Create an asynchronous HTTP handler

ASP. NET How to: create an asynchronous HTTP handler

 

With the asynchronous HTTP processing program, you can start an external process (for example, a method call to a remote server) and continue the processing of the processing program without waiting for the completion of the external process. During asynchronous HTTP processing, ASP. NET puts the threads typically used for external processes back into the thread pool until the handler receives a callback from the external process. This can avoid blocking threads and improve performance, because only a limited number of threads can be executed at a time. If many users are requesting synchronous HTTP processing programs that depend on external processes, the operating system may soon run out of all threads because a large number of threads are blocked and waiting for external processes.

The following code example demonstrates an asynchronous HTTP processing program that processes requests for files with the extension. sampleasync in an ASP. NET application. This example demonstrates the code of the handler and then how to map the. sampleasync extension to the handler in ASP. NET. Finally, this example shows how to map the. sampleasync extension to ASP. NET in IIS so that IIS can forward requests ending with. sampleasync to ASP. NET.

For more information about how ASP. NET runtime interacts with IIS, see ASP. NET application lifecycle overview.

Create helloworldasynchandler HTTP handler class

• Create a directory namedHelloworldasynchandlerAnd add the following code to the class file:

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  Imports Microsoft.VisualBasicImports System.WebImports System.ThreadingPublic Class HelloWorldAsyncHandler    Implements IHttpAsyncHandler    Public ReadOnly Property IsReusable() As Boolean Implements System.Web.IHttpHandler.IsReusable        Get            Return False        End Get    End Property    Public Function BeginProcessRequest( _        ByVal context As System.Web.HttpContext, _        ByVal cb As System.AsyncCallback, _        ByVal extraData As Object) _        As System.IAsyncResult _        Implements System.Web.IHttpAsyncHandler.BeginProcessRequest        context.Response.Write("<p>Begin IsThreadPoolThread is " _            & Thread.CurrentThread.IsThreadPoolThread & "</p>" & vbCrLf)        Dim asynch As New AsynchOperation(cb, context, extraData)        asynch.StartAsyncWork()        Return asynch    End Function    Public Sub EndProcessRequest(ByVal result As _         System.IAsyncResult) _         Implements System.Web.IHttpAsyncHandler.EndProcessRequest    End Sub    Public Sub ProcessRequest(ByVal context _            As System.Web.HttpContext) _            Implements System.Web.IHttpHandler.ProcessRequest        Throw New InvalidOperationException()    End SubEnd ClassClass AsynchOperation    Implements IAsyncResult    Private _completed As Boolean    Private _state As [Object]    Private _callback As AsyncCallback    Private _context As HttpContext    ReadOnly Property IsCompleted() As Boolean _            Implements IAsyncResult.IsCompleted        Get            Return _completed        End Get    End Property    ReadOnly Property AsyncWaitHandle() As WaitHandle _            Implements IAsyncResult.AsyncWaitHandle        Get            Return Nothing        End Get    End Property    ReadOnly Property AsyncState() As [Object] _            Implements IAsyncResult.AsyncState        Get            Return _state        End Get    End Property    ReadOnly Property CompletedSynchronously() As Boolean _            Implements IAsyncResult.CompletedSynchronously        Get            Return False        End Get    End Property    Public Sub New(ByVal callback As AsyncCallback, _            ByVal context As HttpContext, _            ByVal state As [Object])        _callback = callback        _context = context        _state = state        _completed = False    End Sub    Public Sub StartAsyncWork()        ThreadPool.QueueUserWorkItem(New WaitCallback(AddressOf StartAsyncTask), Nothing)    End Sub    Private Sub StartAsyncTask(ByVal workItemState As [Object])        _context.Response.Write("<p>Completion IsThreadPoolThread is " & Thread.CurrentThread.IsThreadPoolThread & "</p>" & vbCrLf)        _context.Response.Write("Hello World from Async Handler!")        _completed = True        _callback(Me)    End Sub 'StartAsyncTaskEnd Class 'AsynchOperation

   

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  using System;using System.Web;using System.Threading;class HelloWorldAsyncHandler : IHttpAsyncHandler{    public bool IsReusable { get { return false; } }    public HelloWorldAsyncHandler()    {    }    public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, Object extraData)    {        context.Response.Write("<p>Begin IsThreadPoolThread is " + Thread.CurrentThread.IsThreadPoolThread + "</p>/r/n");        AsynchOperation asynch = new AsynchOperation(cb, context, extraData);        asynch.StartAsyncWork();        return asynch;    }    public void EndProcessRequest(IAsyncResult result)    {    }    public void ProcessRequest(HttpContext context)    {        throw new InvalidOperationException();    }}class AsynchOperation : IAsyncResult{    private bool _completed;    private Object _state;    private AsyncCallback _callback;    private HttpContext _context;    bool IAsyncResult.IsCompleted { get { return _completed; } }    WaitHandle IAsyncResult.AsyncWaitHandle { get { return null; } }    Object IAsyncResult.AsyncState { get { return _state; } }    bool IAsyncResult.CompletedSynchronously { get { return false; } }    public AsynchOperation(AsyncCallback callback, HttpContext context, Object state)    {        _callback = callback;        _context = context;        _state = state;        _completed = false;    }    public void StartAsyncWork()    {        ThreadPool.QueueUserWorkItem(new WaitCallback(StartAsyncTask), null);    }    private void StartAsyncTask(Object workItemState)    {        _context.Response.Write("<p>Completion IsThreadPoolThread is " + Thread.CurrentThread.IsThreadPoolThread + "</p>/r/n");        _context.Response.Write("Hello World from Async Handler!");        _completed = true;        _callback(this);    }}

  This code implements the beginprocessrequest method. This method writes a string to the response attribute of the current httpcontext object and createsAsyncoperationClass, and then callStartasyncworkMethod. Then,StartasyncworkMethod to add a threadpool objectStartasynctaskDelegate. When a thread is availableStartasynctaskMethod.ResponseAttribute, and then the task is completed by calling the asynccallback delegate.

Register a custom HTTP handler

After creating a custom HTTP handler class, you must register the class in the web. config file so that ASP. NET can process requests for files with the. sampleasync extension.

Register a custom HTTP handler in the web. config file

1. If your website does not have a web. config file, create a web. config file.

2. Add the following code to the Web. config file:

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   <configuration>  <system.web>    
   This code willHelloworldasynchandlerThe handler registers as the handler for the request ending with. sampleasync.

Configure IIS for the HTTP handler Extension

IIS only transmits requests for certain file types to ASP. NET for processing. By default. aspx ,. ascx ,. files with extensions such as asmx have been mapped to ASP.. net to process the file extensions you define, you must register these extensions in IIS. For more information, see ASP. NET application lifecycle overview.

Ing extension in IIS

1. Open Internet Service Manager ".

2. Right-click your application and select "properties ".

3. On the "directory" tab, click "Configure ".

4. Select the ing tab.

5. Add a new association to map. sampleasync to the aspnet_isapi.dll version to be used.

6. If you want the handler to run regardless of whether the user requested file name exists, clear the check box "check whether the file exists.

Test a custom HTTP handler

After creating and registering a custom HTTP handler, you can test the handler by requesting resources with the. sampleasync extension in the application.

Test a custom HTTP handler

• Browse to your application and enter a URL Ending with. sampleasync in your browser.

See