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Automatic JS, CSS Versioning to Update Browser Cache when Files are Changed

By , 11 Jun 2011
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When you update JavaScript or CSS files that are already cached in users' browsers, most likely many users won’t get that for some time because of the caching at the browser or intermediate proxy(s). You need some way to force browser and proxy(s) to download the latest files. There’s no way to do that effectively across all browsers and proxies from the webserver by manipulating cache headers unless you change the file name or you change the URL of the files by introducing some unique query string so that browsers/proxies interpret them as new files. Most web developers use the query string approach and use a version suffix to send the new file to the browser. For example:

<script src="someJs.js?v=1001" ></script>
<link href="someCss.css?v=2001"></link> 

In order to do this, developers have to go to all the html, aspx, ascx, master pages, find all references to static files that are changed, and then increase the version number. If you forget to do this on some page, that page may break because browser uses old cached script. So, it requires a lot of regression test effort to find out whether changing some CSS or js breaks something anywhere in the entire website.

Another approach is to run some build script that scans all files and updates the reference to the JavaScript and CSS files in each and every page in the website. But this approach does not work on dynamic pages where the JavaScript and CSS references are added at run-time, say using ScriptManager.

If you have no way to know what JavaScript and CSS will get added to the page at run-time, the only option is to analyze the page output at runtime and then change the JavaScript, CSS references on the fly.

Here’s an HttpFilter that can do that for you. This filter intercepts any ASPX hit and then it automatically appends the last modification date time of JavaScript and CSS files inside the emitted HTML. It does so without storing the whole generated HTML in memory nor doing any string operation because that will cause high memory and CPU consumption on webserver under high load. The code works with character buffers and response streams directly so that it’s as fast as possible. I have done enough load test to ensure even if you hit an aspx page million times per hour, it won’t add more than 50ms delay over each page response time.

First, you add set the filter called StaticContentFilter in the Global.asax file’s Application_BeginRequest event handler:

Response.Filter = new Dropthings.Web.Util.StaticContentFilter(
    relativePath => 
        if (Context.Cache[physicalPath] == null)
          var physicalPath = Server.MapPath(relativePath);
          var version = "?v=" + 
            new System.IO.FileInfo(physicalPath).LastWriteTime
          Context.Cache.Add(physicalPath, version, null,
            DateTime.Now.AddMinutes(1), TimeSpan.Zero,
            CacheItemPriority.Normal, null);
          Context.Cache[physicalPath] = version;
          return version;
          return Context.Cache[physicalPath] as string;

The only tricky part here is the delegate that is fired whenever the filter detects a script or CSS link and it asks you to return the version for the file. Whatever you return gets appended right after the original URL of the script or css. So, here the delegate is producing the version as “?v=yyyyMMddhhmmss” using the file’s last modified date time. It’s also caching the version for the file to make sure it does not make a File I/O request on each and every page view in order to get the file’s last modified date time.

For example, the following scripts and CSS in the HTML snippet:

<script type="text/javascript" src="scripts/jquery-1.4.1.min.js" ></script>
<script type="text/javascript" src="scripts/TestScript.js" ></script>
<link href="Styles/Stylesheet.css" rel="stylesheet" type="text/css" />

It will get emitted as:

<script type="text/javascript" src="scripts/jquery-1.4.1.min.js?v=20100319021342" ></script>
<script type="text/javascript" src="scripts/TestScript.js?v=20110522074353" ></script>
<link href="Styles/Stylesheet.css?v=20110522074829" rel="stylesheet" type="text/css" />

As you see, there’s a query string generated with each of the file’s last modified date time. Good thing is you don’t have to worry about generating a sequential version number after changing a file. It will take the last modified date, which will change only when a file is changed.

The HttpFilter I will show you here cannot only append version suffix, it can also prepend anything you want to add on image, CSS and link URLs. You can use this feature to load images from a different domain, or load scripts from a different domain and benefit from the parallel loading feature of browsers and increase the page load performance. For example, the following tags can have any URL prepended to them:

<script src="some.js" ></script>
<link href="some.css" />
<img src="some.png" />

They can be emitted as:

<script src="" ></script>
<link href="" />
<img src="" />

Loading JavaScripts, CSS and images from different domains can significantly improve your page load time since browsers can load only two files from a domain at a time. If you load JavaScripts, CSS and images from different subdomains and the page itself on www subdomain, you can load 8 files in parallel instead of only 2 files in parallel.

