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Asynchronous Programming with ASP.Net MVC Futures

Using the AsyncController

Introduction

The AsyncController is an experimental class offered inside the latest MVC Futures dll to allow developers to write asynchronous action methods.  The usage scenario for this is for action methods that have to make long-running requests, such as going out over the network or to a database, and don’t want to block the web server from performing useful work while the request is ongoing.

In general, the pattern is that the web server schedules Thread A to handle some incoming request, and Thread A is responsible for everything up to launching the action method, then Thread A goes back to the available pool to service another request.  When the asynchronous operation has completed, the web server retrieves a Thread B (which might be the same as Thread A) from the thread pool to process the remainder of the request, including rendering the response.  The diagram below illustrates this point.

Configuration

The asynchronous types are declared in the Microsoft.Web.Mvc namespace in the assembly Microsoft.Web.Mvc.dll.  The first change a developer needs to make is to declare a route as asynchronous.  There are MapAsyncRoute() extension methods to assist with this; these are analogs of the normal MapRoute() extension methods already provided by the MVC framework.  In Global.asax:

routes.MapAsyncRoute(

    "Default",

    "{controller}/{action}/{id}",

    new { controller = "Home", action = "Index", id = "" }

);

A route declared with MapAsyncRoute() can correctly handle both synchronous and asynchronous controllers, so there is no need to create complex routing logic such that sync controllers are serviced by the normal MapRoute() handler and async controllers are serviced by the MapAsyncRoute() handler.  However, a sync route [MapRoute()] cannot in general correctly execute async controllers and methods.

Secondly, if you are using IIS6 or IIS7 classic mode, you need to replace the standard *.mvc handler with its asynchronous counterpart.  Replace these lines in Global.asax (they don’t necessarily appear adjacent to one another):

<add verb="*" path="*.mvc" validate="false" type="System.Web.Mvc.MvcHttpHandler, System.Web.Mvc, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>

<add name="MvcHttpHandler" preCondition="integratedMode" verb="*" path="*.mvc" type="System.Web.Mvc.MvcHttpHandler, System.Web.Mvc, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>

With these:

<add verb="*" path="*.mvc" validate="false" type="Microsoft.Web.Mvc.MvcHttpAsyncHandler, Microsoft.Web.Mvc"/>

<add name="MvcHttpHandler" preCondition="integratedMode" verb="*" path="*.mvc" type="Microsoft.Web.Mvc.MvcHttpAsyncHandler, Microsoft.Web.Mvc"/>

Writing asynchronous action methods

In general, any asynchronous controllers that you author should subclass the AsyncController type.  Remember to import the Microsoft.Web.Mvc namespace.

public class MyAsyncController : AsyncController {

    // ...

}

The default constructor of the MyAsyncController sets the ActionInvoker property to an instance of the AsyncControllerActionInvoker.  This specialized invoker can understand several asynchronous patterns.  You can mix and match these patterns inside of your controllers.  If an ambiguity is found, we do our best to throw a detailed exception.

The AsyncController understands three async patterns.  These patterns can be mixed and matched within a single AsyncController.  Additionally, an AsyncController can contain normal (synchronous) action methods.  This makes it easy to change the base class of your controller from Controller to AsyncController in order to add async methods while allowing your original sync methods to run correctly.

The IAsyncResult pattern

The IAsyncResult pattern is a well-known pattern in the .NET Framework and is documented heavily.  A method pair signature which implements the IAsyncResult pattern is shown below.

public IAsyncResult BeginFoo(int id, AsyncCallback callback, object state);

public ActionResult EndFoo(IAsyncResult asyncResult);

This is the asynchronous analog of a synchronous method with the signature public ActionResult Foo(int id).  Note that the BeginFoo() method takes the same parameters as the Foo() method plus two extra – an AsyncCallback and a state object – and returns an IAsyncResult.  The EndFoo() method takes a single parameter – an IAsyncResult – and has the same return type as the Foo() method.

Standard model binding takes place for the normal parameters of the BeginFoo() method.  The invoker will pass a callback and state object for the BeginFoo() method to consume when its asynchronous task has finished.  When the callback is called, we automatically invoke the EndFoo() method and capture the result, then execute the result just as we would have in a synchronous request.

Only filter attributes placed on the BeginFoo() method are honored.  If a filter attribute is placed on EndFoo(), it will be ignored.  If an [ActionName] attribute is placed on the BeginFoo() method in order to alias it, we will look for an EndFoo() method based on the method name of BeginFoo(), not the aliased action name.  For example:

[ActionName("Bar")]

public IAsyncResult BeginFoo(int id, AsyncCallback callback, object state);

public ActionResult EndFoo(IAsyncResult asyncResult);

This will cause the BeginFoo() method to match requests for Bar rather than Foo.  Note that the completion method is still called EndFoo() instead of EndBar() since it matches the name of the entry method, not the entry alias.

