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License: LGPL 3.0 Continuous Integration PlatformIO Registry

Asynchronous HTTP and WebSocket Server Library for ESP32, ESP8266 and RP2040 Supports: WebSocket, SSE, Authentication, Arduino Json 7, File Upload, Static File serving, URL Rewrite, URL Redirect, etc.

This fork is based on yubox-node-org/ESPAsyncWebServer and includes all the concurrency fixes.

Coordinate and dependencies:

WARNING The library name was changed from ESP Async WebServer to ESPAsyncWebServer as per the Arduino Lint recommendations.

mathieucarbou/ESPAsyncWebServer @ 3.1.1


Changes in this fork


Usage and API stays the same as the original library. Please look at the original libraries for more examples and documentation.

AsyncWebSocketMessageBuffer and makeBuffer()

The fork from yubox-node-org introduces some breaking API changes compared to the original library, especially regarding the use of std::shared_ptr<std::vector<uint8_t>> for WebSocket.

This fork is compatible with the original library from me-no-dev regarding WebSocket, and wraps the optimizations done by yubox-node-org in the AsyncWebSocketMessageBuffer class. So you have the choice of which API to use.

Here are examples for serializing a Json document in a websocket message buffer:

void send(JsonDocument& doc) {
  const size_t len = measureJson(doc);

  // original API from me-no-dev
  AsyncWebSocketMessageBuffer* buffer = _ws->makeBuffer(len);
  assert(buffer); // up to you to keep or remove this
  serializeJson(doc, buffer->get(), len);
void send(JsonDocument& doc) {
  const size_t len = measureJson(doc);

  // this fork (originally from yubox-node-org), uses another API with shared pointer
  auto buffer = std::make_shared<std::vector<uint8_t>>(len);
  assert(buffer); // up to you to keep or remove this
  serializeJson(doc, buffer->data(), len);

I recommend to use the official API AsyncWebSocketMessageBuffer to retain further compatibility.

Important recommendations

Most of the crashes are caused by improper configuration of the library for the project. Here are some recommendations to avoid them.

  1. Set the running core to be on the same core of your application (usually core 1) -D CONFIG_ASYNC_TCP_RUNNING_CORE=1
  2. Set the stack size appropriately with -D CONFIG_ASYNC_TCP_STACK_SIZE=16384. The default value of 16384 might be too much for your project. You can look at the MycilaTaskMonitor project to monitor the stack usage.
  3. You can change if you know what you are doing the task priority with -D CONFIG_ASYNC_TCP_PRIORITY=10. Default is 10.
  4. You can increase the queue size with -D CONFIG_ASYNC_TCP_QUEUE_SIZE=128. Default is 64.
  5. You can decrease the maximum ack time -D CONFIG_ASYNC_TCP_MAX_ACK_TIME=3000. Default is 5000.

I personally use the following configuration in my projects because my WS messages can be big (up to 4k). If you have smaller messages, you can increase WS_MAX_QUEUED_MESSAGES to 128.