Subject: RE: RE : Taxonomy draft - web replication problems

From: michael rabinovich <[email-masked]>
Date: Wed, 25 Aug 1999 11:22:03 -0400 (EDT)

> From [email-masked] Wed Aug 25 03:46 EDT 1999
> 
> >As for the distinction between caching and replication, I define 
> >caching as creating a copy of an object at a point that would see 
> >requests for the object regardless of whether or not it has a copy.  
> >Examples: main memory cache on the browser, a proxy cache,
> >main memory cache at the server.  That is, caching creates copies 
> >along the normal path of request and data flow.
> 
> Normal path for a request needs to be clarified :
> For instance transparent caching modifies the normal path of a request.
> Also, when using ICP with siblings, the request may be redirected to a
> sibling cache 
> containing the requested URL, thus modifying the normal path.
> 

Yes, clarification is in order, since one more person had essentially
the same comment.  

 Given the set-up, the normal (i.e., common-case) data path
 does go through the cache in your transparent caching example.  
 It's just like with any other cache - it
 you  do not have it the data does not flow through it.  So, we should
 consider the given environment (in your example, that includes the 
 redirector and the node that receives the redirects).  Given the 
 environment, we ask ourselves whether the redirects recipient is a 
 cache or a replica server, and it is a cache because it is on the 
 normal path of requests: requests arrive at it whther or not it
 has the requested object.
 
 As for siblings, they are on the normal path for *some* requests,
 and they acquire their copies as the result of these requests (this
 is another clarification!).  Thus  they are caches.

 Misha

---

From: George Michaelson <[email-masked]>
Date: Thu, 26 Aug 1999 09:29:29 +1000

Other point: mirrors usually have a legal and published URL in their own
right, irrespective of any synthesized mapping from URI-space to their
instance of the cloned data. 

	http://mirror.aarnet.edu.au/FreeBSD is a documented published
	URL to reach the same resources as http://www.freebsd.org/

Caches do not exist as terminal objects in themselves (for their content
that is) in the same way.

I think this distinction matters because in compiling the set of rewrite
rules for redirection in caches, to supplement any inter-cache ICPlike
behaviour, you need a complete set of the known URL of the mirrors to
make a complete set of the possible (and hence derive the preferential)
rewrite rule to effect.

cheers
	-George
--
George Michaelson         |  DSTC Pty Ltd
Email: [email-masked]    |  University of Qld 4072
Phone: +61 7 3365 4310    |  Australia
  Fax: +61 7 3365 4311    |  http://www.dstc.edu.au