The event pattern

In this pattern, the action method is divided into a setup method and a completion method.  The signatures are below:

public void Foo(int id);

public ActionResult FooCompleted(...);

When a request comes for Foo, we execute the Foo() method.  When the asynchronous operations are completed, we invoke the FooCompleted() method and execute the returned ActionResult.

The invoker will model bind parameters to the Foo() method in the standard way.  Parameters to FooCompleted() are not provided using model binders.  Rather, they come from the AsyncController.AsyncManager.Parameters dictionary, which can be populated as part of the asynchronous setup.  Keys in the dictionary correspond to parameter names of the FooCompleted() method, and any parameters which do not have corresponding keys in the dictionary are given a value of default(T).

The invoker must keep track of the number of outstanding asynchronous operations so that it does not invoke the FooCompleted() method prematurely.  To do this, a counter has been provided, accessible from AsyncController.AsyncManager.OutstandingOperations.  This counter can be incremented or decremented to signal that an operation has kicked off or concluded, and when the counter hits zero the invoker will invoke the completion routine.  For example:

public void Foo(int id) {

    AsyncManager.Parameters["p"] = new Person();

    AsyncManager.OutstandingOperations.Increment();

    ThreadPool.QueueUserWorkItem(o => {

        Thread.Sleep(2000); // simulate some work

        AsyncManager.OutstandingOperations.Decrement();

    }, null);

}

public ActionResult FooCompleted(Person p) {

    // consume 'p'

}

There is also an AsyncController.AsyncManager.RegisterTask() method that is helpful for wrapping calls to IAsyncResult pattern methods from within an event pattern method.  The RegisterTask() method also handles incrementing and decrementing the counter correctly.  For example:

public void Foo(int id) {

    AsyncManager.RegisterTask(

        callback => BeginGetPersonById(id, callback, null),

        ar => {

            AsyncManager.Parameters["p"] = EndGetPersonById(ar);

        });

    AsyncManager.RegisterTask(

        callback => BeginGetTotalUsersOnline(callback, null),

        ar => {

            AsyncManager.Parameters["numOnline"] = EndGetTotalUsersOnline(ar);

        });

}

public ActionResult FooCompleted(Person p, int numOnline) {

    // ...

}

This will kick off two asynchronous tasks and wait for both to finish before executing the FooCompleted() method with the values that were returned.

Only filter attributes placed on the Foo() method are honored.  If a filter attribute is placed on FooCompleted(), it will be ignored.  If an [ActionName] attribute is placed on the Foo() method in order to alias it, we will look for an FooCompleted() method based on the method name of Foo(), not the aliased action name.  For example:

[ActionName("Bar")]

public void Foo(int id);

public ActionResult FooCompleted(...);

This will cause the Foo() method to match requests for Bar rather than Foo.  Note that the completion method is still called FooCompleted() instead of BarCompleted() since it matches the name of the entry method, not the entry alias.

The Foo() method is allowed to return anything.  The invoker ignores the return value of this method; it only cares about the return value of the FooCompleted() method.  This is to allow writing Foo() methods that are easier to unit test.

The delegate pattern

This pattern is very similar to the event pattern, except that the method Foo() returns a parameterless delegate type and there is no FooCompleted() method.  For example:

public Func<ActionResult> Foo(int id) {

    Person p = null;

    int numOnline = 0;

    AsyncManager.RegisterTask(

        callback => BeginGetPersonById(id, callback, null),

        ar => {

            p = EndGetPersonById(ar);

        });

    AsyncManager.RegisterTask(

        callback => BeginGetTotalUsersOnline(callback, null),

        ar => {

            numOnline = EndGetTotalUsersOnline(ar);

        });

    return () => {

        ViewData["p"] = p;

        ViewData["numOnline"] = numOnline;

        return View();

    };

}

Or, more succinctly:

public Func<ActionResult> Foo(int id) {

    AsyncManager.RegisterTask(

        callback => BeginGetPersonById(id, callback, null),

        ar => {

            ViewData["p"] = EndGetPersonById(ar);

        });

    AsyncManager.RegisterTask(

        callback => BeginGetTotalUsersOnline(callback, null),

        ar => {

            ViewData["numOnline"] = EndGetTotalUsersOnline(ar);

        });

    return View;

}

Since this pattern supports only parameterless delegates instead of parameterful delegates, the earlier discussion about AsyncController.AsyncManager.Parameters is not applicable.

