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Newsgroups: mail-lists.delegate-en

[DeleGate-En] Re: refresh file on a cache
18 Jun 2002 21:01:46 GMT (Yutaka Sato)


On 06/09/02(17:30) you michel <> wrote
in <_A1678@delegate-en.ML_>
 |>  |How to be sure that a file on the delegate cache is fresh like the same
 |>  |file on a server ?
 |> EXPIRE=0
 |> will do.  With this, on each request from client, DeleGate always
 |> (0 seconds after the data is cached) asks the server if the cached
 |> data is fresh or no (using If-Modified-Since header).

This works when a HTTP client accesses to a HTTP server via DeleGate
as a HTTP proxy, because the mechanism "If-Modified-Since" (do real
transfer only when the original data is modified since the data is cached) 
is an inherent one in the HTTP protocol to achieve what you (and everyone)
need.  See RFC2616.

 |I'm a newbie , my question is basic but i'm blocked by this .
 |Please help me , i have to defend Delegate's solution in my organisation 
 |and the first step is to responds to this demand (with a ftp - or http - 
 |cache tree structure , how to be sure that the files modifications are 
 |propagating immediately and only this , the others files , not modified 
 |  , reached on the proxy ) .

Unfortunately, there is no such mechanism in FTP protocol as in HTTP
to help caching.  Original protocol does not support method to get
meta-information like file size nor its modified time.  See RFC959.
Recent extensions added "SIZE" (get file size) and "MDTM" (get modified
time).  FTP-proxy DeleGate uses these command to suppress unnecessary
transmission of PLAIN file data.

But for DIRECTORY file, as in your example, this mechanism does not
work.  It is not easy to detect the modification of directory, i.e. if
or not the list of meta-informations of files under a directory is
changed after a copy was made (cached), without comparing the cached
data and current data (which is to be transferred).

  @ @ Yutaka Sato <>
 ( - ) National Institute of Advanced Industrial Science and Technology (AIST)
_<   >_ 1-1-4 Umezono, Tsukuba, Ibaraki, 305-8568 Japan

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