How Do They Do It?

The hardest part of the work in the filter is to intercept writes to the Response stream in chunks of bytes and process those bytes to make sense of them without constructing strings. You have to read one character at a time and understand whether a sequence of characters means a <script> tag or not and then find the src attribute of the tag and then extract the value between double quotes. You have to do all of these without using your favorite string manipulation functions like indexOf, substring, etc.

First, the filter overrides the Write method of Stream.

public override void Write(byte[] buffer, int offset, int count)
  char[] content;
  char[] charBuffer = this._Encoding.GetChars(buffer, offset, count);

  /// If some bytes were left for processing during last Write call
  /// then consider those into the current buffer
  if (null != this._PendingBuffer)
    content = new char[charBuffer.Length + this._PendingBuffer.Length];
    Array.Copy(this._PendingBuffer, 0, content, 0, this._PendingBuffer.Length);
    Array.Copy(charBuffer, 0, content, this._PendingBuffer.Length, charBuffer.Length);
    this._PendingBuffer = null;
    content = charBuffer;

Up to this point, nothing interesting happening but to make sure we always have a complete buffer that has a complete HTML tag. For example, if the last Write call ended with an incomplete buffer that ended half way through a tag like “<script sr”, we want to wait for the next Write call and get more data so that we get a complete tag to process.

The following loop does the real work:

int lastPosWritten = 0;
for (int pos = 0; pos < content.Length; pos++)
  // See if tag start
  char c = content[pos];
  if ('<' == c)
    /* Make sure there are enough characters available in the buffer to finish
     * tag start. This will happen when a tag partially starts but does not end
     * For example, a partial img tag like <img
     * We need a complete tag upto the > character.
    if (HasTagEnd(content, pos))
      if ('/' == content[pos])

        if (HasMatch(content, pos, IMG_TAG))
          lastPosWritten = this.WritePrefixIf(SRC_ATTRIBUTE,
            content, pos, lastPosWritten, this._ImagePrefix);
        else if (HasMatch(content, pos, SCRIPT_TAG))
          lastPosWritten = this.WritePrefixIf(SRC_ATTRIBUTE,
            content, pos, lastPosWritten, this._JavascriptPrefix);

          lastPosWritten = this.WritePathWithVersion(content, lastPosWritten);
        else if (HasMatch(content, pos, LINK_TAG))
          lastPosWritten = this.WritePrefixIf(HREF_ATTRIBUTE,
            content, pos, lastPosWritten, this._CssPrefix);

          lastPosWritten = this.WritePathWithVersion(content, lastPosWritten);

        // If buffer was written beyond current position, skip
        // upto the position that was written
        if (lastPosWritten > pos)
          pos = lastPosWritten;
      // a tag started but it did not end in this buffer. Preserve the content
      // in a buffer. On next write call, we will take an attempt to check it again
      this._PendingBuffer = new char[content.Length - pos];
      Array.Copy(content, pos, this._PendingBuffer, 0, content.Length - pos);

      // Write from last write position upto pos. the rest is now in pending buffer
      // will be processed later
      this.WriteOutput(content, lastPosWritten, pos - lastPosWritten);


The logic is, loop through each character in the character buffer and look for a tag start ‘<’. Then found, look if the buffer has a tag end ‘>’. If not, wait for the next Write call to get a complete buffer. If we have a complete tag, then match the tag name and see if it’s either IMG, SCRIPT or LINK.

It matches the buffer for a tag name using pure character matching, no string operation at all, thus no overhead on garbage collector.

private bool HasMatch(char[] content, int pos, char[] match)
  for (int i = 0; i < match.Length; i++)
    if (content[pos + i] != match[i]
      && content[pos + i] != char.ToUpper(match[i]))
      return false;

  return true;

As you see, there’s no string allocation and thus no new variable is introduced. It does pure character matching.