Timeouts

There is a Timeout property accessible from AsyncController.AsyncManager.Timeout that specifies the number of milliseconds to wait for a response from the action method before canceling the request.  The default value is 30000 (equal to 30 seconds).  If the action method has not returned by the specified time, we throw a TimeoutException.  Action filters and exception filters may handle this particular exception type if they wish.  Setting the Timeout property to System.Threading.Timeout.Infinite signifies that we will never throw this exception.

The timeout duration can be specified on a per-controller or per-action basis by attributing a class or method with [AsyncTimeout] or [NoAsyncTimeout].

Known issues

-          Asynchronous actions generally cannot be called by synchronous invokers or handlers.  If you receive an exception message about an action being unable to be executed synchronously, ensure that you’re using the MapAsyncRoute() extension method in your Global.asax and that your controller subclasses AsyncController.

-          The asynchronous invoker will not match any method beginning with Begin or End or ending with Completed.  This is to prevent web calls to the BeginFoo(), EndFoo(), and FooCompleted() methods directly.  If you need to make an action with this name accessible to web users, use the [ActionName] attribute to alias the method:

[ActionName("Begin")]

public ActionResult DoSomething();

The above is an example of a normal synchronous method that has been renamed to Begin to work around the invoker’s blocking of this name.

-          If the route that normally handles requests for the application root (/) is an asynchronous route, the Default.aspx file should be removed from the web application.  The Default.aspx file included in the template only works with synchronous requests.


Categories: ASP.Net | Asynchronous | C# | MVC
Posted by Williarob on Thursday, June 18, 2009 6:03 AM
Permalink | Comments (0) | Post RSSRSS comment feed

Multi-Threading in ASP.NET

ASP.Net Threading

Inside the ASP.Net Worker Process there are two thread pools. The worker thread pool handles all incoming requests and the I/O Thread pool handles the I/O (accessing the file system, web services and databases, etc.). Each App Domain has its own thread pool and the number of operations that can be queued to the thread pool is limited only by available memory; however, the thread pool limits the number of threads that can be active in the process simultaneously.

  Asp.Net Threading, Threadpools
  Source: Microsoft Tech Ed 2007 DVD: Web 405  "Building Highly Scalable ASP.NET Web Sites by Exploiting Asynchronous Programming Models" by Jeff Prosise.

So how many threads are there in these thread pools? I had always assumed that the number of threads varies from machine to machine – that ASP.NET and IIS were carefully and cleverly balancing the number of available threads against available hardware, but that is simply not the case. The fact is that ASP.Net installs with a fixed, default number of threads to play with: the 1.x Framework defaults to just 20 worker threads (per CPU) and 20 I/O threads (per CPU). The 2.0 Framework defaults to 100 threads in each pool, per CPU. Now this can be increased by adding some new settings to the machine.config file. The default worker thread limit was raised to 250 per CPU and 1000 I/O threads per CPU with the .NET 2.0 SP1 and later Frameworks. 32 bit windows can handle about 1400 concurrent threads, 64 bit windows can handle more, though I don’t have the figures.

In a normal (synchronous) Page Request a single worker thread handles the entire request from the moment it is received until the completed page is returned to the browser. When the I/O operation begins, a thread is pulled from the I/O thread pool, but the worker thread is idle until that I/O thread returns. So, if your page load event fires off one or more I/O operations, then that main worker thread could be idle for 1 or more seconds and in that time it could have serviced hundreds of additional incoming page requests.

  Asp.net Threadpool Saturation
  Source: Microsoft Tech Ed 2007 DVD: Web 405  "Building Highly Scalable ASP.NET Web Sites by Exploiting Asynchronous Programming Models" by Jeff Prosise.

So long as the number of concurrent requests does not exceed the number of threads available in the pool, all is well. But when you are building enterprise level applications the thread pool can become depleted under heavy load, and remember by default heavy load is more than just 200 simultaneous requests assuming a dual CPU Server.

When this happens, new requests are entered into the request queue (and the users making the requests watch that little hour glass spin and consider trying another site). ASP.NET will allow the request queue to grow only so big before it starts to reject requests at which point it starts returning Error 503, Service Unavailable.

If you are not aware of this “Glass Ceiling of Scalability”, this is a perplexing error – one that never happened in testing and may not be reproducible in your test environment, as it only happens under extreme load.

Asynchronous Programming models in ASP.NET

To solve this problem ASP.Net provides four asynchronous Programming models. Asyncronous Pages, Asyncronous HttpHandlers, Asyncronous HttpModules and Asyncronous Web Services. The only one that is well documented and reasonably well known is the asynchronous Web Services model. Since there is quite a lot of documentation on that, and since in future web services should be implemented using the Windows Communication Foundation, we shall concentrate only on the other three.

