nik codes

Archive for the month “March, 2014”

More HTTP Flushing in ASP.NET MVC

Pluralsight If you like my blog, you’ll love my Pluralsight courses:
Tracking Real World Web Performance
WebPageTest Deep Dive

My last post about HTTP flushing in ASP.NET MVC generated quite a bit of buzz and several really good questions. If you haven’t yet, I recommend you read that post before continuing on here. With this post I’ll answer some of the questions that arose about flushing early and offer a few handy tips.

When to Flush?

Steve Souders does a great job of explaining this technique in his book, Even Faster Web Sites, and on his blog, but there still seems to be some confusion about flushing. For example, Khalid Abuhakmeh mentioned that it “seems arbitrary where you would place the flush” and asked:


While one could certainly flush lots of small chunks of content to the client, and there may be circumstances that call for doing just that, this recommendation is specifically aimed at (1) improving the user’s perceived performance and (2) giving the browser’s parser and network stack a head start to download the assets required to render a page.

Given that, the more content that can be flushed early, before any expensive server side processing (e.g. database queries or service calls), the better. Typically this means flushing just after the </head> tag, or just before the first @RenderSection()/@RenderBody() Razor directive.

Here’s an example of the difference that flushing makes on a waterfall chart displaying network activity:


Notice that in the “Before” image, the two style sheets and one JavaScript file that are referenced in the the <head> section of the page aren’t downloaded until the original HTML file has finished downloading itself. When flushing the <head> section, however, the browser is able to start downloading these secondary resources and rendering any available HTML immediately – providing the user with visual feedback even sooner.

What’s more, if the style sheets contain rules that reference tertiary resources (e.g. background images, sprites or fonts) and the flushed HTML matches one of those rules, the tertiary resources will be downloaded early as well. Ilya Grigorik, a developer advocate that works at Google on the Chrome web performance team, recently wrote an a post about font performance with the tip to optimize the critical rendering path – which flushing directly helps with.

So basically, it’s best to flush as much of the beginning of an HTML response as you can to improve perceived performance and give the browser a head start on downloading not only secondary assets, but often times tertiary ones as well.

How can I See the Improvement?

The effects of flushing early is most easily seen on a waterfall chart. (New to waterfall charts? Check out Tammy Everts’ excellent Waterfalls 101 post.) But how does that correlate to the user’s experience? That’s where Speed Index comes in. Speed Index is currently the best metric we have to measure the perceived performance of a page load. measures Speed Index for any given page by doing a series of screen captures of the loading process, analyzing the images over time, and producing the metric.


Screen captures of a loading page

Speed Index comparison of two loads

The WebPageTest documentation covers the algorithm for calculating the metric in depth, and offers suggested targets based on the Alexa Top 300K. The lower a site’s Speed Index, the better.

Personally I’ve never seen flushing early increase a page’s Speed Index. In general, I can’t think of a way that it would hurt performance, but you may not need to use the technique if you don’t have anything in the head to download or any server side processing. As always, your mileage may vary and you should test the results on your own site.

What About Compression?

You should still compress your responses, flushing doesn’t change that recommendation! Compression affects the content that is sent to the client (and adds a Content-Encoding: gzip header).

Flushing, on the other hand, affects how the content is sent to the client (and adds a Transfer-Encoding: chunked header.)

headersHTTP Response with Compression and Flushing in Fiddler

The two options are completely compatible with each other. Souders reported some configuration issues with Apache flushing small chunks of content – but, based on my testing,  IIS doesn’t seem to have these problems. Using the default configuration, IIS compresses each chunk of flushed content and sends it to the client immediately.

How do I Debug?

Flushed HTTP responses are really no different that any other HTTP response. That means the in browser F12 development tools and HTTP proxies like Fiddler work as expected.

One caveat worth mentioning though is that Fiddler, by default, buffers text\html responses, which means that when Fiddler is running the benefits of flushing early won’t be observable in the browser. It’s easy enough to fix this though, simply click the “Stream” icon in the Fiddler toolbar, as covered in the Fiddler documentation.

streamIn Stream mode Fiddler immediately releases response chunks to the client

Are There Any Other Considerations?

Since flushing early is done in the name of giving the browser a head start, be sure to provide the browser’s parser everything it needs to efficiently read the HTML document as soon as possible.

Eric Lawrence, ex-product manager for Internet Explorer, has a post detailing the best practices to reduce the amount of work that the parser has to do to understand a web page. Essentially, begin HTML documents like this and the parser will not have to wait or backtrack:

<!DOCTYPE html>
     <meta charset="utf-8">
     <meta http-equiv="X-UA-Compatible" content="IE=edge">
     <base /><!-- Optional -->
     <!-- Everything else -->

What’s the Easiest Way to do This?

I’ve created a simple open source project called PerfMatters.Flush that attempts to make flushing early in ASP.NET MVC as simple as possible. It’s still very alpha-y, but it’s already live on the homepage. You can play with the bits now, or wait for my follow up post that details how to use it.


