- From: Amos Jeffries <squid3@treenet.co.nz>
- Date: Sat, 29 Jun 2013 20:00:21 +1200
- To: ietf-http-wg@w3.org
On 29/06/2013 5:44 p.m., James M Snell wrote: > I just gave an example that uses content-negotiation so that argument > doesn't really work. The server is crafting the request headers for an > implied GET, so it gets to construct those request headers to be as > specific as it wants them to be, without much need for actual > content-negotiation. > > For example, suppose the server receives a request for an HTML page like this... > > GET /index.html HTTP/1.1 > Accept: text/html, image/jpeg, image/gif > > The server decides that it wants to push jpeg files to the client... it sends > > PUSH_PROMISE > :path = /images/f.jpg > :method = GET > :host: example.org > :if-match: "my-etag1" > accept: image/jpeg > > That's pretty darn unambiguous. I can easily check to see if I have a > representation of "/images/f.jpg" with etag "my-etag1" and > content-type "image/jpeg" in my local cache, without the need to have > *any* response headers in the PUSH_PROMISE. > > Perhaps you have another, more specific example in mind? The above example is a good one. The cache will store the pushed response under: hash(URL)+hash(Vary:"image/jpeg") When looking up future HIT the cache for this client on explicit-GET it will look up hash(URL)+hash(Vary:"text/html, image/jpeg, image/gif") I assume you can spot the difference. Middleware which looks up its cache on future PUSH_PROMISE will also find the does the hash(URL)+hash(Vary:"image/jpeg") version and probably RST_STREAM for teh redundant data. Other clients will however still MISS on that object, and may even MISS on the second version above if they for example drop text/html on their explicit-GET for images. ... PUSH_PROMISE will only work on middleware which supports the proposed Key header. It will not work easily with Vary as you seem to think. Amos > On Fri, Jun 28, 2013 at 10:36 PM, Roberto Peon <grmocg@gmail.com> wrote: >> ... because by then you've opened up a stream., and you're back into >> problematic territorry. >> PUSH_PROMISE exists because we need to indicate to the browser all of the >> information it needs to make a determination about whether or not it wants >> the stream (and to short circuit the inlining/push mechanism when it already >> has what it needs!) >> -=R >> >> >> On Fri, Jun 28, 2013 at 10:34 PM, Roberto Peon <grmocg@gmail.com> wrote: >>> Any content negotiation would be an appropriate example. :) >>> >>> You don't want to have to wait for the HEADERS frame to indicate to the >>> client which resource it might already have (it should have the opportunity >>> to RST_STREAM if it has it in cache, for instance). >>> -=R >>> >>> >>> On Fri, Jun 28, 2013 at 10:25 PM, James M Snell <jasnell@gmail.com> wrote: >>>> Have an example handy? >>>> >>>> Here's an example that shows that response headers in the PUSH_PROMISE >>>> would not be necessary... Let's say I send a PUSH_PROMISE with the >>>> following bits of info... >>>> >>>> PUSH_PROMISE >>>> :path = /images/f.jpg >>>> :method = GET >>>> :host = example.org >>>> :scheme = http >>>> accept = image/jpeg >>>> if-match: "my-etag1" >>>> cache-control: max-age=1000 >>>> >>>> These headers are giving me everything I would need to determine if >>>> there is a matching resource in my local cache. I have the method, I >>>> have the etag, I have the cache-control parameters, accept... There's >>>> no need for response headers at this point. >>>> >>>> Later, once I start accepting the frames for the pushed content, I >>>> would get something like... >>>> >>>> HEADERS >>>> :status = 200 >>>> content-type: image/jpeg >>>> content-length: 123 >>>> etag: "my-etag1" >>>> vary: accept >>>> cache-control: public >>>> >>>> On the off chance that the PUSH_PROMISE doesn't give me what I need, >>>> the follow on HEADERS frame will give me the rest. >>>> >>>> >>>> >>>> On Fri, Jun 28, 2013 at 9:55 PM, Roberto Peon <grmocg@gmail.com> wrote: >>>>> Depending on how the request might have been been constructed, response >>>>> headers may be necessary to identify the resource in the cache, as >>>>> compared >>>>> to the resource specified in the HTML (I'm thinking about vary: stuff). >>>>> >>>>> -=R >>>>> >>>>> >>>>> On Fri, Jun 28, 2013 at 9:44 PM, James M Snell <jasnell@gmail.com> >>>>> wrote: >>>>>> Let's take a step back and consider what a pushed stream is... >>>>>> >>>>>> A pushed stream is essentially an "Implied GET". This means that a >>>>>> server is going to assume that the client was going to send a GET for >>>>>> the pushed