Let’s begin with the first asynchronous programming model, Asynchronous Pages.

Asynchronous Pages

  Lifecycle of Synchronous/Asynchronous Pages in ASP.NET
  Source: Microsoft Tech Ed 2007 DVD: Web 405  "Building Highly Scalable ASP.NET Web Sites by Exploiting Asynchronous Programming Models" by Jeff Prosise.

To make a page Asynchronous, we insert what we refer to as an “Async Point” into that page’s lifecycle, which you can see in green on the right. We need to write and register with ASP.NET a pair of Begin and End Events. At the appropriate point in the page’s lifecycle, ASP.NET will call our begin method. In the begin method we will launch an asynchronous I/O operation, for example an asynchronous database query, and we will immediately return from the begin method. As soon as we return, ASP.Net will drop the thread that was assigned to that request back into the thread pool where it may service hundreds or even thousands of additional page requests while we wait for our I/O operation to complete.

As you’ll see when we get to the sample code, we return from our begin method an IAsyncResult Interface, through which we can signal ASP.NET when the async operation that we launched has completed. It is when we do that, that ASP.NET reaches back into the thread pool, pulls out a second worker thread and calls our end method, and then allows the processing of that request to resume as normal.

So, from ASP.NET’s standpoint it is just a normal request, but it is processed by 2 different threads; and that will bring up a few issues that we’ll need to discuss in a few moments.

Now, none of this was impossible with the 1.1 framework, but it was a lot of extra work, and you lost some of the features of ASP.NET in the process. The beauty of the 2.0 and later frameworks is that this functionality is built right into the Http pipeline, and so for the most part everything works in the asynchronous page just as it did in the synchronous one.

In order to create an Asynchronous page you need to include the Async=”True” attribute in the page directive of your .aspx file. That directive tells the ASP.NET engine to implement an additional Interface on the derived page class which lets ASP.NET know at runtime that this is an asynchronous page.

What happens if you forget to set that attribute? Well the good news is that the code will still run just fine, but it will run synchronously, meaning that you did all that extra coding for nothing. I should also point out that to make an Asynchronous data call, you also need to add “async=true;” or “Asynchronous Processing=true;” to your connection string – If you forget that and make your data call asynchronously, you will get a SQL Exception.

The second thing we need to do in order to create an asynchronous page is to register Begin and End Events. There are 2 ways to register these events. The first way is to use a new method introduced in ASP.NET 2.0 called AddOnPreRenderCompleteAsync:

using System;

using System.Net;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

 

public partial class temp : System.Web.UI.Page

{

    private static readonly Uri c_UrlImage1 = new Uri(@"http://williablog.net/williablog/image.axd?picture=2010%2f1%2fSlide6.JPG");

    private HttpWebRequest request;

 

    void Page_Load(object sender, EventArgs e)

    {

        request = (HttpWebRequest)WebRequest.Create(c_UrlImage1);

 

        AddOnPreRenderCompleteAsync(

            BeginAsyncOperation,

            EndAsyncOperation

        );

    }

 

    IAsyncResult BeginAsyncOperation(object sender, EventArgs e,

        AsyncCallback cb, object state)

    {

        // Begin async operation and return IAsyncResult

        return request.BeginGetResponse(cb, state);

    }

 

    void EndAsyncOperation(IAsyncResult ar)

    {

        // Get results of async operation

        HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(ar);

        Label1.Text = String.Format("Image at {0} is {1:N0} bytes", response.ResponseUri, response.ContentLength);

    }

}

 


The second way is to use RegisterAsyncTask:

using System;

using System.Net;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

 

public partial class temp : System.Web.UI.Page

{

    private static readonly Uri c_UrlImage1 = new Uri(@"http://williablog.net/williablog/image.axd?picture=2010%2f1%2fSlide6.JPG");

    private HttpWebRequest request;

 

    void Page_Load(object sender, EventArgs e)

    {

        request = (HttpWebRequest)WebRequest.Create(c_UrlImage1);

 

        PageAsyncTask task = new PageAsyncTask(

                BeginAsyncOperation,

                EndAsyncOperation,

                TimeoutAsyncOperation,

                null

            );

 

        RegisterAsyncTask(task);

    }

 

    IAsyncResult BeginAsyncOperation(object sender, EventArgs e,

        AsyncCallback cb, object state)

    {

        // Begin async operation and return IAsyncResult

        return request.BeginGetResponse(cb, state);

    }

 

    void EndAsyncOperation(IAsyncResult ar)

    {

        // Get results of async operation

        HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(ar);

        Label1.Text = String.Format("Image at {0} is {1:N0} bytes", response.ResponseUri, response.ContentLength);

    }

 

    void TimeoutAsyncOperation(IAsyncResult ar)

    {

        // Called if async operation times out (@ Page AsyncTimeout)

        Label1.Text = "Data temporarily unavailable";

    }   

}

 

These methods can be called anywhere in the page’s lifecycle before the PreRender event, and are typically called from the Page_Load event or from the click event of a button during a postback. By the way, you can register these methods from within a UserControl, as long as that control is running on a page that has set the async = true attribute. Again, if it runs on a page without that attribute, the code will still run just fine, but it will run synchronously.

