Executing Long Running Operations - Part I

Introduction

In web applications you often end up in triggering a lengthy operation and then getting the "time out" errors. This happens because the operation is taking more time to complete than script timeout value of a page (which defaults to 3 minutes). One easy remedy to this problem might be to increase the script timeout value of the page. However, what if your operation is taking too much of time (say 20 minutes). Would you like your uses to wait for 20 minutes? Of course not. There are more appropriate solutions to this situation and I am going to discuss two of them in this two part article.

Long running tasks in a web service

The part-I of this article discusses about how to handle long running operations in a web service. Here, we are assuming that you have a web application that is acting as a client to this web service. By default when you call a web method the caller waits till the time the web method returns. The problem is - if your web method takes more time than the page script timeout value then you will get timeout error.  Hence, you want to trigger the web service but do not want to wait till the web method completes. In other words you simply want  to call the web method and return immediately to the caller. Let's see how this can be done with a simple example.

The web service

First, we will build a simple web service that will do a task for 5 minutes (time greater than default script timeout value of 3 minutes).

• Create a new ASP.NET web service project in VS.NET
• Add following code to the Service1.asmx code behind file

[WebMethod]
public void StartProcessing()
{
	DateTime dt1=DateTime.Now;
	DateTime dt2=dt1.AddMinutes(5);
	TimeSpan ts=new TimeSpan(0,0,0);
	while(true)
	{
		dt1=DateTime.Now;
		if(dt2.Subtract(dt1)==ts)
		{
			break;
		}
		}
	}
}

Here, we created a web method called StartProcessing() that simply simulates time delay of 5 minutes using a while loop.

• Compile the web service

You are now ready to consume it in a web application

The web application

• Add a new ASP.NET web application project to the above solution
• On the web form drag and drop a button control and a label
• Add "web reference" to the web service we created above
• Write following code in the Click event of the button

private void Button1_Click(object sender, System.EventArgs e)
{
localhost.Service1 s1=new localhost.Service1();
s1.StartProcessing();
Label1.Text="Processing completed!";
}

• Here, we are simply creating an instance of web service proxy and calling the long running web method on that.
• If you run this web application it will wait for the web method to finish and eventually timeout throwing an exception.

The Solution

The solution to solve above problem is to decorate your web method with a special attribute called [SoapDocumentMethod(OneWay=true)]. Once you mark your web method with this attribute the web service will return immediately with response code of 202 after calling the method. This way your client feels that the web method processing is over and becomes free to process further operations. Here is how the web method looks like after decorating with SoapDocumentMethod attribute:

[WebMethod]
[SoapDocumentMethod(OneWay=true)]
public void StartProcessing()
{
	DateTime dt1=DateTime.Now;
	DateTime dt2=dt1.AddMinutes(5);
	TimeSpan ts=new TimeSpan(0,0,0);
	while(true)
	{
		dt1=DateTime.Now;
		if(dt2.Subtract(dt1)==ts)
		{
			break;
		}
	}
}

Note, how we have set the OneWay attribute of the SoapDocumentMethod attribute to true.

By default, SOAP messages are in "document" style of formatting. If you are using Remote Procedure Call (RPC) formatting then use [SoapRpcMethod(OneWay=true)] instead of [SoapDocumentMethod(OneWay=true)].

The limitations

Though above technique works well in many situations it has some disadvantages:

• It required that the web method must not return anything. This is obvious because we are returning immediately without waiting for the web method to finish.
• The use do not get any idea whether the method completed successfully or there were some errors.
• The user do not receive any notification when the method ends.
• This works only for web service because the attribute we used is available only for web methods.

Summary

In this part-I of the two part series, we saw how to execute long running web methods from your web application. The SoapDocumentMethod attribute with OneWay property set to true does the trick for us. In the next part we will see how to use multithreading technique to do similar thing in a web form itself.