Ready for 2010: HTTP Headers and Client Side Caching

There’s a few easy changes you can do to your website setup to speed up content delivery and eat up less bandwidth: configure proper expire values and if possible, keep your static resources on a separate domain.

The HTTP Expires Header

Expires tells the client how long it can keep the current version of a resource as the most recent one. If you set the Expires-header a while into the future, the browser will not make a new request for the file until the resource, well, expires (depending on the cache settings for the browser, requesting a reload (such as shift-reloading in a browser), etc. which can expire the resource earlier). The potential problem is the case where a resource actually changes, such as deploying a change to your stylesheet or external javascript files.

The fix for this is to include something about the file which changes when the file is physically updated on the disk. This can be the last modified time (please keep this cached in your web application, you do not want to hit the disk to retrieve the value for each page view), the current revision number from your revision control system (such as SVN – you can get the current revision of a file by using svn info, and please, cache that value to. You do not want to call svn for each page view :-)) or something else, such as the md5 or crc32 hash of the file. The important part is that you include this value as part of the request, making the URL to the resource unique depending on the version of the resource. You can safely ignore this part of the URL in your rewrite / controller routing magic / handling application, as the only function it has is to tell the browser that it has to request a new file and not use the old one anymore.

Examples of URL-schemes To Get Around Expires:-headers

  1. flickr uses as simple .v in their URLs to indicate the version of the file:
  2. On we use the current SVN revision: /css/main.css?v=1120M
  3. uses the current date, followed with an identifier that probably indicates the current revision for that day: css/frontpage.css?20091203-1

It’s important to remember that the identifier is not used to deliver an older version of the file depending on the parameter, just to make the browser see the new resource. The old URL can still serve the new resource – and if you need to keep old versions around, you’ve probably solved this issue already.

Use a Separate Domain for Static Resources

By using another, separate domain for your static resources, you’re letting browsers fetch the static resources while they’re still processing your HTML. The HTTP/1.1 specification says that browsers never should request more than two files at the same time from the same domain. When you host your static resources on another domain, you tell the browser that it can go ahead and fetch those resources while being busy with downloading other items from your main site.

After you’ve moved your static resources to a separate domain, you’ll usually also end up using less bandwidth. Since you’re now delivering the most requested content from another host, cookies will not be included in the request from the browser. When a browser makes a request for a resource on a certain host, it includes all the cookies that have been set for that domain. This happens independent of which files it’s requesting, and if you have a large number of separate files (which you probably could include into one larger file – resulting in fewer HTTP requests), these Cookie-headers can add up to a significant amount of bandwidth. The HTTP server will also have less work to do, making everyone happier!

If you use www. as a prefix for all your regular HTTP requests and take care of setting your cookies in the domain, you should be able to simply use something like for your static content and avoid leaking cookies into the other subdomain. If you have loads of static content, you can also use several separate subdomains for your files, but be sure to let the request for a certain file point to the same subdomain each time – otherwise you’ll end up with the browser requesting four copies of the same, identical file and actually breaking the regular cache in the browser (which uses If-Modified-Since to tell the server when it last downloaded the file. We want to avoid the browser making the request again at all). At I calculate the crc32 of the filename and use that value to determine which static host the request should use. We also redirect any requests directly to to to make the cookie structure consistent. We do however not set the Expires-header yet, but that might be a part of the next update to the site.

Do you have a particular caching strategy you use for client side content? What kind of URL format works best for you? Leave a comment!

Read all the articles in the Ready for 2010-series