As you can see from just these simple examples, building asynchronous pages is more difficult than building synchronous ones. I’m not going to lie to you. And real world use of these techniques is even more complicated – there is no Business Logic or data layer in the examples above. I don’t want you to leave here believing that you need to make every page asynchronous. You don’t. What I recommend, is doing surgical strikes. Identify that handful of pages in your application that perform the lengthiest I/O and consider converting those into asynchronous pages. The cool thing about this, is that it can improve not only scalability, but also performance, because when you are not holding onto the threads, new requests get into the pipeline faster, they spend less time waiting in that application request queue out there. So, users are happier because pages that they would have had to wait on before – even the ones you have not converted to asynchronous pages, but which might have been delayed while threads were idle, will now load faster. What’s more, as you’ll see in a moment, using RegisterAsyncTask will allow you to perform I/O operations in parallel, which may also improve performance. Having said that, making pages asynchronous is not really about improving performance, it is about improving scalability – making sure that we use the threads in the thread pool as efficiently as we possibly can.

Now I’m sure you are wondering why there are two ways, what the differences are between them, and when you should choose one over the other. Well, there are 3 important differences between AddOnPreRenderCompleteAsync and RegisterAsyncTask.

  1. As we have seen, RegisterAsyncTask allows you to specify a timeout method. It is important to note that the timeout value you specify in the Page Directive of your .aspx page <%@ Page Async="true" AsyncTimeout="5" ... %> is the timeout for ALL asynchronous tasks the page is performing, not 5 secs per async task - all async tasks must be competed within 5 seconds, so be sure to allow enough time here.
  2. If you use AddOnPreRenderCompleteAsync, you may find that some things that worked before, no longer work. For example, if you are using User.Identity.Name in your code to get the authenticated username inorder to personalize a page. If this method is called by the first thread it will work fine. But if you call it on the second thread – in your End method or any of the events that fire after the end method User.Identity.Name will be null. This is because as a Request travels through the ASP.NET Http Pipeline, it is accompanied by an object of type HttpContext that basically encapsulates all of the information that ASP.NET knows about that request. When you use AddOnPreRenderCompleteAsync ASP.NET does not take the extra time to map everything in that context object from thread one to thread two. That’s why User.Identity.Name does not work in thread two. In fact, you will often find that HttpContext.Current is null in thread two. However, if you use RegisterAsyncTask, ASP.Net DOES map everything in that context from thread one to thread two. It does take a few microseconds longer to do this, but it will make your life considerably easier.
  3. The third difference is probably the most important of all. AddOnPreRenderCompleteAsync is a quick and easy way of making a page asynchronous and works well if you have a simple page that needs to perform only 1 asynchronous I/O operation. In real life, a page often needs to perform multiple database queries, or grab data from a webservice and pass it to a database, or something like that. The cool thing about RegisterAsyncTask is that it allows you to quickly and easily queue up multiple Async I/O operations. The last argument is a Boolean value that allows you to specify whether each task can run in parallel. Some times you need to wait for one data call to complete in order to send that data somewhere else, but other times you may need to get data from multiple, unrelated sources and this allows you to fetch them all at the same time, instead of one after the other.

  Controlling Order of Operations
  Source: Microsoft Tech Ed 2007 DVD: Web 405  "Building Highly Scalable ASP.NET Web Sites by Exploiting Asynchronous Programming Models" by Jeff Prosise.

N-Tier Applications

OK. So I expect some of you are thinking “But what if I have a data access layer in my application? My pages can’t go directly to the database, they have to go through that data access layer, or they have to go through my BLL, which calls the Data Access Layer.”

Well, ideally, you should simply add the asynchronous methods to your DAL. If you wrote the DAL yourself or have access to its source code, you should add the Begin and End methods to it Adding the asynchronous methods to your DAL is the best, most scalable solution and doesn’t change the example code much at all: Instead of calling begin and end methods defined inside the page class, you simply call MyDAL.Begin… or MyBll.Begin… when you call RegisterAsyncTask or AddOnPreRenderAsync.

