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From Alexei Kosut <>
Subject Re: cache verification
Date Sun, 10 Mar 1996 19:33:09 GMT
On Sun, 10 Mar 1996, David Robinson wrote:

> 1: I stole the Last-modified factor from the CERN server, so your last
>    statement is false.

No, it's not. As I understand it, CERN uses that directive to determine 
when to get rid of the cached file. i.e. it caches the file until the 
time figured out from the Last-modified header times the factor runs out, 
but it still does a conditional GET each time.

> 2: You can achieve the behaviour you want by setting the last-modified
>   factor to zero; then all documents without an Expiry header will 'expire'
>   immediately.

But this is not very desirable, as it then entails retrieving the whole 
document each time. The point of a cache is to lessen network activity. 
Hence a conditional GET to determine if the file has been updated or not.

> 3: The behaviour of not checking the remote server is very useful;
>    for a lot of people a cache that always checks the origin server can
>    be worse than useless for small documents. (small = few hundred bytes).

Average size of a web document is about 5k. Average size of a 304 
response sent to a conditional GET: 90 bytes.

> There is no overriding reason to treat a missing Expire as 'expire now'.
> In a lot of cases that is the wrong assumption to make; most documents
> don't expire immediately. Of course, that's not say that this isn't the
> right assumption to make sometimes.

It's a question of perferring to get the right document over the wrong 
one. The following scenario:

1. Someone creates a document in June, 1996.
2. Your proxy accesses in February, 1998.
3. The person changes it in March, 1998.

Anyone using your proxy will not see the changes for a whole two months.
This doesn't quite sound like desired behavior to me. And, yes, this isn't
completly hypothetical - I actually do have some web documents that
haven't been updated since 1994, and I'm sure other people do, too. What
if I decide to change them tomorrow?

While I'm admit that for certain applications, an
always-return-cached-version mode may be desirable, I think that, at
least, there needs to be an option to act as I've described.

--// Alexei Kosut // <> // Lefler on IRC --//
-----------------// <> -------// 
"To get the full effect of Pat Buchanan's speeches, they should be
read in the original German." //--------------------------------------

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