- From: Roberto Peon <grmocg@gmail.com>
- Date: Fri, 28 Jun 2013 22:36:10 -0700
- To: James M Snell <jasnell@gmail.com>
- Cc: Martin Thomson <martin.thomson@gmail.com>, HTTP Working Group <ietf-http-wg@w3.org>
- Message-ID: <CAP+FsNcEf6s5s7Jk=NLKdrdU8fV1AsSJ4u-8CZNT8P7YXvxkag@mail.gmail.com>
... 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 05:36:37 UTC