hit-for-pass vs. hit-for-miss

Geoffrey Simmons geoff at uplex.de
Wed Sep 7 21:41:30 CEST 2016

This is worth responding to from vacation.

The "specific requirement we have" is a consequence of applying the HTTP protocol the way it was meant to be used -- responses specify their cacheability, without VCL having to intervene to classify which requests go to lookup or pass. Typically with a sequence of regex matched against URL patterns in vcl_recv.

That may indeed be unusual, but I see that a sad commentary on the state of web developers' knowledge about caching and HTTP. Not as somebody's peculiar requirement.

It would strike me as rather odd if a caching proxy has to treat it as a special case when backends actually do something the right way (like always set Cache-Control to determine TTLs).


Sent from my iPhone

> On Sep 7, 2016, at 1:01 PM, Nils Goroll <slink at schokola.de> wrote:
> Hi,
> TL;DR please shout if you think you need the choice between hit-for-pass and
> hit-for-miss.
>> On 02/09/16 20:10, Nils Goroll wrote:
>> - previously:        hit-for-pass
>> - now:        hit-for-miss
> on IRC, phk has suggested that we could bring back hit-for-pass in a vmod *)
> I would like to understand if bringing back hit-for-pass is a specific
> requirement we have (in which case a vmod producing quite some overhead would be
> the right thing to do) or if others have more cases which would justify a
> generic solution in varnish core like this one:
>>   sub vcl_hit {
>>    if (obj.uncacheable) {
>>        if (obj.http.Criterium) {
>>            return (miss);
>>        } else {
>>            return (pass);
>>        }
>>       }
>>  }
> Thank you, Nils
> *) using a secondary cache index (maybe as in the xkey vmod), mark objects we
> want to pass for in backend_response, check in recv if the object is marked and
> return(pass) if so.
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