Cache busting in ASP.NET

So, what is the reason of "Last modified"? If I set 365 days for caching, the browser doesn`t check the last modified date?

Correct, the "Last-Modified" is used for what is called Cache Validation purposes, but is not read if the file is already in the cache and you don't force a refresh (F5 and shift+F5). So during normal browsing, it has no effect on how the browser serves the file directly from its cache.

That's about the same trick I use. I also added a switch for debug- or min-files. For local debugging I use the debug-file. In production- or staging-environment I use the min-file: if (HttpRuntime.Cache[rootRelativePath] == null) { string absolute = HostingEnvironment.MapPath("~" + rootRelativePath); DateTime date = File.GetLastWriteTime(absolute); int index = rootRelativePath.LastIndexOf('/'); string result = rootRelativePath.Insert(index, "/v-" + date.Ticks); index = result.LastIndexOf('.'); string switch = ConfigurationManager.Appsettings["StaticSwitch"]; //debug or min result = result.Insert(index, "." + switch); HttpRuntime.Cache.Insert(rootRelativePath, result, new CacheDependency(absolute)); }

Mads, I guess there should be some handler implemented which will resolve this '/content/v-634933238684083941/site.css' path. Is that right? Or I'm missing something...

Yes this is what i thought about, too...Or thre should be some kind of iis redirect i guess

I'm having the same question.

You can use <link rel="stylesheet" href="/content/site.css?v-634933238684083941" /> if you want to avoid having a IIS redirect

This is an alternative to implementing a handler you can use the querystring like site.css?version=v-634933238684083941

I know that one. But I'd like to get more details on what Mads is doing.

Could this have something to do if you use IIS Express or the built in web server in VS?

Me too!

Of course!! How could I forget the last part...? Thanks for reminding me, it is now added

Yes, you can do that, but it's not considered a good practice to have URL parameters on static resources. Google Page Speed won't give you maximum points if you do that either.

This works on both full IIS and IIS Express

Still cant get it to work. I get the rewrite tag underlined stating that it is not valid in system.webServer. I'm using VS2012 but when I google it seems to be an old problem solved in VS2010. Do I need to update the intellisense so it compiles correctly or is there something else needed in web.config?

There is no Intellisense for it, so VS is expected to invalidate the <rewrite> element. Just ignore the validation warning. You need to run the AppPool in Integrated Pipeline mode for the <system.webServer> to take effect.

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Don't you also need to install the Microsoft URL Rewrite Module 2.0 for IIS 7 ?

Is it possible to do something similar with MVC4 bundling / minification ... instead of the version querystring?

Yes, potentially. However, most hosters already have it installed and I believe it's pre-installed on IIS 8.

I just asked the bundling team and they don't have a way to change that behavior for now. It's on their backlog. You can write your own Fingerprint class that works with the BundleTable to do the same thing though.

Ok, thanks for asking =)

Made a extension method, which converts the URL ... if anyone else might be interested ... public static class HtmlHelper { private readonly static Regex re_Version = new Regex(@"(\?v=([^$]+)$)", RegexOptions.IgnoreCase | RegexOptions.Compiled); private readonly static Regex re_LastFolder = new Regex(@"(/[^/$]+)$", RegexOptions.IgnoreCase | RegexOptions.Compiled); public static IHtmlString ToTag(this IHtmlString htmlString, int cacheDuration = 10) { string rootRelativePath = htmlString.ToHtmlString(); if (HttpRuntime.Cache[rootRelativePath] == null) { var result = rootRelativePath; if (re_Version.IsMatch(result)) { var versionString = re_Version.Match(result).Groups[2].Value; result = re_Version.Replace(result, ""); if (re_LastFolder.IsMatch(result)) { var lastFolderSegment = re_LastFolder.Match(result).Groups[1].Value; result = string.Concat(re_LastFolder.Replace(result, ""), "/v-", versionString, lastFolderSegment); } } HttpRuntime.Cache.Insert(rootRelativePath, result, null, DateTime.Now.AddMinutes(cacheDuration), new TimeSpan(0)); } return MvcHtmlString.Create(HttpRuntime.Cache[rootRelativePath] as string); } } The web.config rewrite rule shuold be modified to ... <rewrite> <rules> <rule name="fingerprint" stopProcessing="false"> <match url="([\S]+)[b](/v-[^/]+/)[/b]([\S]+)" ignoreCase="true" negate="false" /> <action type="Rewrite" url="{R:1}/{R:3}" /> </rule> </rules> </rewrite> If the razor views (eg. _Layout.cshtml) it can be included as ... <script src="@Scripts.Url("~/bundles/jquery").ToTag()" type="text/javascript"></script>

I'm curious... Instead of globally adding the staticContent/clientCache configuration in the web.config, if you wanted to only perm cache these particular resources, could you modify that rewrite rule somehow to also add a Cache-Control response header set to like "public,max-age=2147472000"?

