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Re: Some data related to the frequency of cache-busting

From: Shel Kaphan <sjk@amazon.com>
Date: Sun, 1 Dec 1996 12:42:46 -0800 (PST)
Message-Id: <199612012042.MAA05371@anaconda.amazon.com>
To: Larry Masinter <masinter@parc.xerox.com>
Cc: snowhare@netimages.com, http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Larry Masinter writes:
 > You said of
 > 
 > 
 > 1 'This page *MUST NOT* ever be displayed from a history'
 > 2 'This page *MAY* be redisplayed from a history but *MUST NOT* be
 >    refetched when displayed from history'
 > 3 'This page *MAY* be redisplayed from a history, but *MUST* be
 >    refetched first'
 > 4 'This page *MAY* be redisplayed from history, unconditionally.' 
 >  
 > that
 > 
 > # The last two cases can be handled by properly implementing the
 > # existing Expires and cache control directives.
 > 
 > but I don't believe there are ANY http directives that place any
 > requirements on the handling of history lists, to the point where HTTP
 > _only_ requires 4.
 > 
 > In fact, there are some browsers where doing much of anything else
 > doesn't make much sense. For example, there was a two-dimensional
 > infinite-plane browser where the 'history' was always completely
 > visible, albeit in perspective.
 > 

(I know I'm repeating myself here, so bear with me):

We just need to define the difference between a cache, which is used
exclusively for performance improvements and is supposed to be
semantically transparent, and __any other client-local storage of
fetched results__, which may be used for whatever purpose desired by the
client (this includes "history").  This was done, to an extent, for
1.1.  The issue is that the rules for controlling the cache should not
be mixed up with the rules for the other local storage.  The design
problem is that nobody wants to constrain browser design more than
necessary to make services predictable and reliable, and that to even
talk about this kind of thing we have to go beyond "bits on the wire".

 > However, I'm a little fuzzy on why lack-of-controls of history makes
 > 'cache-busting' more of a problem, or lessens the value of hit
 > metering.
 > 
 > Larry
 > 
 > 

Use of extra-protocol solutions like unique URLs are a problem for
caching, especially if they can't be combined with appropriate cache
controls for fear of making browsers act badly.  These types of
solutions may not be a problem for hit metering, except they make
accumulating statistics more complex, because now many different URLs
as seen by clients are actually "the same" URL from the server
statistics point of view.

Since caches and other local storage are typically mixed up, certain
uses of certain HTTP headers will have unintended consequences.  So,
people resort to solutions that are outside the protocol, e.g. unique
URLs.

To repeat again the oft-repeated example, let's say a service author
wants to send out a document that must always be refetched on "new"
requests, but should be displayed from a locally stored copy if
someone wants to view previous results.  You set it up to expire
immediately, or you set it up so that it is not cachable.  That's
fine, but what happens when someone hits the BACK button in their
browser to go to this page?  If the history buffer and cache system
are mixed up, hitting BACK will result in the page being re-fetched,
when the service author's goal was to have it be redisplayed from
local storage.  Some browsers can be a bit nasty about it, depending
how the page was generated, and may display results like "DATA
MISSING".  This is no good from a UI perspective, and it will really
freak out naive users, to the point that authors such as myself will
simply avoid using the headers that cause this, and find other ways,
outside the protocol, to approximate the desired result of causing new
requests to get new pages but allow local browser history functions to
work, too.  The problem with this is that using these techniques is
even worse than avoiding caching altogether -- it can cause pages that
should never be cached in the first place to be cached, possibly
displacing usefully cached pages.

It's also a pain from the service design perspective, since you have
to think about all kinds of weird interactions in browsers before
using seemingly obvious and straightforward controls like Expires and
Cache-control.  In the long run this may be worse than a little
caching inefficiency.

--Shel
Received on Sunday, 1 December 1996 12:47:05 EST

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