As soon as it finds the right tag it is looking for, it finds the URL of the file from href or src attribute. Then it checks whether the URL is absolute or relative. If relative, then it prepends the prefix.

/// <summary>
/// Write the prefix if the specified attribute was found and the attribute has a value
/// that does not start with http:// prefix.
/// If atttribute is not found, it just returns the lastWritePos as it is
/// If attribute was found but the attribute already has a fully qualified URL, 
/// then return lastWritePos as it is
/// If attribute has relative URL, then lastWritePos is the starting position 
/// of the attribute value. However,
/// content from lastWritePos to position of the attribute value 
/// will already be written to output
/// </summary>
/// <param name="attributeName"></param>
/// <param name="content"></param>
/// <param name="pos"></param>
/// <param name="lastWritePos"></param>
/// <param name="prefix"></param>
/// <returns>The last position upto which content was written.</returns>
private int WritePrefixIf(char[] attributeName, char[] content, 
			int pos, int lastWritePos, byte[] prefix)
  // write upto the position where image source tag comes in
  int attributeValuePos = this.FindAttributeValuePos(attributeName, content, pos);

  // ensure attribute was found
  if (attributeValuePos > 0)
    if (HasMatch(content, attributeValuePos, HTTP_PREFIX))
      // We already have an absolute URL. So, nothing to do
      return lastWritePos;
      // It's a relative URL. So, let's prefix the URL with the
      // static domain name

      // First, write content upto this position
      this.WriteOutput(content, lastWritePos, attributeValuePos - lastWritePos);

      // Now write the prefix
      if (prefix.Length > 0)
        this.WriteBytes(prefix, 0, prefix.Length);
        // Turn this on if you want to emit an absolute URL from the relative URL
        //this.WriteBytes(this._BaseUrl, 0, this._BaseUrl.Length);

      // If the attribute value starts with the application path it needs to be skipped  
      // as that value should be in the prefix. Doubling it will cause problems. This 
      // occurs with some of the scripts.
      if (HasMatch(content, attributeValuePos, _ApplicationPath))
        // Absolute path starting with / or /Vdir. So, we need to keep the /Vdir/ part.
        attributeValuePos = attributeValuePos + _ApplicationPath.Length;
        // Relative path. So, we need to emit the current folder path. eg folder/
        if (this._CurrentFolder.Length > 0)
          this.WriteBytes(this._CurrentFolder, 0, this._CurrentFolder.Length);

      // Ensure the attribute value does not start with a leading slash 
      // because the prefix is supposed to have a trailing slash.       
      // If value does start with a leading slash, skip it
      if ('/' == content[attributeValuePos]) attributeValuePos++;

      return attributeValuePos;
    return lastWritePos;

The code is heavily documented, so I am not going to repeat what it does.

Similarly, the version is appended by looking at the URL and appending the version on it.

private int WritePathWithVersion(char[] content, int lastPosWritten)
  // We will do it for relative urls only
  if (!HasMatch(content, lastPosWritten, HTTP_PREFIX))
    int pos = lastPosWritten + 1;
    while ('"' != content[pos]) pos++;
    // pos is now right before the closing double quote
    var relativePath = new string(content, lastPosWritten, pos - lastPosWritten);

    // Emit the relative path as is
    this.WriteOutput(content, lastPosWritten, pos - lastPosWritten);
    lastPosWritten = pos;

    // get the last modification date time of the file at relative path
    var version = this._getVersionOfFile(relativePath).ToCharArray();

    // Add a version number at the end of the path
    this.WriteOutput(version, 0, version.Length);

  return lastPosWritten;

It first extracts the path, makes sure it is relative, and then it fires the callback to get the version of the file. Whatever the callback returns, it appends it to the relative path.

How Fast Is It?

It’s pretty fast. I did some Visual Studio performance profiling. VS says the whole code in the filter is faster than .NET Framework code like getting items from cache or calling Server.MapPath() or getting the file’s last modified date time.

