Re: Transparent Content Negotiation and Caching

David Röthlisberger wrote:

> On 21 Oct 2012, at 17:28, Urs Holzer wrote:
>> I'd like to implement Transparent Content Negotiation as described in
>> RFC 2295. Is this a good idea?
> 
> I'm no expert on HTTP, but it seems to me that you would first have to
> implement normal HTTP 1.1 caching of negotiated responses.[1] (See my
> analysis of polipo's current behaviour below.)
> 
> Out of curiosity: What is the status / adoption of RFC 2295? Apache
> calls it an "experimental protocol", and "supports 'transparent'
> content negotiation, [but] does not support [RFC 2295's] 'feature
> negotiation'."[2]
> 
> [1] http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.6
> [2] http://httpd.apache.org/docs/2.4/content-negotiation.html#about

As far as I have found out, it is still experimental. It has never been 
adopted broadly. 

> 
>> I have not yet worked a lot with C, so it
>> would take some time for me to try to implement it.
> 
> The main concern would be not to break existing polipo functionality,
> and the best way to be sure of that is an automated test suite. Before
> making any code changes, I would start with figuring out a way to run
> curl's test suite[3] against polipo. It shouldn't be too difficult;
> I've managed to run a couple of curl's tests against polipo before.[4]
> 
> [3] https://github.com/bagder/curl/tree/master/tests
> [4] http://permalink.gmane.org/gmane.comp.web.polipo.user/2899
> 
> 
>> As I understand it, Polipo caches only one variant for a negotiated
>> resource and serves this one based on the Vary, Content-* and
>> Accept-* headers. Is this correct?
> 
> I take it that you have read what the polipo manual has to say about
> the Vary header.
> 
> Grepping through the sources of polipo reveals the following (all line
> numbers are for revision ec4d6385 on the master branch):
> 
> * The HTTP parser sets cache_control.flags |= CACHE_VARY when the
>   response contains a Vary header (http_parse.c:1206). I couldn't see
>   polipo checking the _value_ of the Vary header anywhere, so it
>   clearly doesn't cache different variation of the response to fulfil
>   subsequent requests based on the new request's Accept-* headers.
> 
> * objectMustRevalidate (object.c) returns true if CACHE_VARY is set,
>   unless you configure mindlesslyCacheVary=true.
> 
> * If the response had CACHE_VARY and an Etag, polipo can serve from
>   cache but first validates with a "conditional" GET
>   (client.c:1244,1252). A "conditional" GET is a GET request with
>   If-Modified-Since / If-None-Match headers (server.c:1661).
> 
> * If the response had CACHE_VARY but no Etag, polipo sets
> CACHE_MISMATCH
>   (http.c:1053). CACHE_MISMATCH forces a full GET request, as opposed
>   to a conditional GET (client.c:1247).
> 
> * There are additional CACHE_VARY checks in validateEntry
> (diskcache.c)
>   and httpTweakCacheability (http.c) that I haven't read through.
> 
> (Superficially at least, it seems there is some redundancy in the
> above checks. But I didn't really study the source code in any depth.)
> 
> To better understand the current behaviour you could also try enabling
> logging levels L_VARY, D_SERVER_REQ and D_CLIENT_REQ (the last two
> require modifying LOGGING_MAX in log.h and recompiling) and run some
> manual tests of your own.
> 
> 
> Hope this helps in some way,
> Dave.
> 
> 
> 
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