri_handler(server, &get_uri);
}

6. Operation Flow

6.1 Build and Flash Flow

ESP-IDF Flash Flow: See ESP-IDF → 1. Before Development → 3. Build and Flash.

6.2 Testing HTTP Server

1. Flash HTTP_Server to ESP32. Note the IP address printed on serial (e.g. 192.168.11.100).

2. Open Postman, create GET request, URL input http://192.168.11.100/ → returns {"msg":"ESP32 HTTP Server is running!"}

3. Same method, GET request http://192.168.11.100/get → returns {"status":"ok","msg":"hello from esp32 http server"}

6.3 Normal Log — HTTP Server

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I (3856) HTTP-SERVER: ========== HTTP SERVER ==========
I (3858) HTTP-SERVER: Waiting for WiFi connection...
I (3872) HTTP-SERVER: Got IP: 192.168.11.100
I (3874) HTTP-SERVER: WiFi connected successfully!
I (3876) HTTP-SERVER: HTTP server started, port: 80
I (5000) HTTP-SERVER: Root / Request processed successfully
I (6000) HTTP-SERVER: GET Request processed successfully

Use Postman to test, create GET request, URL fill ESP32's IP and port:

• GET http://<ESP32_IP>/ → returns {"msg":"ESP32 HTTP Server is running!"}

• GET http://<ESP32_IP>/get → returns {"status":"ok","msg":"hello from esp32 http server"}

7. Common Problems