Quick Google CDN jQuery Tip

Here's a quick tip if your using Google's Content Delivery Network (CDN) for serving up jQuery (or other scripts).

So if you want jQuery 1.4.4 you can use a url like this:

http://ajax.googleapis.com/ajax/libs/jquery/1.4.4/jquery.min.js

Now suppose you just want the latest 1.4.x release, you can use (caution, keep reading):

http://ajax.googleapis.com/ajax/libs/jquery/1.4/jquery.min.js

And if you just want the latest 1.x.x release, you can use this (caution, keep reading):

http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js

I would think that using a less specific version number (like 1.4 or just 1) would result in a higher cache hit ratio for your visitors. One drawback to this would be that when a new version comes out your users will start using it without giving you a chance to do any testing.

Update: However if you take a look at the comments, one of my readers points out that there is a big difference in the Expires http header when using the full version, vs using a version shortcut. Here's what the HTTP headers look like on a request for 1.4.4:

Date: Thu, 13 Jan 2011 16:41:02 GMT
Expires: Fri, 13 Jan 2012 16:41:02 GMT
Cache-Control: public, max-age=31536000

Now here's what they look like when you request just the 1.4:

Date: Fri, 14 Jan 2011 16:45:21 GMT
Expires: Fri, 14 Jan 2011 15:45:21 GMT
Cache-Control: public, must-revalidate, proxy-revalidate, max-age=3600

When requesting 1.4.4 we get a file with an expiration date of next year, and the Cache-Control with a max-age of one year.

Now when you look at the request for 1.4 you will notice that the Expires date is prior to the Date header, and the max-age specified in the Cache-Control header is only one hour.

What are the Pro's and Con's to using Google's CDN to serve up jQuery?

• Pro's
  • Google's CDN has a very low latency, it can serve a resource faster than your webserver can.
  • There is a good chance that the user may have a cached copy of jQuery from google CDN, so they won't need to download it again.
  • Chance are good that the dns lookup for ajax.googleapis.com is cached by the user as well.
  • Because the script is on a separate domain, modern browsers will download the script in parallel with scripts on your domain.
  • Google's CDN also works with HTTPS, so you can offload the SSL processing to google's server.
• Con's
  • Security - If someone hacks google's CDN then they can run javascript on your site.
  • Additional DNS request may be required, but I'm betting the Pro's make this negligible in most cases.
  • May be slower if your site is internal (an intranet for example).

  Do you have any other Pro's or Con's to using Google's CDN for common javascript files?