If you look at the breakdown of the time spent on the Write function, majority of the time spent is in getting the version number added:


All the for loops, if conditions, etc. are negligible compared to the call to WritePathWithVersion which fires the callback to get the version for each file.


Inside the WritePathWithVersion, you see all the time is spent on calling the callback to get the version number.


Finally, it shows that all the time is spent in doing the cache operations and getting the LastWriteTime of the file. it proves that all the code written in the filter is faster than reading an item from cache or getting the last modification date of a file.

When I do load testing by producing 20 concurrent users each making 30 consecutive calls, then the CPU consumption without the filter shows:


This is without the filter. You can see there’s nothing but ASP.NET stuff doing their work. CPU consumption is avg between 40% to 60%.

Now when I turn on the filter, the CPU consumption looks like:


The CPU consumption is still between 40% to 60%. So, there’s no visible impact on CPU when the filter is added. I have made sure the filter does enough work by producing a page output around 200 KB. This ensures there were many calls to Write and the filter code did a lot of work.

I am Convinced, How Do I Use It?

Go to and download the sample project. In App_Code, you will find the filter. All you need to do is to register the filter in Application_BeginRequest in Global.asax as shown in the example. That’s it!


You need to cache JavaScripts, CSS, images on the browser and proxies to provide as fast browsing experience as possible. But that means you can't update the static files and deliver them to all the browsers unless you change the URL of the file. Manually updating file references throughout the website is difficult and error prone. This HttpFilter does it automatically for you.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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Comments and Discussions

GeneralMy vote of 5 PinmemberAmir Mehrabi-Jorshari2-Aug-13 9:29 
QuestionFilter for MVC Pinmemberdineshd873-May-13 4:41 
Questionsimpler approach Pinmembergiammin13-Jun-12 3:47 
GeneralMy vote of 5 PinmemberHumaid Ashraf7-Feb-12 23:22 
QuestionUsing HTML filter with ASP Pinmembersuhumar6-Dec-11 3:53 
QuestionWhere is the cacheable settings get set? PinmemberBlaine Trimmell20-Sep-11 10:50 
AnswerRe: Where is the cacheable settings get set? PinmvpOmar Al Zabir21-Sep-11 9:22 
BugModifications to support Inline Scripts AND Autogenerated scripts Pinmemberlogmeinhere31-Jul-11 22:31 
GeneralRe: Modifications to support Inline Scripts AND Autogenerated scripts PinmemberHumaid Ashraf8-Feb-12 3:55 
Suggestionstrip out .js/.css, combine, hash, re-add Pinmembertobias42127-Jun-11 22:39 
QuestionHTTPS; Version Number via Routed Urls; HttpModule Pinmembertobias4217-Jun-11 23:26 
AnswerRe: HTTPS; Version Number via Routed Urls; HttpModule PinmvpOmar Al Zabir7-Jun-11 23:39 
GeneralRe: HTTPS; Version Number via Routed Urls; HttpModule Pinmembertobias4218-Jun-11 9:26 
GeneralRe: HTTPS; Version Number via Routed Urls; HttpModule PinmvpOmar Al Zabir9-Jun-11 3:59 
GeneralInline Javascript PinmemberRex Mahel7-Jun-11 7:21 
GeneralRe: Inline Javascript PinmvpOmar Al Zabir10-Jun-11 23:47 
GeneralRe: Inline Javascript PinmemberRex Mahel12-Jun-11 12:45 
GeneralRe: Inline Javascript PinmemberESTAN28-Jun-11 9:59 
GeneralMy vote of 4 PinmemberL Hills31-May-11 1:53 
GeneralMy vote of 4 PinmemberMatt Sollars30-May-11 17:54 
GeneralRe: My vote of 4 PinmvpOmar Al Zabir10-Jun-11 23:50 
GeneralRe: My vote of 4 PinmemberMatt Sollars11-Jun-11 5:16 
GeneralIllegal Characters In Path PinmvpAspDotNetDev29-May-11 9:55 
GeneralRe: Illegal Characters In Path PinmvpOmar Al Zabir30-May-11 7:24 
GeneralRe: Illegal Characters In Path PinmvpAspDotNetDev30-May-11 10:08 

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