resource. This also means that the server has to make some >>>>>> assumptions about the make up of that implied GET. >>>>>> >>>>>> Now, consider how HTTP caching works. When a cache receives a request >>>>>> for a resource, how does it determine whether or not it has a >>>>>> representation of the resource already available? Does it look at the >>>>>> request headers or the response headers? Obviously, it looks at the >>>>>> request headers. It uses the response headers when populating the >>>>>> cache. >>>>>> >>>>>> So, if we look at the pushed resource sent by the server, what we need >>>>>> is for A) the server to first let us know about the implied GET >>>>>> request.. which means pushing down a set of request headers then B) >>>>>> the server to send the actual resource, which means pushing down the >>>>>> response headers. >>>>>> >>>>>> Already in our design for pushed resources, we have the server sending >>>>>> a PUSH_PROMISE frame that contains a header block, followed by a >>>>>> HEADERS frame that also contains a headers block. It stands to reason >>>>>> that the PUSH_PROMISE frame would contain the set of request headers >>>>>> that the server is assuming for the implied GET. These are delivered >>>>>> to the client, which uses those to determine whether or not a cached >>>>>> representation of the resource is already available (just as any cache >>>>>> would do using the request headers). The server would then send it's >>>>>> response headers in a HEADERS frame, just as it would any response to >>>>>> any other kind of GET. >>>>>> >>>>>> Two examples to show how this naturally fits... First, let's look at a >>>>>> normal GET request sent by the client to the server... >>>>>> >>>>>> Client Server >>>>>> ------ ------ >>>>>> | | >>>>>> | ---------------------> | >>>>>> | HEADERS | >>>>>> | GET | >>>>>> | /images/f.jpg | >>>>>> | If-Match: etag1 | >>>>>> | Accept: image/jpeg | >>>>>> | | >>>>>> | <--------------------- | >>>>>> | HEADERS | >>>>>> | 200 | >>>>>> | Content-Type: | >>>>>> | image/jpeg | >>>>>> | Content-Length: | >>>>>> | 123 | >>>>>> | | >>>>>> | <--------------------- | >>>>>> | DATA....DATA.... | >>>>>> | | >>>>>> >>>>>> Now consider the same resource being pushed by the server using >>>>>> PUSH_PROMISE... >>>>>> >>>>>> Client Server >>>>>> ------ ------ >>>>>> | | >>>>>> | <--------------------- | >>>>>> | PUSH_PROMISE | >>>>>> | GET | >>>>>> | /images/f.jpg | >>>>>> | If-Match: etag1 | >>>>>> | Accept: image/jpeg | >>>>>> | | >>>>>> | <--------------------- | >>>>>> | HEADERS | >>>>>> | 200 | >>>>>> | Content-Type: | >>>>>> | image/jpeg | >>>>>> | Content-Length: | >>>>>> | 123 | >>>>>> | | >>>>>> | <--------------------- | >>>>>> | DATA....DATA.... | >>>>>> | | >>>>>> >>>>>> >>>>>> Note that the only difference here is the direction and type of the >>>>>> first frame. Everything else is identical. The PUSH_PROMISE contains >>>>>> everything the client needs to determine whether or not it already has >>>>>> the resource in it's local cache (request URI, etag, content-type...). >>>>>> >>>>>> There's no need to get any more complicated than this. We already >>>>>> require two distinct header blocks for every request. We already send >>>>>> two distinct header blocks for each pushed stream. We already indicate >>>>>> that a pushed stream is an implied GET. To make it work, we simply >>>>>> state that the PUSH_PROMISE contains the Request headers that the >>>>>> server has assumed for the implied GET request, while the HEADERS >>>>>> frame sent later contains the Response headers. If the request headers >>>>>> in the PUSH_PROMISE end up not being adequate enough to properly >>>>>> determine if the resource is already cached, then we treat it as just >>>>>> another cache miss. >>>>>> >>>>>> On Fri, Jun 28, 2013 at 5:21 PM, Martin Thomson >>>>>> <martin.thomson@gmail.com> wrote: >>>>>>> https://github.com/http2/http2-spec/issues/153 >>>>>>> >>>>>>> The current text describes PUSH_PROMISE as having a few request >>>>>>> headers, plus some response headers, but it's quite vague. >>>>>>> >>>>>>> I think that if this is going to be properly workable across a wide >>>>>>> range of uses with lots of different headers, PUSH_PROMISE needs to >>>>>>> include two sets of headers: the ones that it overrides from the >>>>>>> associated request (:path being foremost of those) and the ones that >>>>>>> it provides as a "preview" of the response (e.g., ETag might allow >>>>>>> caches to determine if they were interested in the rest of the >>>>>>> response). >>>>>>> >>>
Received on Saturday, 29 June 2013 08:00:58 UTC