CS193H:%20High%20Performance%20Web%20Sites%20Lecture%207:%20Add%20an%20Expires%20Header

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CS193HHigh Performance Web SitesLecture 7 Add
an Expires Header

• Steve Souders
• Google
• souders_at_cs.stanford.edu

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Announcements

• No more hardcopy handouts view/annotate online

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Expires header
GET /v-app/scripts/107652916-dom.common.js
HTTP/1.1 Host www.blogger.com User-Agent
Mozilla/5.0 () Gecko/2008070208
Firefox/3.0.1 Accept-Encoding gzip,deflate
HTTP/1.1 200 OK Content-Type application/x-javasc
ript Last-Modified Mon, 22 Sep 2008 211435
GMT Content-Length 2066 Content-Encoding
gzip Expires Mon, 12 Oct 2009 145734
GMT Cache-Control max-age31536000 XmoÛHþ\ÿFÖvã
wØoq...

• Expiration date determines freshness.
• Can also use Cache-Control max-age

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Expires vs. max-age

• Expires works in HTTP/1.0, max-age in HTTP/1.1
• Expires is an absolute date
• 12 Oct 2009 145734 GMT
• max-age is of seconds until expiration
• 31536000
• Expires relies on clock synchronization between
  client and server for short expirations
• max-age takes precedence over Expires

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Sending Expires (Apache)

• mod_expires
• ltFilesMatch "\.(gifjpgjscss)"gt
• ExpiresDefault "access plus 1 year"
• lt/FilesMatchgt
• sends both Expires and max-age
• Expires Mon, 12 Oct 2009 145734 GMT
• Cache-Control max-age315360000

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Expires in the Wild 2007
Images Scripts Stylesheets with Expires Median Age
amazon.com 0/62 0/3 0/1 0 114 days
aol.com 23/43 6/18 1/1 48 217 days
cnn.com 0/138 2/11 0/2 1 227 days
ebay.com 16/20 0/7 0/2 55 140 days
froogle.google.com 1/23 0/1 0/1 4 454 days
msn.com 32/35 3/9 1/1 80 34 days
myspace.com 0/18 0/2 0/2 0 1 day
wikipedia.org 6/8 2/3 1/1 75 1 day
yahoo.com 23/23 4/4 1/1 100 na
youtube.com 0/32 0/7 0/3 0 26 days
average 10/40 (25) 2/5 (38) 0.5/2 (27) 12/46 (26)

• March 2007

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Expires in the Wild 2008
Images Scripts Stylesheets with Expires Median Age
aol.com 26/35 13/20 1/1 71 189 days
ebay.com 48/48 6/7 2/2 98 1 day
facebook.com 93/97 20/22 20/20 96 121 days
google.com/search 1/1 0/1 0/0 50 1 day
search.live.com/results 6/6 1/1 4/4 100 na
msn.com 45/45 3/3 3/3 100 na
myspace.com 21/21 7/7 4/4 100 na
en.wikipedia.org/wiki 7/32 5/5 9/9 46 310 days
yahoo.com 23/23 4/4 1/1 100 na
youtube.com 8/27 1/1 1/1 34 unk
average 28/34 (83) 6/7 (85) 5/5 (100) 38/45 (85)

• October 2008

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Revving Filenames

• really, this is independent of Expires headers
• once you make a resource public, you can never
  change it gt aggressive proxies prevent 100 of
  users from getting the update
• best solution change the filename
• date trough_062308.gif
• version onload_1.6.1.js
• checksum 1076572916-dom.common.js
• don't use querystring
• wikibits.js?179
• won't be cached by some proxies

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Homework

• "Improving Top Site" class project
• first set of improvements due by 2359 Mon 10/22
• THIS WILL AFFECT YOUR GRADE
• includes improvements from Rules 1-4
• measure improvements using Hammerhead
• record results in your personal Web 100 sheet
• include URLs that I can check (in your sheet)
• read Chapter 4 of HPWS for 10/15

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Questions

• What's are some differences between Expires and
  max-age?
• What types of resources should an Expires header
  be used with?
• Once a resource is cached with a far future
  expiration date, how can you push updates and
  ensure users get the new version?