Streaming text output for long-running action? - asp.net-mvc

Question

I have a few utility actions that return text output via return Content("my text","text/plain").
Sometimes these methods take a few minutes to run (i.e. log parsing, database maintenance).
I would like to modify my action method so that instead of returning all of the output at once, the text is instead streamed to the client when it is ready.
Here's a contrived example:
public ActionResult SlowText()
{
var sb = new System.Text.StringBuilder();
sb.AppendLine("This happens quickly...");
sb.AppendLine("Starting a slow 10 second process...");
System.Threading.Thread.Sleep(10000);
sb.AppendLine("All done with 10 second process!");
return Content(sb.ToString(), "text/plain");
}
As written, this action will return three lines of text after 10 seconds. What I want is a way to keep the response stream open, and return the first two lines immediately, and then the third line after 10 seconds.
I remember doing this 10+ years ago in Classic ASP 3.0 using the Response object. Is there an official, MVC-friendly way to accomplish this?
--
Update: using Razor .cshtml in the app; but not using any views (just ContentResult) for these actions.

Answer 1

Writing directly to the Response object should work, but only in some simple cases. Many MVC features depend on output writer substitution (e.g. partial views, Razor view engine, and others) and if you write directly to the Response your result will be out of order.
However, if you don't use a view and instead write straight in the controller then you should be fine (assuming your action is not being called as a child action).

Answer 2

I would skip the MVC controller entirely since you are going to break encapsulation anyway. In it's place I'd use a barenaked IHttpHandler implementation, streaming directly to the aforementioned output stream.

Answer 3

You are exposing yourself to a browser timeout if the process takes longer than originally intended. Then you don't have a way to recover what happened / unless you implement a separate method that gives the information on the long running process.
Given that you want the other method anyway, you can start a long running process and return immediately. Have the browser check the other method that gives the latest information on the long running process. On the last time I had to do this, I kept it simple and just set the refresh header from the controller before returning the view.
As for starting a long running process, you can do something like this:
// in the controller class
delegate void MyLongProcess();
//...
// in the method that starts the action
MyLongProcess processTask = new MyLongProcess(_someInstance.TheLongRunningImplementation);
processTask.BeginInvoke(new AsyncCallback(EndMyLongProcess), processTask);
//...
public void EndMyLongProcess(IAsyncResult result)
{
try{
MyLongProcess processTask = (MyLongProcess)result.AsyncState;
processTask.EndInvoke(result);
// anything you needed at the end of the process
} catch(Exception ex) {
// an error happened, make sure to log this
// as it won't hit the global.asax error handler
}
}
As for where do you put the log of the actions that happened, it's up to you to how long lived you want it to be. It can be as simple as a static field/class where you add the info of the ongoing process, or instead saving it to a data store where it can survive an application recycle.
The above assume this is all about a long running process that goes on reporting the actions that has been done. Streaming is a different subject, but the above might still play a role in keeping the operations in your controller & only the piece responsible of streaming what becomes available to the client in the action result.

Answer 4

You can implement your custom ActionResult like ContentStreamingResult and use HttpContext, HttpRequest and HttpResponse in the ExecuteResult method.
public class ContentStreamingResult : ActionResult
{
private readonly TextReader _reader;
public ContentStreamingResult(TextReader reader)
{
_reader = reader;
}
public override void ExecuteResult(ControllerContext context)
{
var httpContext = context.HttpContext;
//Read text from the reader and write to the response
}
}
public class YourController : Controller
{
public ContentStreamingResult DownloadText()
{
string text = "text text text";
return new ContentStreamingResult(new System.IO.StringReader(text));
}
}

Answer 5

Try Response.Flush and BufferOutput to false. Note it would work with the different action results, you have to directly write into the response object. Probably you can use it with conjunction with AsyncController.