Force browser to reload all cache after site update

Question

Is there a way to force the clients of a webpage to reload the cache (i.e. images, javascript, etc) after a server has been pushed an update to the code base? We get a lot of help desk calls asking why certain functionality no longer works. A simple hard refresh fixes the problems as it downloads the newly updated javascript file.

For specifics we are using Glassfish 3.x. and JSF 2.1.x. This would apply to more than just JSF of course.

To describe what behavior I hope is possible:

Website A has two images and two javascript files. A user visits the site and the 4 files get cached. As far as I'm concerned, no need to "re-download" said files unless user specifically forces a "hard" refresh or clears their cache. Once a site is pushed an update to one of the files, the server could have some sort of metadata in the header informing the client of said update. If the client chooses, the new files would be downloaded.

What I don't want to do is put meta-tag in the header of a page to force nothing from ever being cached...I just want something that tells the client an update has occurred and it should get the latest once something has been updated. I suppose this would just be some sort of versioning on the client side.

Thanks for your time!

Answer 1

The correct way to handle this is with changing the URL convention for your resources. For example, we have it as:

/resources/js/fileName.js

To get the browser to still cache the file, but do it the proper way with versioning, is by adding something to the URL. Adding a value to the querystring doesn't allow caching, so the place to put it is after /resources/.

A reference for querystring caching: http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.9

So for example, your URLs would look like:

/resources/1234/js/fileName.js

So what you could do is use the project's version number (or some value in a properties/config file that you manually change when you want cached files to be reloaded) since this number should change only when the project is modified. So your URL could look like:

/resources/cacheholder${project.version}/js/fileName.js

That should be easy enough.

The problem now is with mapping the URL, since that value in the middle is dynamic. The way we overcame that is with a URL rewriting module that allowed us to filter URLs before they got to our application. The rewrite watched for URLs that looked like:

/resources/cacheholder______/whatever

And removed the cacheholder_______/ part. After the rewrite, it looked like a normal request, and the server would respond with the correct file, without any other specific mapping/logic...the point is that the browser thought it was a new file (even though it really wasn't), so it requested it, and the server figures it out and serves the correct file (even though it's a "weird" URL).

Of course, another option is to add this dynamic string to the filename itself, and then use the rewrite tool to remove it. Either way, the same thing is done - targeting a string of text during rewrite, and removing it. This allows you to fool the browser, but not the server :)

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UPDATE:

An alternative that I really like is to set the filename based on the contents, and cache that. For example, that could be done with a hash. Of course, this type of thing isn't something you'd manually do and save to your project (hopefully); it's something your application/framework should handle. For example, in Grails, there's a plugin that "hashes and caches" resources, so that the following occurs:

• Every resource is checked
• A new file (or mapping to this file) is created, with a name that is the hash of its contents
• When adding <script>/<link> tags to your page, the hashed name is used
• When the hash-named file is requested, it serves the original resource
• The hash-named file is cached "forever"

What's cool about this setup is that you don't have to worry about caching correctly - just set the files to cache forever, and the hashing should take care of files/mappings being available based on content. It also provides the ability for rollbacks/undos to already be cached and loaded quickly.