[펌] ViewState Compression

>> 출처: http://www.codeproject.com/useritems/ViewStateCompression.asp

.NET Framework에서 IIS 6.0의 HTTP Compression을 응용하여 ViewState만 압축하는 방법이 잘 정리되어 있어 퍼왔다.

 

Introduction

 

Recently I developed a huge ASP.NET page, with more than 30 controls. As we all know, it's a good idea to disable the ViewState for the controls that don't actually need it, say Literals or Labels.
After doing that, I noticed that the hidden ViewState field was still a few KB big.
This is obviously a big problem for the users that still don't have a broadband connection, because uploading to the server 40 KB is really a bad issue, especially when they begin to click the "submit" button again and again because they don't notice any response. So, after a few searches through the Internet, I built a simple solution to compress the ViewState and therefore save a rough 50% of bandwidth. This post by Scott Hanselman has been particularly useful.
Although it's possible to use external libraries to perform compression tasks, I think the better solution is to use the GZipStream or DeflateStream that the .NET Framework 2.0 includes.

Compressing and Decompressing Data in Memory
First of all, we need a way to compress and decompress an array of bytes in memory. I put together this simple static class that exposes two methods: Compress and Decompress. The two available classes, GZipStream and DeflateStream, according to MSDN, use the same algorithm, so it's irrelevant which one you chose. The code below is really simple and doesn't need further explaination.

using System.IO;
using System.IO.Compression;

public static class Compressor {

  public static byte[] Compress(byte[] data) {
    MemoryStream output = new MemoryStream();
    GZipStream gzip = new GZipStream(output, CompressionMode.Compress, true);
    gzip.Write(data, 0, data.Length);
    gzip.Close();
    return output.ToArray();
  }

  public static byte[] Decompress(byte[] data) {
    MemoryStream input = new MemoryStream();
    input.Write(data, 0, data.Length);
    input.Position = 0;
    GZipStream gzip = new GZipStream(input, CompressionMode.Decompress, true);
    MemoryStream output = new MemoryStream();
    byte[] buff = new byte[64];
    int read = -1;
    read = gzip.Read(buff, 0, buff.Length);
    while(read > 0) {
      output.Write(buff, 0, read);
      read = gzip.Read(buff, 0, buff.Length);
    }
    gzip.Close();
    return output.ToArray();
  }
}
 

You need to save this class in a .cs file and put it in the App_Code directory of your ASP.NET application, making sure it's contained in the proper custom namespace (if you don't specify any namespace, the class will be available in the built-in ASP namespace).

 

Compressing the ViewState

 

Now we can actually compress the ViewState of the Page. To do that, we have to override the two methods LoadPageStateFromPersistenceMedium and SavePageStateToPersistenceMedium.
The code simply uses an additional hidden field, __VSTATE, to store the compressed ViewState. As you can see by viewing the HTML of the page, the __VIEWSTATE field is empty, while our __VSTATE field contains the compressed ViewState, encoded in Base64. Let's see the code.

 

public partial class MyPage : System.Web.UI.Page {

  protected override object LoadPageStateFromPersistenceMedium() {
    string viewState = Request.Form["__VSTATE"];
    byte[] bytes = Convert.FromBase64String(viewState);
    bytes = Compressor.Decompress(bytes);
    LosFormatter formatter = new LosFormatter();
    return formatter.Deserialize(Convert.ToBase64String(bytes));
  }

  protected override void SavePageStateToPersistenceMedium(object viewState) {
    LosFormatter formatter = new LosFormatter();
    StringWriter writer = new StringWriter();
    formatter.Serialize(writer, viewState);
    string viewStateString = writer.ToString();
    byte[] bytes = Convert.FromBase64String(viewStateString);
    bytes = Compressor.Compress(bytes);
    ClientScript.RegisterHiddenField("__VSTATE", Convert.ToBase64String(bytes));
  }

  // The rest of your code here...
}

 

In the first method, we just decode from Base64, decompress and deserialize the content of the __VSTATE and return it to the runtime.
In the second method, we perform the opposite operation: serialize, compress and encode in Base64. The Base64 string is then saved into the __VSTATE hidden field.
The LosFormatter object performs serialization and deserialization tasks.

You may also want to create a new class, for example CompressedPage, inheriting from System.Web.UI.Page in which override the tow methods and then inherit your page from that class, for example MyPage : CompressedPage. Just remember that .NET has only single inheritance, and by following this way you "spend" your only inheritance chance to use the ViewState compression. on the other hand, overriding the two methods in every class is a waste of time, so you have to chose the way that best fits your needs.

 

Performances and Conclusions

 

After a few tests, I noticed that the ViewState has been reduced from 38 KB to 17 KB, saving 44%. Supposing you have an average of 1 postbacks per minute per user, you could save more than 885 MB of bandwith per month on every single user. That's an excellent result: you save bandwidth (and therefore money), and the user notices a shorter server response time.

I wanted to point out that this solution has a performance hit on the server's hardware. Compressing, decompressing, encoding and decoding data is quite a heavy work for the server, so you have to balance the number of users with your CPU power and RAM.