Do asynchronous operations in ASP.NET MVC use a thread from ThreadPool on .NET 4

Question

After this question, it makes me comfortable when using async operations in ASP.NET MVC. So, I wrote two blog posts on that:

• My Take on

Answer 1

There are two concepts at play here. First of all we can make our code run in parallel to execute faster or schedule code on another thread to avoid making the user wait. The example you had

public ViewResult Index() { 

    Task.Factory.StartNew(() => { 
        //Do an advanced looging here which takes a while
    });

    return View();
}

belongs to the second category. The user will get a faster response but the total workload on the server is higher because it has to do the same work + handle the threading.

Another example of this would be:

public ViewResult Index() { 

    Task.Factory.StartNew(() => { 
        //Make async web request to twitter with WebClient.DownloadString()
    });

    Task.Factory.StartNew(() => { 
        //Make async web request to facebook with WebClient.DownloadString()
    });


    //wait for both to be ready and merge the results

    return View();
}

Because the requests run in parallel the user won't have to wait as long as if they where done in serial. But you should realize that we use up more resources here than if we ran in serial because we run the code at many threads while we have on thread waiting too.

This is perfectly fine in a client scenario. And it is quite common there to wrap synchronous long running code in a new task(run it on another thread) too keep the ui responsive or parallize to make it faster. A thread is still used for the whole duration though. On a server with high load this could backfire because you actually use more resources. This is what people have warned you about

Async controllers in MVC has another goal though. The point here is to avoid having threads sittings around doing nothing(which can hurt scalability). It really only matters if the API's you are calling have async methods. Like WebClient.DowloadStringAsync().

The point is that you can let your thread be returned to handle new requests untill the web request is finished where it will call you callback which gets the same or a new thread and finish the request.

I hope you understand the difference between asynchronous and parallel. Think of parallel code as code where your thread sits around and wait for the result. While asynchronous code is code where you will be notified when the code is done and you can get back working at it, in the meantime the thread can do other work.