Persisting View State to the File System
page 6 of 7
by Justin Lovell
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Scott's and My Problem

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There is one problem that both Scott's code and my code run into: garbage collection! Over time, your view state might take a hundred megabytes of disk space and you will have to devise a way on how to clean it up. There are a few solutions on how to schedule the clean-up of the view state files but most of them have one thing in common.

The one thing in common is the clean up code. The code is rather simple and I have commented the code as well in a self explanatory fashion:

public sealed class CleanUpViewStateFiles {
   private const int DaysToLeaveFiles = 2;</font>


<font face="Verdana" size="2">   /// <summary>
   /// Cleans up all the files that are found in a given directory.
   /// </summary>
   /// <param name="path">
   /// The absolute file path to the directory which contains the view state files.
   /// </param>
   public void CleanUp(string path) {
      TimeSpan timeSpan = new TimeSpan(DaysToLeaveFiles, 0, 0, 0);

      if (!path.EndsWith(@"\"))
         path += @"\";

      foreach (string filePath in Directory.GetFiles(path)) {
         FileInfo file = new FileInfo(path + filePath);

         // if the difference between now and the last access time is greater than the time span
         // delete the file.
         if (DateTime.Now.Subtract(file.LastAccessTime) >= timeSpan)
            file.Delete();
      }
   }</font>


<font face="Verdana" size="2">   private CleanUpViewStateFiles() {}
}

I think it is a good period of keeping view state files for two days. Busier sites might want to reduce the amount the amount of time that the view state file stays on disk because those disks can get filled rather quickly.

I mentioned that there are a couple solutions to the clean-up. One of them is to implement a simple windows service that runs the common code every thirty minutes. Obviously, most of us do not have that type of control over the server (web or file server).

The other solution is, which I prefer, is to run one thread in the background. I do have some code on some "practice/honing sites" that runs a thread in the background that does the odd site maintenance (like keeping the ASP.NET application compiled so there are no delays). For an example of how to implement the ASP.NET-keep-alive, you may read Paul Wilson's article. And while you are at it, you may integrate the clean-up code into the keep-alive.

I have one tip: do not use the Global.asax approach -- HttpModule's will work just as well. In fact, even better because you will not be restricted by single inheritance. A sample of an HttpModule has been included in the downloadable source code - it is located in the CleanUpHttpModule.cs file. To set up the clean up module, this is what  you configuration file should look something like this (I recommend that you put the clean up code and module in a seperate assembly):

<configuration>
   <system.web>
      <httpModules>
          <add name="ViewStateCleanUp"
             type="ViewStateToFS.CleanUpHttpModule, ViewStateToFS.dll"/>
      </httpModules>
    </system.web>
</configuration>


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User Comments

Title: Will fail for ASP.NET 2.0   
Name: TomP
Date: 2007-12-11 9:13:31 AM
Comment:
We used this for ASP.NET 2.0 and found that you will lose your control state. A good article exists here: http://blog.arctus.co.uk/articles/2007/04/23/advanced-asp-net-storing-viewstate-in-a-database
Title: Problem with Microsoft Ajax   
Name: AnupT
Date: 2007-06-13 12:17:38 AM
Comment:
Excellent article and i was able to successfully use this in a project.

But I ran in to problem in another project that uses Microsoft Ajax 1.0.

I have a page which uses updatepanel control. In this updatepanel, i have 2 dropdownlist with autopostback enabled. For some weird reason the second dropdownlist is not maintaing the selected value. As soon as i change the 2nd dropdownlist, its value changes back to first item in the list.

This problem is happening for all the page that has 2 or more dropdownlist.
Title: Some problems occurs during long sessions   
Name: Raul
Date: 2006-01-16 12:22:06 PM
Comment:
\
Title: Httpmodule problem   
Name: Ian
Date: 2005-05-25 12:47:33 PM
Comment:
Great article.
I did run into one problem with the HttpModule. I kept getting the error "Server operation is not available in this context". I think it was because the the httpmodule is created in the Application_OnStart and at that point the context does not contain a value for Server.MapPath. Anyway, I got around this by using System.AppDomain.CurrentDomain.BaseDirectory instead.