Flushing in ASP.NET MVC

Pluralsight If you like my blog, you’ll love my Pluralsight courses:
Tracking Real World Web Performance
WebPageTest Deep Dive

I’ve written a follow up to this post that answers many of the common questions about flushing early. Be sure to check it out.

The Setting

Before the beginning of this decade, Steve Souders released two seminal books on the topic of web performance: High Performance Web Sites and Even Faster Web Sites. The findings and subsequent suggestions that came out of those books changed the face of web development and have been codified into several performance analysis tools including Yahoo YSlow and Google PageSpeed.

High Performance Web Sites Even Faster Web Sites

Most professional web developers that I’ve met over the past five years are familiar with Souder’s recommendations and how to implement them in ASP.NET MVC. To be fair, they aren’t that difficult:

  • HTTP Caching and Content Compression can both be enabled simply via a few settings in web.config.
  • Layout pages make it easy to put stylesheets at the top of a page and scripts at the bottom in a consistent manner.
  • The Microsoft.AspNet.Web.Optimization NuGet package simplifies the ability to combine and minify assets.
  • And so on, and so forth…

The recommendation to “Flush the Buffer Early” (covered in depth in chapter 12 of Even Faster Web Sites), however, is not so easy to implement in ASP.NET MVC. Here’s an explanation of the recommendation from Steve’s 2009 blog post:

Flushing is when the server sends the initial part of the HTML document to the client before the entire response is ready. All major browsers start parsing the partial response. When done correctly, flushing results in a page that loads and feels faster. The key is choosing the right point at which to flush the partial HTML document response. The flush should occur before the expensive parts of the back end work, such as database queries and web service calls. But the flush should occur after the initial response has enough content to keep the browser busy. The part of the HTML document that is flushed should contain some resources as well as some visible content. If resources (e.g., stylesheets, external scripts, and images) are included, the browser gets an early start on its download work. If some visible content is included, the user receives feedback sooner that the page is loading.

A few months ago Steve revisited this guidance and provided examples of the difference that flushing can make to an application’s performance. His post inspired me to try the same on an ASP.NET MVC project that I’m working on, which led to this post.

The Conflict

Since .NET 1.1, ASP.NET has provided a mechanism to flush a response stream to the client with a simple call to HttpResponse.Flush(). This works quite well when you are incrementally building up a response, but the architecture of MVC, with its use of the command pattern, doesn’t really allow for this. (At least not in a clean manner.) Adding a call to Flush() inside a view doesn’t do much good.


This is because MVC doesn’t execute the code in the view until all the other work in the controller has completed – essentially the opposite of what the czar of web performance recommends.

The Climax

Because I’m a believer that #PerfMatters, I decided to take matters into my own hands to see if I could do anything better.

First, I realized that I could can get around a few issues by leveraging partial results, manually executing and flushing them, like so:

public ActionResult FlushDemo()

       // do other work
       return PartialView("RemainderOfPage");

I think that looks pretty ugly, so I’ve taken things a step farther and removed the cruft around executing the result by creating a base controller with its own Flush() method:

public class BaseController : Controller
     public void Flush(ActionResult result)

I think my new Flush() method clarifies the intent in the action method:

public ActionResult FlushDemo()

     // do other work
     return PartialView("RemainderOfPage");

What I’d really like to be able to do is leverage the yield keyword. Yield seems like the natural language and syntax to express this. I was able to cobble together this example:

public IEnumerable<ActionResult> Test()
      yield return PartialView("Header");
      Thread.Sleep(2000); // or other work

      yield return PartialView("Lorem");
      Thread.Sleep(2000); // or other work

      yield return PartialView("Vestibulum");
      Thread.Sleep(2000); // or other work

      yield return PartialView("Sed");
      Thread.Sleep(2000); // or other work

      yield return PartialView("Footer");

I got that working with a pretty ugly hack, but leveraging the yield keyword and IEnumerable<ActionResult> like this should theoretically be possible by making a few changes to MVC’s default action invoker, no hack necessary.  Unfortunately, C# throws a few curve balls at this since you can’t combine the usage of yield with async/await – which I think would be a pretty common usage scenario.

The Resolution?

It looks like splitting up a layout file into multiple partial views and using my Flush() helper method is the best that we can do right now.


  • Yielding multiple views isn’t currently supported by MVC, and even if it was it would be incompatible with the current C# compiler.
  • Partial views are the best we can get to break up a response into multiple segments – which is painful. It’d be nice if I could ask Razor to render just a section of a layout page, which would reduce the need for partial views.

I’m hoping that the ASP.NET team, or my readers, can come up with a better way to handle flushing in the future, but I wanted to at least walk you through what your options are today.

For those interested, I might pull together some of this thinking into a NuGet package that leverages action filter attributes to simplify the syntax even further. If you like the sounds of that, encourage me on Twitter: @nikmd23

Post Navigation