Re: What is The logic of Vary Headers cachiness?

ons 2013-07-24 klockan 18:46 -0600 skrev Alex Rousskov:
>
> > The full logics (still missing from squid-3) involves an n-m map of
> > requests to response variants.
>
> I thought Squid already tries to support that. Or are you talking about
> responses with varying Vary header values (discussed further below)?

Squid-2 does, but Squid-3 only keeps a 1-1 map.

> > This has direct impact on where Vary logics can be performed.
>
> Can you elaborate on this point? I think there are ways to support full
> Vary logics with and without "all variants are in one store" restrictions.

It has an impact on how many store api accesses is needed to resolve a
request for a varying URL.

> > * Add mapping of request headers to specific response variant
> > (identified by ETag).
>
> AFAIK, this is already supported by adjusting the store key.

No, there is no way to query the store which ETags are known for a URL.
All it knows is that the URL response varies.

> > * Lookup the response matching this specific request.
>
> This is already supported by looking up the computed key in the store index.

Sort of. That lookup is today based on URL + response headers which is
correct for objects with no ETag.

> > To complicate matters further different responses MAY have different
> > Vary header value. But it is not very likely.
>
> True. To support that, we would need to add a list of Vary header values
> to the special Vary object and iterate it, looking up a cache object for
> every listed Vary header value.

It's a little more complicated than that.

> > Or better yet allow the same response body to be
> > referenced by multiple responses (simplifies validation logics and aging
> > considerably). Response headers is the original response updated with
> > header data from the 304 response in response to If-None-Match.
>
> It feels like this is kind of separate optimization. It can be
> restricted to Vary transactions, but does not have to be.

I don't think it's separate. It has direct impact on how you can store
responses to a varying URL and how to handle cache validation.

Regards
Henrik
Received on Thu Jul 25 2013 - 06:33:19 MDT