Unfortunately, neither Llblgen nor the Enterprise library (nor LINQ for that matter) supports asynchronous data calls natively. However, I believe that you can modify the generated code in llblgen to enable asynchronous data calls. You could also crack open the source code of the Enterprise library and add the asynchronous methods yourself, but before you try check to see if it has already been done.

Asynchronous HTTP Handlers

The 2nd Asynchronous Programming model in ASP.NET is for HttpHandlers and has been around since .Net 1.x, but was not documented any better in version 2 than it was in version 1. Http Handlers are one of the two fundamental building blocks of ASP.NET, an http handler is an object that is built to handle http requests and convert them into http responses. For the most part, each handler corresponds to a file type. For example, there is a built in handler in ASP.NET that handles .aspx files. It is that handler that knows how to instantiate a control tree and send that tree to a rendering engine. The ASMX Handler knows how to decode SOAP and allows us to build web services.

Basically an HTTP Handler is just a class that implements the IHttpHandler interface, which consists of an IsResuable Boolean function and a ProcessRequest method which is the heart of an httphandler as its job is to turn a request into a response. The ProcessRequest method is passed an HttpContext Object containing all the data asp.net has collected about the request, as well as exposing the Session, Server, Request and Response objects that you are used to working with in page requests.

 

using System.Web;

 

public class HelloHandler : IHttpHandler

{

    public void ProcessRequest(HttpContext context)

    {

        string name = context.Request["Name"];

        context.Response.Write("Hello, " + name);

    }

 

    public bool IsReusable

    {

        get { return true; }

    }

}

 

There are 2 ways to build them. One way is to add the class to your project and register is in the web.config. If you want to register it for any file extension not currently handled by asp.net, you would need to add that extension to IIS. The easier way, is to deploy them as .ASHX files. The ASHX extension has already been registered in IIS, it is auto compiled, no changes are required in the web.config and performance is the same. Ok. So you know what they are and how to build one, when is an appropriate time to use one?

Handlers are commonly used to generate custom XML and RSS feeds, to unzip and render files stored as BLOB fields in the database including image files or logos, HTTP Handlers can also be used as the target of AJAX calls.

A common mistake that programmers new to .net, especially those like myself who came from classic ASP or PHP, is to use the Page_Load event of a page to create a new http response. For example, before I learned about httphandlers, I would use the page load event to create an xml document or a dynamic PDF file and output it to the response stream with a response.End() to prevent the page continuing after I output my file. The problem with that approach is that you are executing a ton of code in asp.net that doesn’t need to execute. When ASP.NET sees that request come in, it thinks it is going to need to build and render a control tree. By pointing the link at the handler instead, you will gain a 10-20% performance increase every time that request is fetched, just because of the overhead you have reduced. Put simply, Http Handlers minimize the amount of code that executes in ASP.NET.

To implement an Asynchronous handler you use the interface IHttpAsyncHandler, which adds BeginProcessRequest and EndProcessRequest methods. The threading works the same way as with an async page. After the begin method is called, the thread returns to the thread pool and handles other incoming requests until the I/O thread completes its work, at which point it grabs a new thread from the thread pool and completes the request.

Page.RegisterAsyncTask cannot be used here, so if you need to run multiple async tasks you will need to implement your own IAsyncResult Interface and pass in your own callbacks to prevent the EndProcessRequest method being called before you have completed all your async operations.

Asynchronous HTTP Modules

HTTP Modules are another fundamental building block of ASP.NET. They don’t handle requests, instead they sit in the HTTP Pipeline where they have the power to review every request coming in and every response going out. Not only can they view them, but they can modify them as well. Many of the features of ASP.NET are implemented using httpmodules: authentication, Session State and Caching for example, and by creating your own HTTP Modules you can extend ASP.NET in a lot of interesting ways. You could use an HTTP Module for example to add google analytics code to all pages, or a custom footer. Logging is another common use of HTTP Modules.

E-Commerce web sites can take advantage of HTTP Modules by overriding the default behavior of the Session Cookie. By default, ASP.NET Session Cookies are only temporary, so if you use them to store shopping cart information, after 20 minutes of inactivity, or a browser shut down they are gone. You may have noticed that Amazon.com retains shopping cart information much longer: You could shut down your laptop, fly to Japan and when you restart and return to Amazon your items will still be there. If you wanted to do this in ASP.NET you could waste a lot of time writing your own Session State Cookie Class, or you could write about 10 lines of code in the form of an HTTP Module that would intercept the cookie created by the Session Object before it gets to the browser, and modify it to make it a persistent cookie. So, there are lots and lots of practical uses for HTTP Modules.

An Http Module is nothing more than a class that implements the IHttpModule Interface, which involves an Init method for registering any and all events that you are interested in intercepting, and a dispose method for cleaning up any resources you may have used.

