Caching Images (was: Daily Bulletin, Monday, May 14)
Alex Lopez-Ortiz
Well I don't use IE explorer but since late February I've also noticed
"very aggressive" caching behaviour on Opera and Firefox.
If the problem is common to all three browsers, it seems to me that the
explanation must then lie elsewhere. To give an example, my yahoo home
page has some charts on it. In the past those charts were either up to
date or a simple reload command would update them. Right now
Opera/Firefox completely refuse to download new versions of it, but a
freshly started IE session will download the new images.
Has there been a change on the If-Modified-Since syntax that might lead
the browser to believe the image is fresh? Could it be that a layer 3
proxy server was silently deployed upstream from UW?
Alex
> Almost every day lately, I hear from at least one person who's having
> trouble seeing the Daily Bulletin. Usually what I'm told is that they
> clicked in the right place on the UW home page, but instead of today's
> Daily Bulletin, they're seeing yesterday's. The problem isn't at
> [Refresh button circled] my end, I explain, it's at yours -- click the
> "reload" or "refresh" button on your browser and you'll see the
> latest. (The Daily Bulletin is updated at 9:00 each morning.) I asked
> a technical expert why this kind of thing is happening more often than
> it used to. The answer seems to be the arrival of Internet Explorer 7,
> which was released last fall and now accounts for about half of all
> web traffic, depending on whose statistics you believe. IE 7 has "very
> aggressive caching behaviour", I was told, which means that IE 7 users
> who read yesterday's Daily Bulletin are more likely to have it stored
> in their machines, where the browser will find it, than users of other
> software such as Firefox. So, I repeat, if such a thing happens to
> you, _try hitting the refresh button_. In IE 7 it's near the top right
> of the screen _(typical screenshot pictured)_.
--
Alejandro (Alex) Lopez-Ortiz alop...@uwaterloo.ca
http://www.cs.uwaterloo.ca/~alopez-o Associate Professor
David R. Cheriton School of Computer Science University of Waterloo
Doug Payne
> [...] Could it be that a layer 3
> proxy server was silently deployed upstream from UW?
Yahoo and lots of others are Akamai customers.
Alex Lopez-Ortiz
I've been poking around with charts all day trying to reproduce the
behaviour in a reliable way. Here's an example:
This chart cames up the same in Firefox, no matter how many times I press
reload.
http://ichart.finance.yahoo.com/b?s=USDCNY=X
Yet if I go to Tools->Options and clear the cache then press reload,
the image is updated.
My best guess is that Firefox sends a GET request with an
if-modified-since header (i.e. "GET chart if it has changed since last
access") from the previous download, and someone up stream has a newer
version of the graph but with the wrong (older) time stamp, so it
replies "you have the latest version".
When I clear the cache, Firefox issues the GET request without the
if-modified-since header and the upstream server simply replies with
the latest chart.
Alex
Jem Berkes
> "very aggressive" caching behaviour on Opera and Firefox.
Did you check to see if you are browsing from behind some kind of web
proxy? There could be a transparent proxy (it's very annoying and bad
practice but the university could be using one). I haven't checked myself.
--
Jem Berkes
www.sysdesign.ca