Comments on: Should You Use JavaScript Library CDNs?

By: Dave Sargent

Dave Sargent — Mon, 28 Nov 2011 19:39:32 +0000

One more thought regarding the DNS request issue. Many larger businesses, ours included, utilize proxy servers and edge routers with caching. Even with clearing my local DNS I cannot get my computer to show the DNS penalty. My conclusion is the the local proxy server is caching the Google CDN. It only has to do this for the first employee, every other employee gets the benefits of the locally cached copy of the DNS record, regardless if they do a Ctrl+F5 or ipconfig /flushdns

I do not know if any ISP’s do this. I.e. run local dns resolver and therefor speed up the results.

I may be wrong, as I haven’t done the research, but it seems to me that the TTL on the DNS resolution would need to be low for mobile users. If you are on the west coast and get a 1 year cached copy of the actual resolution to a CDN resource and then fly to the east cost your computer would still try and load files from the CDN on the west coast. Again, that’s just my theory, I have not done the research.

Last point: Statistics do not always tell the whole story. Even if a very small percentage of sites are the ones using the CDN we must look at the average use of those sites. For example, the following sites use the Google repository: Break.com, FAIL Blog, Foursquare, Twitter, Posterous, SitePoint, Stack Overflow, Stanford.edu, and even the jQuery site itself. If we can get Facebook on board, that would take care of allot of people.

One very salient point has to do with your business needs. We have customers all over the world. It’s far better for companies over seas to load these libraries from Google then from our west coast location.

By: Billy Hoffman

Billy Hoffman — Sat, 16 Jul 2011 13:23:25 +0000

Rincewind42,

We do need to re-examine the data. However remember its not just an increase in Google’s JavaScript CDN usage that matters. Its CDN usage, library usage, usage of a specific library version, and finally the specific URL used to access that library (/jquery/latest.js vs. /jquery/jquery.1.5.0.js).

By: Rincewind42

Rincewind42 — Thu, 14 Jul 2011 05:24:26 +0000

Thank you for an informative article. It was just the information I had been looking for. However, you wrote this in Jan 2010 and it is now a year and a half later. The data on builtwith.com shows that, especially Googles AJAX Libraries API is now at 13.07% compared with the 3.89% figure you quoted a year ago. Clearly this increase in use of the CDN will change the numbers and balance of your article. Would it be possible for you to do an update of this page?

By: Michael Stillwell

Michael Stillwell — Sat, 16 Apr 2011 13:57:37 +0000

Only if you say you want, say, the latest version in the 1.7.x series. (As it must: a new version might be released.) If you say you want a specific version you get a max-cache of a year.

By: Billy Hoffman

Billy Hoffman — Wed, 27 Oct 2010 16:40:12 +0000

Andy,

You absolutely can do that.

By: Andy Corpes

Andy Corpes — Tue, 12 Oct 2010 12:09:49 +0000

Just found this thread while searching for “why can’t i download googles java script libraries”. I’ve played around with yahoo’s yui & CI, but the first thing i did was downloaded it and unipped it onto my own server. So, why can’t i do the same for google’s libraries?

By: Billy Hoffman

Billy Hoffman — Mon, 30 Aug 2010 13:53:38 +0000

Fred,

We are not in the business of helping Microsoft”s or Google’s ideas hit critical mass.. We are in the business of helping people make their websites faster. We are not going to tell our customers to do something that would hurt performance for their website, on the off-chance that one day it might help their site’s performance. If or when JavaScript library CDNs provide a clear performance advantage we will start recommending them.

By: fred

fred — Thu, 26 Aug 2010 13:34:56 +0000

Although I found this a very rich and usefull article + comments, there’s one thing I do not get.
Since one of the points is that we do not benefit from these cached versions due to the fact that too few sites actually use these methods. This doesn’t make sense to me. You end with stating :
“Zoompf recommends the vast majority of websites avoid using JavaScript Library CDNs until they gain more market penetration”.
But that will never happen if everyone waits for this.
Should this not be:
“Zoomppf recommends the vast majority of websites start using JavaScript Library CDNs in order to gain more market penetration” ? and therefor everyone gains..right ?

By: Dave Ward

Dave Ward — Fri, 02 Apr 2010 19:52:09 +0000

Of the top Alexa sites using the CDN, the overwhelming majority are using Google’s CDN and are explicitly referencing either 1.3.2, 1.4.2, or 1.2.6. There’s very little fragmentation there to worry about. It really only takes a tiny handful of these top-tier sites to prime a tremendous number of caches on a daily basis.

Also, the implicit “latest” references (e.g. 1.3 and 1.4) will continue to be less and less common as awareness is raised about the caching drawbacks. Since those are only served with a +1 hour expires header, that’s detrimental even to the single site referencing it that way. That’s typically the result of migrating to 1.4 (or jQuery UI 1.8 now) early and not realizing it should be referenced as 1.4.0 to get the correct expires header.

By: Billy Hoffman

Billy Hoffman — Tue, 30 Mar 2010 14:38:20 +0000

Dave,

Great feedback. Your point about where your traffic sources come from is a good one. Just remember there are 3 factors that affect whether a JavaScript file from a JavaScript CDN will be in a visitor’s cache.

-Do your main traffic sources use the same CDN that you do?
-Do they use the JavaScript CDN for the same libraries that you do?
-Do they use the same version of the same libraries as you do?
-Do they use the same URL to fetch the same library with the same version that you do?

You mentioned Break.com as an example. However they do a horrible job. They are using jQuery 1.3.2, but are referencing it using the very uncommon URL:

http://ajax.googleapis.com/ajax/libs/jquery/1.3/jquery.min.js

instead of the much more common:

http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js

So unless your site are also using jQuery 1.3.2 and referencing (using the exact same URL) you get no benefit.

Because of this, I think your point that it only takes a few high traffic sites to prime a large number of caches is too simplistic. It actually takes high traffic sites using the same libraries, with the same versions, from the same CDN to prime a large number of caches. Based on the trends I’ve seen so far, the latest version of jQuery is the only JavaScript library with the potential to benefit from these CDNs.