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Using the AsyncController in ASP.NET MVC 2

On the Web server, the .NET Framework maintains a pool of threads that are used to service ASP.NET requests. When a request arrives, a thread from the pool is dispatched to process that request. If the request is processed synchronously, the thread that processes the request is blocked while the request is being processed, and that thread cannot service another request.

If you have multiple long-running requests, all available threads might be blocked and the Web server rejects any additional request with an HTTP 503 status (Server Too Busy).

To solve this issue in ASP.NET MVC, actions can be processed asynchronously by deriving your controllers from the AsyncController class.

For example, let’s convert this code sample into a more efficient asynchronous alternative:

   1:  public class HomeController: Controller 
   2:  {
   3:      public ActionResult LongRunningAction() 
   4:      {
   5:          DoLengthyOperation();
   6:          return View();
   7:      }
   8:   
   9:      private void DoLengthyOperation()
  10:      {
  11:          Thread.Sleep(5000);
  12:      }
  13:   
  14:  }

To convert this synchronous action method to an asynchronous action method involves the following steps:


  1. Instead of deriving the controller from Controller, derive it from AsyncController. Controllers that derive from AsyncController enable ASP.NET to process asynchronous requests, and they can still service synchronous action methods.


  2. Create two methods for the action. The method that initiates the asynchronous process must have a name that consists of the action and the suffix "Async". The method that is invoked when the asynchronous process finishes (the callback method) must have a name that consists of the action and the suffix "Completed". In the previous example, the LongRunningAction method has been turned into two methods: LongRunningActionAsync and LongRunningActionCompleted.

    The LongRunningActionAsync method returns void. The LongRunningActionCompleted method returns an ActionResult instance. Although the action consists of two methods, it is accessed using the same URL as for a synchronous action method (for example, Home/LongRunningAction). Methods such as RedirectToAction and RenderAction will also refer to the action method as LongRunningAction and not LongRunningActionAsync.

    The parameters that are passed to LongRunningActionAsync use the normal parameter binding mechanisms. The parameters that are passed to LongRunningActionCompleted use the Parameters dictionary.


  3. Replace the synchronous call in the original ActionResult method with an asynchronous call in the asynchronous action method. 

   1:  public class HomeController : AsyncController 
   2:  {
   3:      public void LongRunningActionAsync() 
   4:      {
   5:          AsyncManager.OutstandingOperations.Increment();
   6:          Task.Factory.StartNew(() => DoLengthyOperation());
   7:      }
   8:   
   9:      private void DoLengthyOperation()
  10:      {
  11:          Thread.Sleep(5000);
  12:          AsyncManager.Parameters["message"] = "hello world";
  13:          AsyncManager.OutstandingOperations.Decrement();
  14:      }
  15:   
  16:   
  17:      public ActionResult LongRunningActionCompleted(string message) 
  18:      {
  19:          return View();
  20:      }
  21:  }

More information here.

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