 

using System;

using System.Web;

 

public class BigBrotherModule : IHttpModule

{

    public void Init(HttpApplication application)

    {

        application.EndRequest +=

            new EventHandler(OnEndRequest);

    }

 

    void OnEndRequest(Object sender, EventArgs e)

    {

        HttpApplication application = (HttpApplication)sender;

        application.Context.Response.Write

            ("Bill Gates is watching you");

    }

 

    public void Dispose() { }

}

 

The events you can intercept in an HTTP Module:

 

  HttpApplication Events 
  Source: Microsoft Tech Ed 2007 DVD: Web 405  "Building Highly Scalable ASP.NET Web Sites by Exploiting Asynchronous Programming Models" by Jeff Prosise.

 Notice the HTTP Handler at the end there that converts the request into a response. These events will always fire in this order, in every request. The Authenticate Request event is the one fired by ASP.NET when a requested page requires authentication. It checks to see if you have an authentication cookie and if you do not, redirects the request to the login page. In the simple example, I was using that End Request event, which is the last one before the response is sent to the browser.

So, that is what HTTP Modules are for, and how they work. Why do we need an Asynchronous version? Well if you really want to see how scalable your application is, add an HTTP Module that makes a synchronous call to a webservice or a database. Since the event you register will be fired on every request, you will tie up an additional thread from the asp.net thread pool on every single request that is just waiting for these I/O processes to complete. So, if you write a synchronous HTTP Module that inserts a record into a database for every single request, and that insert takes 1 second, EVERY single request handled by your application will be delayed by 1 second. So if you need to do any type of I/O from within an HTTP Module, I recommend you make the calls asynchronously and if you are retrieving data, cache it!

To Register Async Event Handlers in an http module - In the Init Method, simply register your begin and end methods using AddOnPreRequestHandlerExecuteAsync:

 

using System.Web;

 

public void Init (HttpApplication application)

{

    AddOnPreRequestHandlerExecuteAsync (

        new BeginEventHandler (BeginPreRequestHandlerExecute),

        new EndEventHandler (EndPreRequestHandlerExecute)

    );

}

 

IAsyncResult BeginPreRequestHandlerExecute (Object source,

    EventArgs e, AsyncCallback cb, Object state)

{

    // TODO: Begin async operation and return IAsyncResult

}

 

void EndPreRequestHandlerExecute (IAsyncResult ar)

{

    // TODO: Get results of async operation

}

Error Handling while multithreading

Errors can happen at any point during the execution of a command. When ASP.NET can detect errors before initiating the actual async operation, it will throw an exception from the begin method; this is very similar to the synchronous case in which you get the exceptions from a call to ExecuteReader or similar methods directly. This includes invalid parameters, bad state of related objects (no connection set for a SqlCommand, for example), or some connectivity issues (the server or the network is down, for example).

Now, once we send the operation to the server and return, ASP.NET doesn’t have any way to let you know if something goes wrong at the exact moment it happens. It cannot just throw an exception as there is no user code above it in the stack when doing intermediate processing, so you wouldn't be able to catch an exception if it threw one. What happens instead is that ASP.Net stores the error information, and signals that the operation is complete. Later on, when your code calls the end method, ASP.Net detects that there was an error during processing and the exception is thrown.

The bottom line is that you need to be prepared to handle errors in both the begin and the end methods, so it is wise to wrap both events in a try – Catch block.

Conclusion

Now you have seen three of the asynchronous programming models ASP.NET has to offer, hopefully I have impressed upon you how important it is to at least consider using them when creating pages that do I/O if you expect those pages to be heavily trafficked. Remember you can also create asynchronous web services. I didn’t cover those here because there is pretty good documentation for that already.

The good thing about Asynchronous Programming models is that it enables us to build scalable and responsive applications that use minimal resources (threads/context switches).

What is the down side? Well it forces you to split the code into many callback methods, making it hard to read, confusing to debug and difficult for programmers unfamiliar with asynchronous programming to maintain.

With this in mind, whenever I add an asynchronous method to an object in the my projects, I also add a traditional Synchronous version. For example, if I had created a BeginUpdatexxx() Method in the BLL, there would also be a traditional Updatexxx() Method, so that if anyone else finds themselves having to use that object, they won’t be left scratching their heads, wondering “how on earth do I use that?”

Asynchronous command execution is a powerful extension to.NET. It enables new high-scalability scenarios at the cost of some extra complexity.

For more information on multi-threading in ASP.NET I highly recommend you read "Custom Threading in ASP.Net".


