Skip to main content

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.

Labels:

Popular posts from this blog

Podman– Command execution failed with exit code 125

After updating WSL on one of the developer machines, Podman failed to work. When we took a look through Podman Desktop, we noticed that Podman had stopped running and returned the following error message: Error: Command execution failed with exit code 125 Here are the steps we tried to fix the issue: We started by running podman info to get some extra details on what could be wrong: >podman info OS: windows/amd64 provider: wsl version: 5.3.1 Cannot connect to Podman. Please verify your connection to the Linux system using `podman system connection list`, or try `podman machine init` and `podman machine start` to manage a new Linux VM Error: unable to connect to Podman socket: failed to connect: dial tcp 127.0.0.1:2655: connectex: No connection could be made because the target machine actively refused it. That makes sense as the podman VM was not running. Let’s check the VM: >podman machine list NAME         ...
Read more

Cache stampede: when our cache turned against us

While investigating some performance issues, we ran into an ASP.NET Core API that cached a fairly expensive aggregation query for 60 seconds. Under normal load, that was fine: one request rebuilds the cache, everyone else reads from it. Under peak load, dozens of requests would arrive in that same expiry window, all see a cache miss, and all fire the same expensive query in parallel. The database didn't like that. That was the moment when our caching layer stopped helping and started hurting. A burst of requests comes in at the same time, all miss the cache, and all go hammer the database or the downstream API at once. That's a cache stampede . The cache was supposed to protect our backend, and for a few hundred milliseconds it did the opposite. Why this happens IMemoryCache.GetOrCreate (and its async sibling) looks like it protects you, but it doesn't add any locking on its own. Look at the naive version: public async Task<Report> GetReportAsync(string key) ...
Read more

A complex system designed from scratch never works

A few years ago, I worked as an architect on a big mainframe rewrite. I still count it as one of my failures. Not because the technology was wrong, but because I couldn't convince the management team to simplify the approach. Years later, the organization is still struggling to get the new system up and running. I left the project at the time, because I couldn't put my name behind an approach that would take very long and cost a lot of money without a working system to show for it along the way. Gall’s Law That memory keeps coming back to me, because it's a textbook case of Gall's Law playing out in real life. Gall's Law , from John Gall's Systemantics , states it plainly: A complex system that works is invariably found to have evolved from a simple system that worked. A complex system designed from scratch never works, and it cannot be patched to make it work. You have to start over with a simple system that works. What does that mean in practice,...
Read more