How to check if there is a (web) client connected.

[HenkHoldijk]

Hi,

After several evenings of googling and reading, I'm about to give up and implement a very dirty solution in order to check if there is a client connected to the web-server I am running. I build my own little ESP32-C3 board with an OLED display and I just want to display if there is a client connected or not. I have looked at the examples and digged through a large part of the lib trying to understand how things work. I also tried a few things, but none of them worked.

Is there a way, using AsyncWebServer to check if a client is connected? Is there a status or client count I can access, or a callback I can use? I can see the information itself in the lib, but I can't find a neat way to access it.

Cheers, Henk

[mathieucarbou]

if there is a client connected or not.

You need to define what this means for you.
SSE, WebSocket, HTTP request, HTTP Download, Chunk Encoding, Request continuations are all forms of different requests / connections that are ranging from short-lives to long-lived.

Is there a way, using AsyncWebServer to check if a client is connected?

Yes, but depending on what you mean and/or can be done easily (i.e. with a middleware)

You just need to precise your intention, use case, meanings and we can help you ;-)

[HenkHoldijk]

Hi,

Thanks for the quick response. I'm only interested in the HTTP requests, so if someone is looking / using the web-pages. I'll have a try with your middleware example. I will actually have a better look at the middleware part, as I ignored it more or less as I have a different understanding of what middleware is (for me like Splice / OpenSplice or O2 / O2N).

Cheers, Henk

[mathieucarbou]

Here is for example a middleware that you can use to count the number of clients at one point on a handler (or set of handler):

class ClientMonitorMiddleware : public AsyncMiddleware {
public:
  void run(AsyncWebServerRequest *request, ArMiddlewareNext next) override {
    _count++;
    request->onDisconnect([this]() {
      _count--;
    });
    next();
  }

  size_t clientCount() const {
    return _count;
  }

private:
  size_t _count;
};

static ClientMonitorMiddleware myDownload; // will be applied to one handler
static ClientMonitorMiddleware allAPI; // will be applied to several ones

that you can use like this:

  server.on("/api/foo", HTTP_GET, [](AsyncWebServerRequest *request) {
    // [...]
  }).addMiddleware(&allAPI);

  server.on("/api/bar", HTTP_GET, [](AsyncWebServerRequest *request) {
    // [...]
  }).addMiddleware(&allAPI);
  
  server.on("/download", HTTP_GET, [](AsyncWebServerRequest *request) {
    // [...]
  }).addMiddleware(& myDownload);

• allAPI.clientCount() => returns the number of concurrent requests on ALL api
• myDownload() => returns the number of concurrent requests on download only

[HenkHoldijk]

Hi,

Thanks! It sort of works like this, just the onDisconnect part (decreasing the counter) creates an issue. The number of clients is only available while the request is being handled. It looks like as soon as the response has been provided, the client is disconnected again and the counter decreased. So polling clientCount mostly results in 0 and on a very rare occasion it will show 1 client connected. Commenting the onDisconnect call-back out, makes the counter increase with every request (as expected). I could add a time-out that resets the counter if there has been no update of it for a "while". Any other suggestion?

Cheers, Henk

[mathieucarbou]

If you want to know the number of connected client request, the code as it is is ok.
An http request is fast.
Maybe if you have more slow requests like file downloads, you will see the increased count.
Using a timeout is wrong because the client is gone.

If your want to know how many requests were handled per handler then you just need an always increasing counter .

i think you really need to rethink your use case and expose it more clearly because what you suggest doing for the counter goes against your first description.

[HenkHoldijk]

I will try to make more clear what I want to achieve. I would like to know if a client (or better user/person) is using the web-interface in order to indicate that on an OLED. That is why I suggested using a time-out. Most part of the web pages use meta refresh. So if the user has not requested a (new) page for a "while", he or she can be considered to have left.

[mathieucarbou]

Use concept on a webapp is usually translated by an Http Session and a cookie.
ESPAsyncWebSerevr is request based, but nothing prevents you from adding this concept and managing a cookie header with some expiration.