Can a http server detect that a client has cancelled their request?

Question

My web app must process and serve a lot of data to display certain pages. Sometimes, the user closes or refreshes a page while the server is still busy processing it. This

Answer 1

HTTP is stateless, hence the only way to detect a disconnected client is via timeouts.

See the answers to this SO question (Java Servlet : How to detect browser closing ?).

Answer 2

While @Oded is correct that HTTP is stateless between requests, app servers can indeed detect when the underlying TCP/IP connection has broken for the request being processed. Why is this? Because TCP is a stateful protocol for reliable connections.

A common technique for .Net web apps processing a resource intensive request is to check Response.IsClientConnected (docs) before starting the resource intensive work. There is no point in wasting CPU cycles to send an expensive response to a client that isn't there anymore.

private void Page_Load(object sender, EventArgs e)
{
    // Check whether the browser remains
    // connected to the server.
    if (Response.IsClientConnected)
    {
        // If still connected, do work
        DoWork();
    }
    else
    {
        // If the browser is not connected
        // stop all response processing.
        Response.End();
    }
}

Please reply with your target app server stack so I can provide a more relevant example.

Regarding your 2nd alternative to use XHR to post client page unload events to the server, @Oded's comment about HTTP being stateless between requests is spot on. This is unlikely to work, especially in a farm with multiple servers.