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Question

Why does content served from jQuery's CDN lack far-future expire headers? (or, "What am I doing wrong here?")

Background

I'm using jQuery and jQuery Mobile on a web project. To serve these scripts, I use jQuery's CDN, per their directions. However, I've noticed that these resources are served without far-future expires headers, which prevents browsers from being able to cache them intelligently.

Although the jQuery servers will respond with 304 Not Modified, that's not the same as an expire header the negates that request altogether. I realize the simple answer here is "because they don't" however, I'm wondering why they don't, and if there's some way I can serve this content, ideally from a CDN, with far-future expires headers.

Thanks for your time.

Examples

jQuery CDN

http://code.jquery.com/jquery-1.6.4.min.js

Response:

HTTP/1.1 304 Not Modified
Date: Wed, 16 May 2012 00:05:27 GMT
ETag: "9e68e84-16615-6ad9bfc0+gzip"
Last-Modified: Mon, 12 Sep 2011 22:55:03 GMT
Server: ECS (dca/532A)
Vary: Accept-Encoding
X-Cache: HIT

Google CDN

https://ajax.googleapis.com/ajax/libs/jquery/1.6.4/jquery.min.js

Response:

HTTP/1.1 200 OK
age:134505
cache-control:public, max-age=31536000
content-encoding:gzip
content-length:32103
content-type:text/javascript; charset=UTF-8
date:Mon, 14 May 2012 10:45:15 GMT
expires:Tue, 14 May 2013 10:45:15 GMT
last-modified:Mon, 02 Apr 2012 18:24:28 GMT
server:sffe
status:200 OK
vary:Accept-Encoding
version:HTTP/1.1
x-content-type-options:nosniff
x-xss-protection:1; mode=block

Note the far-future expires date in expires:Tue, 14 May 2013 10:45:15 GMT

Solution

I'm using Microsoft's CDN hosted version which features a 1 year expire date:

http://ajax.aspnetcdn.com/ajax/jquery.mobile/1.1.0/jquery.mobile-1.1.0.min.js

Full list of CDN hosted files.

@zuul explained below why some CDN hosted items have longer expire headers than others. Thanks!

Jeff
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2 Answers2

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The whole caching issue depends on the links you're using. I've observed that quite often people aren't linking to exact version numbers of jQuery.

For example, from code.jquery.com you can reference this file...

http://code.jquery.com/jquery-latest.min.js

... and you'll always get the latest copy of jQuery.

Or you can do something similar from Google...

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

That one from Google will get the latest version, assuming it starts with "1". The trouble is that when you use these references you don't get a far-future expiration date set in the cache-control header. Therefore the user's browser will be checking for updates much too frequently, slowing down your site's load times.

In fact here's a breakdown of several options and their expiration settings...

http://code.jquery.com/jquery-latest.min.js (no cache)

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

http://ajax.googleapis.com/ajax/libs/jquery/1.7/jquery.min.js (1 hour)

http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js (1 year)

By explicitly stating the exact version of jQuery you want Google sends back a max-age header of 1 year. So you're greatly increasing the chances that the client will simply use a cached copy. And if a new version of jQuery comes along with bug fixes or features you really want, then just update your link.

This explanation is originated from BucketSoft Blog

Zuul
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    Should I be writing something well written before? For that, I could just omit the link and make that explanation my own... There are great articles everywhere, the answer is given and the link to enhance the knowledge base as well! (the other way around would be bad!) – Zuul May 16 '12 at 01:19
  • @Zuul Any idea where I can find a CDN hosted version of jQuery Mobile 1.1.0 that has far-future expire headers? jQuery's CDN doesn't have them and Google doesn't appear to host jQuery Mobile's content. – Jeff May 16 '12 at 02:42
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    @Jeff, sorry, don't know of any. But keep watching [this page](https://developers.google.com/speed/libraries/devguide?hl=en#Libraries), it lists Google's CDN libraries! (as far as my knowledge goes, Google is the one provider with long expiration dates) – Zuul May 16 '12 at 02:57
  • @Jeff - It looks like Microsoft's CDN provides a 1 year max-age for jQuery Mobile 1.1.0. All of the CDN options are listed here... http://jquerymobile.com/download/ By the way, Microsoft's CDN is fast and reliable and would be good to use. The only reason I'd normally suggest Google's CDN is that it's the overwhelming favorite on websites for jQuery proper, and so the chances of a user already having a copy from Google are very high. – Steve Wortham Jul 03 '12 at 20:15
  • @SteveWortham Good point. I've set the answer to that of Oliver Kurmis as the correct answer since it is now the more accurate one. Sorry Zuul :) – Jeff Jul 04 '12 at 22:42
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    @Jeff Hey, where's my green tick? :) Just kidding, time goes by, things change, new solutions come up! My happiness is seeing problems solved ;) Glad You've got a more accurate solution! – Zuul Jul 04 '12 at 22:47
  • Thank you! I didn't know about the expiry time / jQuery version connection – David Taiaroa Feb 18 '15 at 01:41
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Microsoft also has a CDN wich is hosting the jquery mobile stuff:

http://www.asp.net/ajaxLibrary/CDNjQueryMobile110.ashx

Expire-Date is about 1 year in the future!

Oliver Kurmis
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  • Selected as the correct answer as this now solves the original question best. Thanks! – Jeff Jul 04 '12 at 22:42
  • +'d your answer, couldn't find it when placing my answer... didn't even remember about the Microsoft CDN. Nice one :) – Zuul Jul 04 '12 at 22:49