ASP.NET：这个在线用户计数器出了什么问题？

Question

I'm trying to count the amount of online users.

This is the code:

protected void Application_Start()
{
    ...
    Application["OnlineUsers"] = 0;
}

private void Session_Start(object sender, EventArgs e)
{
            Application.Lock();
            Session["O"] = "OO"; // Need to have something in the session
            Application["OnlineUsers"] = (int)Application["OnlineUsers"] + 1;
            Application.UnLock();
}


private void Session_End(object sender, EventArgs e)
{
            Application.Lock();
            Application["OnlineUsers"] = (int)Application["OnlineUsers"] - 1;
            Application.UnLock();
}

There are <b>@Context.ApplicationInstance.Application["OnlineUsers"].ToString()</b> users online

It kind of works, but I always have the value that is greater than the actual amount of users online.

And even worse: in Opera refreshing the page N times increases the amount of online users by N!

Answer 1

It is important to note here that the ASP.NET is trying to be extremely efficient storing sessions for users. If ASP.NET doesn't have a reason to remember who you are, it won't .

When we request a page first time, a session object will be created and its session identifier will be sent to web-browser so browser can store session identifier in cookie (for identiity of request). If that page is again submitted/postedback then the same session identifier from the cooike will be available to the app-server and it assume that this is an old-request. But in your case (you are refereshing a page), it means web-browser issue a fresh request (and of-course the request type will be GET) without sending cookies. So, it is better to redirect the user to a specific page on first request.

The Session_End event handle will be called when Session get timeout (default value is 20 minutes) even after that client (browser) is closed (or ends the session).

Answer 2

I know its not quite what you are asking but you can query the PerformanceCounter on IIS for this info

(Razor Example)

@using System.Diagnostics
@{
  var perf = new PerformanceCounter("ASP.NET", "State Server Sessions Active"); 
}
<h2>About</h2>
<p>
   @perf.NextValue()
</p>

I didn't check but your access to this might need a windows/service account in your app pool.

You can also Increment and Decrement you own Performance counters and make them available to system admins via the tools they use to monitor Websites etc.

The SqlMembershipProvider has a facility for counting logged on users, which would mean you probably already have the data sitting in your database if you are using it to manage your forms authentication.

You could also consider having your pages emit an ajax pulse every 'period of time' and count that. or have some applet, silverlight, flash etc doing the same.