Re: "Valid document was not found in the cache" forwarded to clients

[Corentin Delcourt <codl@xxxxxxx>]

On 20/02/15 13:29, Amos Jeffries wrote:

On 20/02/2015 10:55 p.m., Corentin Delcourt wrote:

I run two squid servers, siblings with each other, let's call them A and
B. When clients send requests to A, and A thinks B has the resource
cached, but in reality B doesn't,

Use HTCP protocol between the peers, far less false positives than ICP
or digest.

Will do, thanks for the tip.

the client will get a 504 error with
the message: “Valid document was not found in the cache and
‘only-if-cached’ directive was specified.” Vice versa if clients send
requests to B.

I believe that A should catch this error and fetch the resource
directly, but instead it forwards it to the client, and I cannot figure
out why. I am not using allow-miss.

It should be but there are conditions to that:

* another source of the URL is actually accessible (think IPv6-only
websites, routing outages, DNS lookup failures, Path-MTU failures,
traffic blackholes, etc)

If I re-send a failing request with Cache-Control: no-cache, or if I remove the cache_peer directive, I don't get any errors at all so it doesn't seem like this is the problem here.

* there is time remaining within the forward_timeout for the alterative
route to be contacted. If the timeout occurs the last error state
encountered will be reported, if there is no earlier error the timeout
itself is mentioned.

I believe the default timeout is 4 minutes? I set it explicitely to 10 minutes, and forward-max-tries to 25 for good measure, and requests are still failing within hundreds of milliseconds so I don't think this is related.

* the 504 is not being cached. That error response is one which is
allowed to be cached by proxies if it gets delivered with cacheability
headers. NP: It could be cached and delivered by either proxy, so even
coming from the other one *it* may still have been cached and what
caused the "HIT" to be identified.

I don't think the error is being served from cache. If I look at the access logs for both machines I see:

1424437827.131	6 127.0.0.1 	TCP_MISS/504		4913	GET http://example.net/ - SIBLING_HIT/10.0.1.2	text/html

… on A and

1424437827.882	0 10.0.1.1	TCP_MISS/504		4801	GET http://example.net/ - HIER_NONE/- 		text/html

… on B, 10.0.1.1 being A and 10.0.1.2 being B.

What confuses me even more is that this didn't happen a month ago, with
the same setup.

... what version of Squid?
... and is it the same setup AND version(s) of Squid from a month ago?

I'm running Squid 3.5.2 on both servers. I was running 3.5.1 on both a week ago and observing the same problem.

I can't say for sure which version I was running before this started happening, but I believe it was 3.4.8 on one machine and 3.4.10 on the other.

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