[H-GEN] Squid cache selection

[Bruce Campbell]

[ Humbug *General* list - semi-serious discussions about Humbug and ]
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On Thu, 24 Feb 2000, Michael Anthon wrote:

mca> I set this up yesterday and I'm a bit confused as to why it didn't work the
mca> way I expected it to...
mca> 
mca> cache_peer proxy.provider1.com.au parent 3128 3130 no-query weight=2
mca> cache_peer proxy.provider2.com.au parent 3128 3130 no-query weight=1
mca> 
mca> I then disconnected the link to provider1 to see if it would
mca> then get pages from provider2, and it didn't.  All I got was a
mca> page from squid with something like "(101) network unreachable",
mca> which was perfectly correct, but not what I had expected.  I had
mca> expected squid to then attempt the proxy from provider2.
mca> 
mca> Can anyone point out what I may have done wrong?  Or is there a different
mca> setting somewhere I have missed?

The 'no-query' options does, for the most part, what it says.  

Without 'no-query', Squid will send a quick ICP query using UDP to its
parent or sibling caches.  This ICP query has two functions, it sees
whether a given remote server has the object requested, and whether the
remote server is there.

There are two replies to an ICP query, a definitive "I've got
it" (ICP_HIT) or a "I ain't got it" (ICP_MISS) message.

Squid will open a TCP connection to the cache that replies first with
an ICP_HIT, +/- weight loadings, and then try to retrieve the object.

If an ICP_MISS is received, Squid will not attempt a TCP connection to
that parent/sibling cache (unless it is the only one available because of
other rulesets).  If *no* ICP response is received from a given cache (or
host unreachable) Squid will prefer one that does respond.

Of course, if no-query is turned on, Squid is not receiving immediate
feedback, and naturally assumes that a given cache is still there, and
blindly opens a TCP connection to that cache.  If it happens to try a
cache which has a downed link, then...  (luck of the draw)

Remove no-query.  Check also the exact semantics of the 'weight' command.

Note that the behaviour of Squid without any specific tcp_outgoing_address
statements and differing IP addresses of the squid machines between
providers is undefined (Squid is designed for Servers.  Servers generally
don't change IP addresses every 5 minutes ;) )

-- 
  Bruce.                                                         | VP/BOFH
  Infinite Monkeys.  Infinite Keyboards.  I see no Shakespeare.  |  HUMBUG



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