Re: ONLINE-ADS>> Cache Busting, in Detail

Tom Hukins (tom_at_eborcom.com)
Mon, 09 Mar 1998 19:17:35 +0000

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Many thanks to Rick Bruner for reporting his findings on cache busting to
this list.

Rick's autoresponder message distinguished cache counting (as used by
Imgis, NetGravity et al) from cache busting. Cache busting prevents caches
from re-using previously-viewed data, in order to generate more accurate
statistics. Cache counting, on the other hand, allows the actual banner
images to be cached. Rather than directly loading the banner images,
however, cache counting makes browser software first contact a
non-cacheable address (Rick refers to this as a "ping"), which then refers
the browser software to the banner image.

Rick concludes:
>I would urge all publishers to move towards the cache counting
>technique described above. Not only is it better for network
>bandwidth in the long run, it provides users with a faster
>loading page and sites with a more accurate measure of
>impressions.

I disagree that this cache counting is a good idea. A large proportion of
Internet users around the world pay per minute charges, either in the form
of local telephone calls, ISP charges, or both. Such people quickly get
into the habit of offline browsing; going online for a short amount of
time, quickly grabbing interesting Web sites, then going offline and
reading the content of these sites offline.

Because offline browsers cannot contact the non-cacheable address used by
cache counting, they will not be referred to the banner image. When the
user was online, a banner impression would have been counted, but the value
of that impression would be low. A far better impression would be made if
the user could see the banner when leisurely browsing offline.

A far better approach seems to me to be that employed by SAFE-Audit. I
suspect others use this technique too. Please Note: What follows is my
interpretation of how SAFE-Audit's system works, it may or may not be
totally accurate.

All banners are sent in a cacheable form as plain images. A separate, very
small image (a 1x1 transparent GIF, I believe) is placed on the Web page
next to the banner. This small image uses cache-busting techniques to
ensure more accurate statistical information is recorded. The banners
themselves can be viewed offline because the cache-busting element
necessary to measure cached impressions is a completely separate entity to
the banner itself.

With my limited knowledge in this area, it seems that this is a far better
technique than cache counting. Comments, anyone?

Regards,
Tom

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