Re: Caching responses with "Vary" responses

On 07/19/2012 08:46 PM, Amos Jeffries wrote:
> On 19/07/2012 9:50 p.m., Alexander Komyagin wrote:
>> On Wed, 2012-07-18 at 11:10 -0600, Alex Rousskov wrote:
>>> On 07/17/2012 09:28 AM, Alexander Komyagin wrote:
>
>>>> It seems that on the first HIT we have successfully loaded data from
>>>> the
>>>> swap store, but then we save the data to shared memory. So the next
>>>> request takes data from shm. And it looks like Squid doesn't save
>>>> mem_obj->vary_headers to shm!!
>>> Shared memory cache is meant to be just like a disk cache in this
>>> aspect. It does not know anything about mem_obj->vary_headers and just
>>> packs/unpacks the STORE_META_STD TLV (see StoreMapSlot
>>> src/ipc/StoreMap.h) part of StoreEntry.
>>>
>>> I do not see vary_headers used by any src/fs/* code. Do you know how the
>>> disk cache gets them? Perhaps there is a higher level call to pack and
>>> unpack Vary request headers that the MemStore is missing. We can add
>>> special field/code to StoreMapSlot but I would prefer to see how it is
>>> done for the disk storage first, in hope that we can reuse the same
>>> code.
>>>
>> For the disc cache, STORE_META_VARY_HEADERS is used (see
>> store_swapmeta.cc). Appropriate tlv is added to the list.
>>
>> But for shm I don't see any SwapMeta work! There are some calls that are
>> writing reply headers and body to the internal mem_hdr object and then I
>> can see this object data copied to shm!

The current SwapMeta work is in populating StoreMapSlot with
STORE_META_STD TLV. Clearly, that is not enough to support Vary.

>> In the meantime, those who are in need can use my small dirty hack
>> (squidv3-vary-headers-shm-hack.patch) that packs mem_obj->vary_headers
>> as an additional reply header, so it can be flawlessly saved to shm and
>> recovered afterwards.
>
> Isn't vary_headers just a copy of Vary: reply header anyway? so it is
> already in the response headers once.

AFAIK, you need to store the matching _request_ headers to tell whether
the new request headers are the same.

HTH,

Alex.
Received on Fri Jul 20 2012 - 16:25:41 MDT