I tried using the URL in the outbound rule, but by then it's already been rewritten. I had to save off the original request path in a server variable and use that in the outbound rule. This is what i came up with. It seems to work but the drawback is you have to remember to allow the server variable (X_REQUESTED_URL_PATH / whatever you want to call it) in IIS/applicationHost.config. <rewrite> <rules> <rule name="fingerprint"> <match url="([\S]+)(/v-[0-9]+/)([\S]+)" /> <action type="Rewrite" url="{R:1}/{R:3}" /> <serverVariables> <set name="X_REQUESTED_URL_PATH" value="{R:0}" /> </serverVariables> </rule> </rules> <outboundRules> <rule name="fingerprint cache header"> <match serverVariable="RESPONSE_Cache-Control" pattern=".*" /> <conditions> <add input="{X_REQUESTED_URL_PATH}" pattern="([\S]+)(/v-[0-9]+/)([\S]+)" /> </conditions> <action type="Rewrite" value="public,max-age=2147472000" /> </rule> </outboundRules> </rewrite> But this way you don't have to globally enable the staticContent/clientCache configuration. Thoughts?

What if you use this on stylesheet A that @import's another stylesheet B? If you make changes to stylesheet B - they'll never be picked up because only A is fingerprinted, right?

Doesnt seem to work with the devserver in VS, only with IIS Express.

Is there a reason to not use etags? http://en.wikipedia.org/wiki/HTTP_ETag , http://www.dotnetscraps.com/dotnetscraps/post/ETag-and-IIS-demystified.aspx

You can add "public" to the cache-control header like so, but it would be for all static resources: <clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="365:00:00" cacheControlCustom="public" />

No, you can use ETags. That's not a problem unless you run in a web farm and you are using the default ETag generation performed automatically by IIS. And even then, it's not that big a problem

Here's how to remove ETags if you want to just use the Last-Modified header instead: http://mark.mymonster.nl/2011/10/18/improve-the-yslow-score-remove-the-etags

I always use the build number assigned by the build server for cache busting. In TeamCity it is really simple to peek into AssemblyInfo.cs and change the revision part of the version to the build number. There's a built-in feature that can do just that: http://confluence.jetbrains.net/display/TCD7/AssemblyInfo+Patcher Then I have a Razor HTML Helper that adds the Assembly's revision number to the URL: public static class UrlHelperExtensions { private static int _revisionNumber; public static string ContentVersioned(this UrlHelper urlHelper, string contentPath) { string url = urlHelper.Content(contentPath); int revisionNumber = GetRevisionNumber(); return String.Format("{0}?v={1}", url, revisionNumber); } public static int GetRevisionNumber() { if (_revisionNumber == 0) { Version v = Assembly.GetExecutingAssembly().GetName().Version; _revisionNumber = v.Revision; } return _revisionNumber; } } And in my Razor layout page, I include scripts like this: <script type="text/javascript" src="@Url.ContentVersioned("/Scripts/libs/backbone-min.js")"></script>

Any advice on how to handle images referenced inside a stylesheet? Ex: background: url('/Images/background.jpg'); Right now we add a version number to the name, but it doesn't seem like the best approach. background: url('/Images/background-1.jpg');

Will GetLastWriteTime work even when the file goes trough a VCS, or when in different time zones/different server times?

In my web.config file, as default is: [b]<clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="365:00:00" cacheControlCustom="public" />[/b] But when check header [b]http://prohost.vn/ Date: Mon, 28 Jan 2013 04:23:59 GMT Content-Encoding: deflate Server: Microsoft-IIS/6.0 X-Powered-By: ASP.NET Content-Type: text/html; charset=utf-8 Content-Script-Type: text/javascript Cache-Control: private Content-Style-Type: text/css Content-Length: 12943 200 OK [/b] Can you help me!

Might be worth noting that if you have any relative URLs in your stylesheets, you'll need to add an extra '../' to them, or convert them to absolute URLs.

Or you can add a rewrite rule.. For my own project, this is what it looks like.. <rule name="cssImages"> <match url="(stylesheets/images/)([\S]+)" /> <action type="Rewrite" url="images/{R:2}" /> </rule>

Combining JavaScript bundling, minification, cache busting, and easier debugging Combining JavaScript bundling, minification, cache busting, and easier debugging

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Well, you can use Long Path Tool for such issues, it works good.

Mads, I believe you are setting the maxage to 365 hours and not days above. Shouldn't cacheControlMaxAge="365:00:00" be cacheControlMaxAge="365.00:00:00"?

Interesting technique. However, it isn't bullet proof, especially with a very large web farm which has to serve traffic 24x7. Care must be taken that requests for "/content/v-634933238684083941/site.css" is truly served by a server with that content and not another server. In addition, there is a risk of a DoS attack. All I have to do as a hacker is to change the fingerprint for every request. For smaller sites, low bandwidth sites this may not be a problem, but for critical systems it is. To mitigate risk, its better to upload artifacts to a cookie-less domain hosted by your CDN. If the fingerprint is modified, a 404 will be returned without any traffic making it to the origin servers.

I use a similar cache-busting technique, except rather than the timestamp, I use the assembly version number of the web application project (and make sure my vendor static files always include their version number). Quick note to the person who suggested query string parameters: that won't necessarily work if you're serving your static content through a CDN. Munging version info into the path itself forces the CDN to do a fetch-through if it doesn't have that precise version of the file.

Doesn't the asp.net bundling and minification support do this?