Caching web contents in the browser

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Caching web contents in the browser
Nacho Coloma
Java Architect
Extrema Sistemas de
Información

Overall Presentation Goal
Learn to think like the browser

Speaker’s qualifications
Nacho Coloma is a Java Architect at Extrema Sistemas de Información
Since 1999 Nacho has designed air traffic systems, e-banking platforms, Document Management Systems and a full load of enterprise applications
… during which he has dealt with browsers more than he would like to :)

Ye olde cache
We tend to avoid what we do not understand.
Source: http://www.sawse.com/

304 (Not Modified) responses
Expires headers
Strong and weak cache indicators
Three things to know

Means “the browser already has the latest version of this file”
Will happen with every click unless Expires is used
304

If the server includes Last-Modified the browser will include If-Modified-Since with each further request
Example: any static file with tomcat
304 with Last-Modified Browser Server <No header> Last-Modified: Fri, 02 Nov 2007 00:36:01 GMT If-Modified-Since: Fri, 02 Nov 2007 00:36:01 GMT 200 OK / 304 Not Modified First request Subsequent requests

If server includes ETag the browser will include If-None-Match with each further request
Example: <inode>-<file-size>-<last-modified> with apache
304 with ETag Browser Server <No header> ETag: 2e88051-8c-43f0b6dd972c0 If-None-Match: 2e88051-8c-43f0b6dd972c0 200 OK / 304 Not Modified First request Subsequent requests

Indicates for how long the file should not be retrieved again.
Cache-control (max-age)
http 1.1
seconds to expire
used by the browser
Expires header Browser Server <No header> Expires: Mon, 15 Dec 2008 19:11:50 GMT Cache-Control: max-age=1209600 (No more requests until the resource expires) First request Subsequent requests
EExpires
http 1.0
Date
used by proxies installed by The Roman Empire.

Refresh : a new request will be sent for each file, ignoring the Expires header.
Any If-Modified-Since or If-None-Match headers will be sent.
The server may return a 304 response.
Shift + Refresh : a new request will be sent for each file, ignoring the Expires header.
Any If-Modified-Since or If-None-Match headers will not be sent .
The server cannot return a 304 response.
The Refresh button

Weak : Last-Modified, default ETag implementations.
Strong : MD5, version numbers
Strong indicators can be inserted in the URL or used as ETag headers
Weak and strong indicators

Version numbers
http://ajax.googleapis.com/ajax/libs/jquery/1.2.6/jquery.min.js
http://yui.yahooapis.com/2.6.0/build/reset/reset-min.css
Revision numbers
http://localhost/myentity?id=5&revision=20
MD5
http://localhost/images/photo.gif?checksum=9e0bee8ace86b2230140642adcf57c4c
Strong indicators

Using strong indicators, you can set your Expires header to cache forever:
HTTP spec : 1 year
Yahoo! recommendation: 10 years
Use only with immutable resources
Expires & strong indicators

DEMO
You can try this yourself:
Go to
http://loom.extrema-sistemas.com/loomdemo/
Look the javascript file names that are retrieved; recognize the MD5 hash.
Look the files downloaded for the first and further requests (use LiveHttpHeaders or Firebug)
Click Refresh and see what happens

Summary
The browser will ask for every resource unless you specify an Expires header
You may return 304 responses
You can use strong and weak cache indicators, in the URL or as HTTP headers
With strong cache indicators, you can cache immutable resources forever.

Improve your performance!
Set your goal to < 1 sec per request

Thanks for your attention
Nacho Coloma
[email_address]
http://icoloma.blogspot.com
Source: http://www.sawse.com/ http://www.extrema-sistemas.com/

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Caching web contents in the browser

by Ignacio Coloma on Feb 06, 2009

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Caching web contents in the browser — Presentation Transcript