Posted by Williarob on Tuesday, December 16, 2008 5:30 PM
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Asynchronous DataSet

If you haven't yet read the article "Scalable Apps with Asynchronous Programming in ASP.NET" by Jeff Prosise or seen his presentation at TechEd then you should cetainly take the time to do so, however I'll summarize the key points briefly here. Basically, there is a finite number of Threads available to ASP.Net for request handling, and by making database calls the way that most textbooks and articles recommend, many of the available threads that should be handling requests are actually tapping their feet waiting for your database request to complete, before it can serve your page and return to the thread pool. When all the threads are busy, incoming requests are queued, and if that queue becomes too long then users start to see HTTP 503 Service unavailable errors. In other words there is a glass ceiling to scalability, with synchronous IO requests in asp.net.

 To make optimum use of the thread pool all IO requests that you know could take a second or more to process should be made asynchronously and the links above will give you plenty of examples of how you should do this. The purpose of this article is to demonstrate how you can return a DataSet asynchronously, which is not something I could find an example of anywhere. Another thing I could not find in my research was how to use asynchronous database calls when you have a datalayer, as opposed to a page or usercontrol that contacts the database directly, and I will provide you with both here.

If you have looked at examples of asynchronous database calls elsewhere on the web before arriving here you have probably become familair with the BeginExecuteReader and EndExecuteReader methods of the SQLClient namespace, but where is the BeginExecuteDataSet method? If you need a dataset, why should that have to be a synchronous request? I could not find any asynchronous methods for the DataAdapter and while I did find ways to call a WebMethod or WebService that returns a dataset asynchronously, why should you have to break your methods that return datasets out to webservices? I also found some examples of how to create delegates or use System.Threading.Thread.Start to create datasets in their own thread, but, according to Mr. Prosise these are worthless in ASP.Net because both of these methods actually steal threads for the same thread pool ASP.Net is using anyway! So by using System.Threading.Thread.Start to create your dataset all you are doing is returning a thread to the threadpool and immediately grabbing another one. So how can you do it?

For this example I borrowed the Datalayer from the Job Site Starter Kit and added some Asynchronous methods to it. Here is my BeginExecuteDataSet method:

       Public Function BeginExecuteDataSet(ByVal callback As System.AsyncCallback, _
          ByVal stateObject As Object, ByVal behavior As CommandBehavior) As System.IAsyncResult   
             Dim res As IAsyncResult = Nothing
             Me.Open()
             res = cmd.BeginExecuteReader(callback, stateObject, behavior)
             Return res
       End Function

But how is that different to BeginExecuteReader? It is not it is exactly the same, I don't see the need to rewrite the DataAdapter class from scratch to support Asynchronous functions when I can simply use a datareader to populate a dataset. The key differences are in the EndExecuteDataSet Method:

       Public Function EndExecuteDataSet(ByVal asyncResult As System.IAsyncResult) As DataSet   
             Dim ds As Dataset = Nothing
             Dim rdr As SqlClient.SqlDataReader = cmd.EndExecuteReader(asyncResult)
             Dim dt As DataTable = New DataTable()
             dt.Load(rdr)
             ds = New DataSet
             ds.Tables.Add(dt)
             Return ds
       End Function

Calling these methods is therefore no different to calling BeginExecuteReader. For example the following code would work from both an .aspx page or an .ascx control.


    Dim db As Classes.Data.DAL
    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        'Append async attribute to connection string
        db = New Classes.Data.DAL(String.Concat _
        (ConfigurationManager.ConnectionStrings("MyDB")ConnectionString, ";async=true"))
        db.CommandText = "asyncTest"
        ' Launch data request asynchronously using page async task  
        Page.RegisterAsyncTask(New PageAsyncTask(New BeginEventHandler(AddressOf BeginGetData), _
        New EndEventHandler(AddressOf EndGetData), New EndEventHandler(AddressOf GetDataTimeout), _
        Nothing,True))
    End Sub
    Function BeginGetData(ByVal sender As Object, ByVal e As EventArgs, _
        ByVal cb As AsyncCallback, ByVal state As Object) As IAsyncResult
       Return db.BeginExecuteDataSet(cb, state)  
    End Function
    Sub EndGetData(ByVal ar As IAsyncResult)
        Try
           gv1.DataSource = db.EndExecuteDataSet(ar)
           gv1.DataBind()
        Catch ex As Exception
           lblMsg.Text = ex.ToString()
        Finally
           db.Dispose()
        End Try
    End Sub
    Sub GetDataTimeout(ByVal ar As IAsyncResult)
        db.Dispose()
        lblMsg.Text = "Async connection timed out"
    End Sub

There you have it, a DataSet returned asynchronously, from a Data Access Layer. Download the entire Data Access Layer (zip file contains both Visual Basic and C# versions - 3.05 kb).


Posted by Williarob on Wednesday, December 19, 2007 8:07 AM
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