Good stuff though
Title: Cache   
Name: Wesley
Date: 2004-08-07 5:58:02 AM
Comment:
Justin,

I thought it was all about minimizing site traffic due to the sometimes very large viewstate. Just a little misunderstanding. But...

Your mathwork showed me somthing else as wel. 50 x 8 x 15 = 6000KB an hour! If we're talking about shared hosting most hosts give you a harddisk space of 50MB to 100MB which means that in the best case(not counting object allready there) we got 16.6 hours of viewstate left. So it's not very usefull for the average.

I would like to thank you both(scott and you) an awfull lot for explaning viewstate in such an extended way. I managed to solve my viewstate problems on the composite control mentioned in the url.

Cheers,

Wes
Title: Postback problem still exists   
Name: Justin Lovell
Date: 2004-08-07 5:57:14 AM
Comment:
Richard,

As I have already said to, the problem that you are experiencing is due to the fact that my HTML code that I posted is directing you to Scott's example page.

I have upload the new source code with the fixed HTML
Title: re: Cache   
Name: Justin Lovell
Date: 2004-08-06 5:43:53 PM
Comment:
Hi Wesley,

You can use the cache object to store the view state. However, it totally depends on what type of site you are running. The main issue that was addressed by persisting the view state to the file system is that an user can fill a form and then leave it to stand over night before he submits his changes.

If you are using the cache object, you will then have to counter that time-restriction by placing the cache time to a couple of hours and sometimes as long as a day. If your site gets an average of 50 users who all visit eight pages on average, and assuming that the average view state size is 15KB, then do the maths: 50 x 8 x 15 = 6000KB of memory (~6MB). That is a low traffic site... now let's multiply the amount of sites doing exactly that (shared hosting for low traffic sites) to about 40: that is ~240MG just for just holding view state into memory.

That is a bit inefficient. However, it becomes even more dramatized if you have to run off medium trafic site. And to make it worst: you can't scale your application to a web farm.

It is all about keeping your options open... and your memory for better use :-)
Title: Cache   
Name: Wesley
Date: 2004-08-06 2:57:28 PM
Comment:
As I am quit new on programming in .Net I tend to read a lot just to learn. So don't blame me if this is a stupid idea

One thing I truly don't understand: Why not cache the viewstate by the name of the SessionID+Page and the time a session exists? Something like: cache.insert(SessionId+Page, Viewstate,,DateTime.Now.AddMinutes(SessionTTL))

In that case your sure to get the viewstate corresponding to the current session, it auto cleans up the mess and it's a lot quicker I guess... You just have to make sure the cache isn't cleared.

Cheers,
Wes
Title: Postback problem still exists   
Name: Richard Lemmon
Date: 2004-08-06 2:41:38 PM
Comment:
I followed your article and was able to duplicate the same error that was being caused by Scott's code.

Try the following:

1.) http://localhost/ViewStateToFS/MySolution.aspx
2.) Click on the "Show User Control A"
3.) Select Hello world
4.) right-click on "Show user Control B" and select Open in new window
5.) In the new window click Postback
6.) Return to the initial window & click Postback

same error.

Regards,

Rich Lemmon
rich@lindenstreet.com
Title: A couple of very small points on an otherwise excellent article   
Name: Scott Galloway
Date: 2004-08-06 11:37:13 AM
Comment:
Great article, I do have a couple of small points though - firstly, as written your code writes the filepath into the HTML source of the generated page; this is not really a great idea as path disclosure is generally to be avoided in web apps. I made a simple modification to allow just the GUID portion to be stored in the hidden field - which is also smaller :-). Second point is to do with the HttpModule you use for cleanup, I tend to use a different approach here, using a Timer object stored as a static object - it has the nice side effect of avoiding using a separate module but it does have the disadvantage of requiring a code change.

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