Session State Locks cause pending requests

In some scenario, while there is a long pending request exist, it may cause other request pending because session data is locked by the first request. Here is a example of session lock.

First I have a controller with default session policy

using System;
using System.Collections.Generic;
using System.Drawing.Imaging;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using WebApplication2.Models;

namespace WebApplication2.Controllers
{
    public class HomeController : Controller
    {
        public ActionResult Index()
        {
            return View();
        }

        public ActionResult LongTimeout()
        {
            System.Threading.Thread.Sleep(30000);
            return Json(new { code = 200 }, JsonRequestBehavior.AllowGet);
        }

        public ActionResult ShortTimeout()
        {
            return Json(new { code = 200 }, JsonRequestBehavior.AllowGet);
        }

        public ActionResult Captcha()
        {
            var captcha = MathCaptcha.Generate();
            Session["captcha"] = captcha.Answer;

            return File(captcha.GetImageData(ImageFormat.Png), "image/png");
        }
    }
}

And then create another controller that make session state readonly

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;

namespace WebApplication2.Controllers
{
    [SessionState(System.Web.SessionState.SessionStateBehavior.ReadOnly)]
    public class SessionLessController : Controller
    {
        public ActionResult LongTimeout()
        {
            System.Threading.Thread.Sleep(30000);
            return Json(new {code=200 }, JsonRequestBehavior.AllowGet);
        }

        public ActionResult ShortTimeout()
        {
            return Json(new { code = 200 }, JsonRequestBehavior.AllowGet);
        }
    }
}

Request is pending because the session of first request is locked

After first request completed, other requests will go on

Request won't be block when session state is set as readonly