caching - Force refresh of cached CSS data

Is it possible force the browser to fresh the cached CSS?

This is not as simple as every request. We have a site that has had stable CSS for a while.

Now we need to make some major updates to the CSS; however, browsers that have cached the CSS will not receive the new CSS for a couple of days causing rendering issues.

Is there a way to force refresh of the CSS or are we better just opting for version specific CSS URLs?

Answer

There are several things to consider and a variety of ways to approach this. First, the spec

What are we trying to accomplish?

Ideally, a modified resource will be unconditionally fetched the first time it is requested, and then retrieved from a local cache until it expires with no subsequent server interaction.

Observed Caching Behavior

Keeping track of the different permutations can be a bit confusing, so I created the following table. These observations were generated by making requests from Chrome against IIS and observing the response/behavior in the developer console.

In all cases, a new URL will result in HTTP 200. The important thing is what happens with subsequent requests.

+---------------------+--------------------+-------------------------+
|        Type         |   Cache Headers    |     Observed Result     |
+---------------------+--------------------+-------------------------+

| Static filename     | Expiration +1 Year | Taken from cache        |
| Static filename     | Expire immediately | Never caches            |
| Static filename     | None               | HTTP 304 (not modified) |
|                     |                    |                         |
| Static query string | Expiration +1 Year | HTTP 304 (not modified) |
| Static query string | Expire immediately | HTTP 304 (not modified) |
| Static query string | None               | HTTP 304 (not modified) |
|                     |                    |                         |
| Random query string | Expiration +1 Year | Never caches            |
| Random query string | Expire immediately | Never caches            |

| Random query string | None               | Never caches            |
+---------------------+--------------------+-------------------------+

However, remember that browsers and web servers don't always behave the way we expect. A famous example: in 2012 mobile Safari began caching POST requests. Developers weren't pleased.

Query String

^{Examples in ASP.Net MVC Razor syntax, but applicable in nearly any server processing language.}

...since some applications have traditionally used GETs and HEADs with
query URLs (those containing a "?" in the rel_path part) to perform
operations with significant side effects, caches MUST NOT treat
responses to such URIs as fresh unless the server provides an explicit
expiration time. This specifically means that responses from HTTP/1.0
servers for such URIs SHOULD NOT be taken from a cache.

Appending a random parameter to the end of the CSS URL included in your HTML will force a new request and the server should respond with HTTP 200 (not 304, even if it is hasn't been

modified).

Of course, if we randomize the query string with every request, this will defeat caching entirely. This is rarely/never desirable for a production application.

If you are only maintaining a few URLs, you might manually modify them to contain a build number or a date:

@{

    var assembly = Assembly.GetEntryAssembly();
    var name = assembly.GetName();
    var version = name.Version;
}

This will cause a new request the first time the user agent encounters the URL, but subsequent requests will mostly return 304s. This still causes a request to be made, but at least the whole file isn't served.

Path Modification

A better solution is to create a new path. With a little effort, this process can be automated to rewrite the path with a version number (or some other consistent identifier).

This answer shows a few simple and elegant options for non-Microsoft platforms.

Microsoft developers can use a HTTP module which intercepts all requests for a given file type(s), or possibly leverage an MVC route/controller combo to serve up the correct file (I haven't seen this done, but I believe it is feasible).

Of course, the simplest (not necessarily the quickest or the best) method is to just rename the files in question with each release and reference the updated paths in the link tags.

TLDR

• Change the file name or query string


• Use a change that only occurs once per release


• File renaming is preferable to a query string change


• Always set HTTP headers to maximize the benefits of caching