[isp-caching] Re: isp-caching digest: January 18, 2003

[Duane Wessels <3695@...]

> - if i have two caches, can they both be setup as sibblings without a parent ?

Yes.

> (What I am after is that if the client request is a cache miss on the
> cache that received the request and also on the sibling, then the requester
> cache goes direct to origin server, since there is no parent)

That is how it works.

When the sibling returns ICP_MISS, the first cache forwards the request
to the origin server.

There is, however, a small chance that the sibling returns ICP_HIT
and, when the first cache forwards the HTTP request to the sibling,
the request becomes a cache miss.  This could happen, for example,
if the client's request has some cache-control directive (such as
min-fresh) that makes the sibling's cached response unusable for
this particular request.

Whenver Squid forwards a request to a sibling, it adds an
'only-if-cached' directive.  So, in the above case, the sibling
would return 504 (gateway timeout) and the first cache would
re-forward the request, this time to the origin server.

> - if a client request is a cache miss, but the sibling has the content (ie a
> cache hit on the sibling), does the requested content go from sibling to
> client ? (What I am after here is that as much as possible cached content is
> not repliacted on both caches, since this would result in a waste of disk
> space)

In this case, the reponse comes from the sibling, through the cache
that originally received the request, then to the client.  By default,
the first cache will store the response as well.

With Squid you can add the 'proxy-only' option to the 'cache_peer'
line.  This option instructs squid to never store responses fetched
from the sibling.