Henrik, Thanks so much for your very informative reply! On Thu, Oct 02, 2008 at 12:31:03PM +0200, Henrik Nordstrom wrote: > By default Squid tries to use a parent 10 times before declaring it > dead. Ah, I never would have guessed that I needed to try 10 times before negative_ttl would take effect for a dead host. That wouldn't be bad at all. I just tried this now by having two squids, one a cache_peer parent of the other. I requested a URL while the origin server was up in order to load the cache, with a CC max-age of 180. Both squids have max_stale 0 and negative_ttl of 3 minutes. Next, I put the origin server name as an alias for localhost in /etc/hosts on both machines the squids were on, so they both see connection refused when they try to connect to the origin server. I also restarted nscd and did squid -k reconfigure to make sure the new host name was seen by squid. After the (small) object in the cache expired, I retried the request 20 times in a row. Every time I still saw the request get sent from the child squid to the parent squid and return a 504 error. This is unexpected to me; is it to you, Henrik? I would have thought the 504 error would get cached for three minutes after the tenth try. > Each time Squid retries a request it falls back on the next possible > path for forwarding the request. What that is depends on your > configuration. In normal forwarding without never_direct there usually > never is more than at most two selected active paths: Selected peer if > any + going direct. In accelerator mode or with never_direct more peers > is selected as candidates (one sibling, and all possible parents). > > These retries happens on > > * 504 Gateway Timeout (including local connection failure) > * 502 Bad gateway > > or if retry_on_error is enabled also on > > * 401 Forbidden > * 500 Server Error > * 501 Not Implemented > * 503 Service not available > > Please note that there is a slight name confusion relating to max-stale. > Cache-Control: max-stale is not the same as the squid.conf directive. > > Cache-Control: max-stale=N is a permissive request directive, saying > that responses up to the given staleness is accepted as fresh without > needing a cache validation. It's not defined for responses. > > The squid.conf setting is a restrictive directive, placing an upper > limit on how stale content may be returned if cache validations fail. > > The Cache-Control: stale-if-error response header is equivalent the > squid.conf max-stale setting, and overrides squid.conf. That's very good to know. I didn't see that in the HTTP 1.1 spec, but I see that Mark Nottingham submitted a draft protocol extension with this feature. > The default for stale-if-error if not specified (and squid.conf > max-stale) is infinite. > > Warning headers is not yet implemented by Squid. This is on the todo. Sounds good. - Dave
