Re: Snapshot-date

Roy T. Fielding (fielding@simplon.ICS.UCI.EDU)
Mon, 18 Jul 1994 17:08:13 +0200

Karl wrote:

> Before launching into the following (long) reply, the point that I really
> want to make is that I would like master servers, but *not* cache servers to
> generate the following header line whenever they send a document.
> snapshot-date: <date>

They already do -- it's called the "Date:" header. How the cache managers
interpret/use that information is what matters.

>> should reflect the date/timestamp of last comparison with the original
>> (i.e. the Date: header returned by the source document's HTTP server
>> in that comparison). Note that this should not be changed by cache
>> managers more than once-removed from the origin. This is because the
>> act of checking the cache consistency with the original document is
>> equivalent to getting a new copy of that document, but the act of
>> checking cache consistency with a higher-level cache is only equivalent
>> to copying that cache.
> Cache-last-updated isn't quite what we need. What is needed is the
> time the copy in the cache was separated from the master source.
> While it might be nice to know how long something has resided in a
> cache, for management or statistical purposes, what is useful to users
> is how long it has been since the cache copy has been unlinked (as it
> were) from the original.

I'm sorry, but that is exactly what Cache-last-updated would provide.
It represents the exact timestamp which the master server generated when
it sent the original copy. To get the "how long" info, just subtract
Cache-last-updated from the current timestamp (after conversion, of course).
If there is no Cache-last-updated header, then the message came straight
from the master source.

...Roy Fielding ICS Grad Student, University of California, Irvine USA
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