Re: [squid-users] TCP_HIT/504 on fetch after UDP_HIT

I know it's bad form to reply to your own post, but I found a partial
explanation.

from
http://linuxdevcenter.com/pub/a/linux/2001/09/17/squidpeering.html?page=2

FALSE HITS: (ICP only) Because ICP does not communicate request headers
(only the URI is presented in an ICP query), it is possible for a peer
to return an affirmative for a given URI but not be able to satisfy the
request from cache.

      * cache1 sends an ICP query to cache2 for
        http://www.example.org/index.html.
        
      * cache2 has a cached copy of the object (87,376 seconds old), and
        answers in the affirmative.
        
      * cache1 then issues the request to cache2, but the request
        headers contain "Max-Age: 86400"). cache2's copy is too old to
        satisfy this request.
        
      * If cache1 has miss_access on cache2, then cache2 will go forward
        to the origin server (or a parent) and fetch a new copy,
        If not, cache2 will return a 504 HTTP response and cache1 will
        have to select a new source for the object.

But why is it then recommended that miss_access be disabled for
siblings? Would it be a bad thing for cache2 to fetch a new copy ?

As far as I can see cache1 doesn't cache the object retrieved from the
new source after the 504.

On Wed, 2010-03-24 at 13:40 +0100, Taylan Develioglu wrote:
> Hi,
>
> I'm trying to set up two squid siblings in front of lighttpd as reverse
> proxies and have a question about some behavior I'm seeing.
>
> Squid versions are 2.7.STABLE7-1~bpo50+1 from the debian backports
> repository.
>
> My goal is to create a setup with two cache siblings and one origin
> server (lighttpd) where no duplicate cached entries between siblings
> exist.
>
> I am getting these TCP_HIT/504's and I can't understand why they are
> happening.
>
> When I do a refresh in firefox and request a picture from sibling1,
> requested before on sibling2, I see the following:
>
> sibling1 is 5.5.5.5
>
> - The original request on sibling 1:
>
> 1269431848.626 4 1.1.1.1 TCP_MISS/304 378 GET
> http://pictures.something.com/pics/d/3/c/picture.jpg -
> DEFAULT_PARENT/pict-dev image/jpeg
>
> - A UDP_HIT occurs on sibling2, sibling1 tries to fetch the file but it
> received a 504 instead (excerpt below).
>
> 269431848.881 0 5.5.5.5 UDP_HIT/000 76 ICP_QUERY
> http://pictures.something.com/pics/d/3/c/picture.jpg - NONE/- -
> 1269431848.883 1 5.5.5.5 TCP_HIT/504 1752 GET
> http://pictures.something.com/pics/d/3/c/picture.jpg - NONE/- text/html
>
> ----start----
>
> HTTP/1.0 504 Gateway Time-out
> Expires: Wed, 24 Mar 2010 09:55:19 GMT
> X-Squid-Error: ERR_ONLY_IF_CACHED_MISS 0
> Age: 1269424520
> Warning: 113 localhost (squid/3.0.STABLE8) This cache hit is still fresh
> and more than 1 day old
> X-Cache: HIT from localhost
> X-Cache-Lookup: HIT from localhost:80
>
> The requested URL could not be retrieved
> Valid document was not found in the cache and 'only-if-cached'
> directive was specified.
>
> You have issued a request with a 'only-if-cached' cache control
> directive. The document was not found in the cache, or it required
> revalidation prohibited by 'only-if-cached' directive.
>
> ----end----
>
> Why does sibling2 respond with a UDP_HIT but then sends a 504 error page
> to sibling1 when sibling1 tries to fetch the picture?
>
> Is this normal behavior or am I doing something wrong here? Please see
> my squid.conf below, suggestions are very much appreciated and thanks in
> advance.
>
> acl all src all
> acl manager proto cache_object
> acl localhost src 127.0.0.1/32
> acl to_localhost dst 127.0.0.0/8
> acl SSL_ports port 443
> acl Safe_ports port 80 #http
> acl Safe_ports port 21 #ftp
> acl Safe_ports port 443 #https
> acl Safe_ports port 70 #gopher
> acl Safe_ports port 210 #wais
> acl Safe_ports port 1025-65535 #unregistered ports
> acl Safe_ports port 280 #http-mgmt
> acl Safe_ports port 488 #gss-http
> acl Safe_ports port 591 #filemaker
> acl Safe_ports port 777 #multiling http
> acl CONNECT method CONNECT
>
> acl sites dstdomain pictures.something.com
> acl siblings src 6.6.6.6
>
> cache_peer 4.4.4.4 parent 80 0 default no-query no-digest originserver
> name=lighttpd-server login=PASS
> cache_peer 6.6.6.6 sibling 80 9832 proxy-only
> name=sibling2
>
> cache_peer_access lighttpd-server allow sites
> cache_peer_access sibling2 allow sites
>
> http_access allow sites
> http_access allow siblings
> http_access allow manager localhost
> http_access deny manager
> http_access deny !Safe_ports
> http_access deny CONNECT !SSL_ports
> http_access allow localhost
> http_access deny all
>
> icp_access allow siblings
> icp_access deny all
>
> htcp_access deny all
>
> miss_access deny siblings
>
> http_port 80 act-as-origin accel vhost
> icp_port 9832
>
> access_log /var/log/squid/access.log
> debug_options ALL,3
> cache_mgr cachmgr_at_domain.com
>
> cache_dir aufs /var/spool/squid 4000 32 128
>
> refresh_pattern ^ftp: 1440 20% 10080
> refresh_pattern ^gopher: 1440 0% 1440
> refresh_pattern -i (/cgi-bin/|\?) 0 0% 0
> refresh_pattern . 0 20% 4320
>
>
Received on Wed Mar 24 2010 - 14:10:55 MDT