Force browser to clear cache

Question

Is there a way I can put some code on my page so when someone visits a site, it clears the browser cache, so they can view the changes?

Languages used: ASP.NET, VB.N

Answer 1

Here is the MDSN page on setting caching in ASP.NET.

Response.Cache.SetExpires(DateTime.Now.AddSeconds(60))
Response.Cache.SetCacheability(HttpCacheability.Public)
Response.Cache.SetValidUntilExpires(False)
Response.Cache.VaryByParams("Category") = True

If Response.Cache.VaryByParams("Category") Then
   '...
End If

Answer 2

I had a case where I would take photos of clients online and would need to update the div if a photo is changed. Browser was still showing the old photo. So I used the hack of calling a random GET variable, which would be unique every time. Here it is if it could help anybody

<img src="/photos/userid_73.jpg?random=<?php echo rand() ?>" ...

EDIT As pointed out by others, following is much more efficient solution since it will reload images only when they are changed, identifying this change by the file size:

<img src="/photos/userid_73.jpg?modified=<? filemtime("/photos/userid_73.jpg")?>"

Answer 3

For static resources right caching would be to use query parameters with value of each deployment or file version. This will have effect of clearing cache after each deployment.

/Content/css/Site.css?version={FileVersionNumber}

Here is ASP.NET MVC example.

<link href="@Url.Content("~/Content/Css/Reset.css")?version=@this.GetType().Assembly.GetName().Version" rel="stylesheet" type="text/css" />

Don't forget to update assembly version.

Answer 4

A lot of answers are missing the point - most developers are well aware that turning off the cache is inefficient. However, there are many common circumstances where efficiency is unimportant and default cache behavior is badly broken.

These include nested, iterative script testing (the big one!) and broken third party software workarounds. None of the solutions given here are adequate to address such common scenarios. Most web browsers are far too aggressive caching and provide no sensible means to avoid these problems.

Answer 5

Update 2012

This is an old question but I think it needs a more up to date answer because now there is a way to have more control of website caching.

In Offline Web Applications (which is really any HTML5 website) applicationCache.swapCache() can be used to update the cached version of your website without the need for manually reloading the page.

This is a code example from the Beginner's Guide to Using the Application Cache on HTML5 Rocks explaining how to update users to the newest version of your site:

// Check if a new cache is available on page load.
window.addEventListener('load', function(e) {

  window.applicationCache.addEventListener('updateready', function(e) {
    if (window.applicationCache.status == window.applicationCache.UPDATEREADY) {
      // Browser downloaded a new app cache.
      // Swap it in and reload the page to get the new hotness.
      window.applicationCache.swapCache();
      if (confirm('A new version of this site is available. Load it?')) {
        window.location.reload();
      }
    } else {
      // Manifest didn't changed. Nothing new to server.
    }
  }, false);

}, false);

See also Using the application cache on Mozilla Developer Network for more info.

Update 2016

Things change quickly on the Web. This question was asked in 2009 and in 2012 I posted an update about a new way to handle the problem described in the question. Another 4 years passed and now it seems that it is already deprecated. Thanks to cgaldiolo for pointing it out in the comments.

Currently, as of July 2016, the HTML Standard, Section 7.9, Offline Web applications includes a deprecation warning:

This feature is in the process of being removed from the Web platform. (This is a long process that takes many years.) Using any of the offline Web application features at this time is highly discouraged. Use service workers instead.

So does Using the application cache on Mozilla Developer Network that I referenced in 2012:

Deprecated
This feature has been removed from the Web standards. Though some browsers may still support it, it is in the process of being dropped. Do not use it in old or new projects. Pages or Web apps using it may break at any time.

See also Bug 1204581 - Add a deprecation notice for AppCache if service worker fetch interception is enabled.