Display output from command line program realtime in asp.net - asp.net

Question

I am writing a web app where the application runs a command on the system using System.Diagnostics class.
I wanted to display realtime output from a command which takes a lot of time to complete. After searching a bit, I found that BeginOutputReadLine can stream output to an event handler.
I am also using jquery ajax to call this method and have the process run asynchronously.
So far, I am trying to do it this way:
Process p2= new Process();
p2.OutputDataReceived += new DataReceivedEventHandler(opHandler);
p2= Process.Start (psi2);
p2.BeginOutputReadLine();
I have declared a class with a static variable to store the output of the command as a Label on the page wont be accessible from a static method.
public class ProcessOutput
{
public static string strOutput;
[WebMethod]
public static string getOutput()
{
return strOutput;
}
}
In the event handler for BeginOutputReadLine, set the variable with the line from output.
private static void opHandler(object sendingProcess,DataReceivedEventArgs outLine)
{
if (!String.IsNullOrEmpty(outLine.Data))
{
ProcessOutput.strOutput= outLine.Data;
}
}
and from the aspx page, I am calling the method to get the value of strOutput
$(document).ready(function() {
setInterval(function() {
$.ajax({
type: "GET",
url: "newscan.aspx/getOutput",
data: "",
success: function(msg){
$('#txtAsyncOp').append(msg.d);
}
});
}, 1000);
});
I dont know why, but the lable is not getting updated. If I put alert, I get 'undefined' in the alert box every 10 seconds.
Can anybody suggest me how to do it correctly?

Answer 1

Each request begins a new thread as a part of the Request pipeline. This is by design. Each thread has its own stack and can't access each others stacks. When a thread starts running a new method it stores the arguments and local variables in that method on its own stack. Long story short you won't be able to assign that variable and expect to retrieve its value from another Request.
There are a couple approaches you can take, you can scope it to the session variable (most common) with:
System.Web.HttpContext.Current.Session["variable"] = value ;
Or you set it to application scope using:
if (System.Web.Caching.Cache["Key1"] == null)
System.Web.Caching.Cache.Add("Key1", "Value 1", null, DateTime.Now.AddSeconds(60), Cache.NoSlidingExpiration, CacheItemPriority.High, onRemove);
Alternatively, you can log the output to a database or file and echo out the results via the WebMethod. If your long running process is running asynchronously, you won't have access to the HttpContext -- so the Session state bag will not be available; the application Cache could be used, however it is generally not used for this type of mechanism (cache is available for performance reasons, not a persistence mechanism -- its important to remember that you cannot control when your web application recycles).
I'd highly suggest writing to a database or log file. Asynchronous processes commonly require logged output or trace to diagnose potential problems and to validate results.
Furthermore, because you cannot control when the web app recycles, you can easily lose control of that child process you're launching. A better design would start an asynchronous method in-process, or an out-of-process application or service that polls a database to pick up jobs (possibly use the task scheduler or cron depending on your platform).