ASP.NET MVC 5 concurrent requests are queued even with disabled Session

Question

Before thinking about downvoting or telling me "google it", please read the problem more carefully. This is old/classic problem but old/classic solution is no longer working. Here is very simple scenario to reproduce in Visual Studio 2013/2015:

1) Create ASP.NET Web application using MVC template:

2) Open Controllers\HomeController.cs and add attribute to controller and "Sleep" action:

[SessionState( System.Web.SessionState.SessionStateBehavior.Disabled)]
public class HomeController : Controller
{
    public ActionResult Sleep(int? time)
    {
        System.Threading.Thread.Sleep(time ?? 3000);
        return Content("OK");
    }

    public ActionResult Index()
    {
...

3) Open file: Views\Home\Index.cshtml and add/replace content html with the following :

<script>
    function ReqClick() {
        var startTime = Date();

        $.ajax("/Home/Sleep")
        .success(function () {
            var log = $("#log");
            var endTime = Date();
            log.text(log.text() + "Start: " + startTime.toString() + "  === " + endTime.toString());
        });
    };
</script>

<button type="button" onclick="ReqClick();">
    Request
</button>
<div>
    <textarea id="log" style="width:640px; height:480px"></textarea>
</div>

4) Run it (does not matter if you're using IIS or IIS Express or Vs Dev Server) - Open Home/Index. Click F12 to open dev tool, open network tab. On the Home page click "Request" button twice fast. You can see that second request takes almost 6 seconds:

In Debug mode in controller you can see that Session is null:

Cookies are totally empty (ASP.NET Session Id is absent)

Please let me know what I'm missing?

Adding the setting below to web.config does not help either:

<sessionState mode="Off"/>
<pages enableSessionState="ReadOnly"/>

Answer 1

Concurrent parallel requests worked for me when I decorated my controller with this attribute

[SessionState(System.Web.SessionState.SessionStateBehavior.ReadOnly)]

It works better than the above disabling session state and was added back in MVC 3. More info here

Answer 2

I spent a lot of time on this issue and found much of the information that you saw above also did not correct the issue. My parallel requests were always queuing, I saw this in Internet Explorer as well as Firefox.

I found this article related to am using an MVC application this article

This shows that you can use Disabled [SessionState(SessionStateBehavior.Disabled)] instead of setting it to ReadOnly.

I moved my code that was performing the AJAX GET call to its own controller since I had other code in the current controller that used Session via TempData. This corrected my issue and my $.get() call to the controller with session disabled would process in parallel with the $.post() to the other controller that used session.