Re: [#153] PUSH_PROMISE headers

[Roberto Peon <[e-mail]>]

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 <[e-mail]> 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 <[e-mail]> 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 <[e-mail]>
> 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
> >> <[e-mail]> 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).
> >> >
> >>
> >
>