Re: CT Proxies and Forward Caches

Indeed, the use of a "Vary: User-Agent" header generates much more 
entries than a more typical use of Vary such as "Vary: Accept-Language", 
and is thus not a really cache-friendly directive.

The solution Bryan suggested to create representation-specific URIs for 
each UA group, coupled with a redirect response from a canonical 
representation is much better from a cache perspective but it has a 
cost: that of a round-trip between the server and the client to serve 
the redirect response to the representation-specific URI. This solution 
is recommended by the W3C Technical Architecture Group in a finding "On 
Linking Alternative Representations To Enable Discovery And Publishing" [1].

We only mention the use of the "Vary" header in current version of the 
Content Transformation Guidelines document, but we have a long-running 
discussion (internally named ISSUE-222) on the above mentioned TAG 
finding. We may include that possibility in the document as well.

[1] http://www.w3.org/2001/tag/doc/alternatives-discovery.html#id2261672


Sullivan, Bryan wrote:
> Hi Umesh,
> As you mention, meta-group assignment (e.g. good/better/best) is a 
> deployment-specific function, i.e. one Content Provider (CP) may choose 
> a different set of groups and UA assignment as compared to another. 
> Without the direct involvement of the CT proxy in group selection, the 
> only way I see to reduce the cached representations is for the CP to 
> provide a distinct URI to UA's in a group (e.g. a URI parameter or 
> unique path), so the various UA's naturally get served one of a fewer 
> variations of the page from the cache.
>  
> "direct involvement of the CT proxy in group selection" implies some 
> kind of metadata exchange between CP and CT proxy, through which 
> group-related pages can be indicated, and maybe a tighter integration of 
> the CT proxy and cache. Both appear (to me) to be less desirable to 
> standardize, and at least more complex to consider.
>  
> Best regards,
> Bryan Sullivan | AT&T
> ------------------------------------------------------------------------
> *From:* public-bpwg-ct-request@w3.org 
> [mailto:public-bpwg-ct-request@w3.org] *On Behalf Of *Umesh Sirsiwal
> *Sent:* Monday, May 19, 2008 8:12 AM
> *To:* public-bpwg-ct@w3.org
> *Subject:* CT Proxies and Forward Caches
> 
> Several content transformation proxies and the Internet in general 
> includes forward caches. Current definition of HTTP includes indication 
> of transformation using Vary header. In most cases the Content 
> Transformation proxies and servers vary their responses based on 
> User-Agent header. The number of User-Agent string in is very high and 
> caches cannot possibly store these mean copies of the response. Most 
> servers are likely to classify the devices in certain meta-groups for 
> the purpose of content transformation. However, this meta-group is 
> expected to be server specific. In absence of formal method, the caches 
> will be left to guess the meta-group. What will be the method to solve this?
> 
>  
>

Received on Wednesday, 21 May 2008 08:35:18 UTC