On 23/09/2013 7:21 p.m., Ron Klein wrote:
> My example of favicons was to simplify the question. The real case is
> different.
Then please tell us the real details. In full if possible.
favicon is one of the special-case type of URLs and like Eliezer and I
already mentioned there are some specific usage for them which directly
causes problems with your stated goals or even using it as a simplified
test case. Perhapse your real case is also using similar special-case
URLs with other problems - but nobody can assist with that if you hide
details.
So please at least avoid "favicon" references for the remainder of this
discussion. You have indicated that they are irrelevant.
> I want to cache all "favicons" (that is, other resources, internally
> used) for 60 minutes.
> For a given "favicon", I'd like to have the following caching policy:
Anywho, ignoring all the protocol and UA special-case behaviour factoids
because you said that was a fake example...
> The period of 60 minutes should start when the first consumer consumes
> the favicon. Let's mark the time for that first request as T0 (T Zero).
Your policy assumes and requires that your proxy is the only one between
users and the origin server. If your upstream at any stage have a proxy
the object age will not meet your T0 criterion - this is why
Last-Modified and Age headers are used in HTTP. To indicate an objects
time since creation regardless of whether the object might have been
newely generated by the origin, altered by an intermediary or stored for
some time by an intermediary or the origin itself (server-side caching
or static archive).
FWIW: I am working with a client at present who want to do this type of
caching for every URL in existence, but only for a few minutes. They
have a growing list of domain names where the policy has to be disabled
due to problems it causes to user traffic.
> During T0 until T0+60minutes, this favicon should be considered as
> "fresh", in terms of caching.
The single value of 60 in the refresh_pattern line "max" field along
with override-expire override-lastmod meets the above criteria.
However as I said earlier, freshness does not guarantee a HIT. There are
many other HTTP features which need to be considered on top of that
freshness to determine whether it HITs or MISSes.
> After T0+60minutes, this favicon should be considered as "stale", in
> terms of caching, and should be re-fetched by Squid, upon request.
There is no such thing as a refetch in HTTP caching.
There is only MISS or REFRESH. The revalidation may happen transparently
at any time and you never see it.
> The favicon would be cached even if the original server explicitly
> instructed not to cache nor store the favicon.
The refresh_pattern ignore-private and ignore-no-store meet that
criteria in a way. The object result from the current transaction will
be left in the cache regardless of what might happen to it on any future
or past ones.
> Yes, I know it might be considered a bad practice,
As stated your caching policy is not particularly bad. The use/need of
ignore-private and ignore-no-store is the only bad thing and the strong
sign that you are possibly violating some law...
> and perhaps illegal to some readers,
... so consulting a lawyer is recommended.
We provide those controls in Squid for specific use-cases. Yours may or
may not be one of those it is hard to tell from a fake example.
> but I assure you that the other servers (the real web servers) that
> provide the responses, are business partners and they gave me their
> approval to override their caching policy. However, they don't want to
> change their configuration and it's totally up to me to create my
> caching layer.
They may not be willing to alter their public cache controls, but
Surrogate-Control features available in Squid offer an alternative
targeted caching policy to be emitted by their servers for your proxy.
This assumes they are willing to setup such alternative policy and you
configure your proxy as a reverse-proxy for their traffic.
Your whole problem would be solved by the upstream simply sending:
Surrogate-Control: max-age=3600;your_proxy_fqdn
> And another thing: the clients are not web browsers. The clients
> consuming these resources ("favicons" for sake of simplicity) are
> software components using HTTP as their transport protocol.
>
> Thanks for any advice on the subject.
Well...
you have a set of URLs with undefined behaviour differences from the
notably special-case ones in your example ...
being fetched by clients with undefined but very big behaviour
differences from the UA which would be fetching your example URLs ...
... and you want us to help with specific details about why your config
is not working as expected?
As the old cliche goes "insufficient data".
Amos
Received on Mon Sep 23 2013 - 08:21:09 MDT
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