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RE: HTTP/2 Stream Priority and Dependency

From: Lucas Pardue <Lucas.Pardue_at_bbc.co.uk>
Date: Fri, 5 Jun 2015 12:43:49 +0000

´╗┐Hi Daniel,

> (HTTP/2 Server push works pretty good now:
> http://daniel.haxx.se/blog/2015/06/03/server-push-to-curl/)
>
> I've moved further and started to think about how to set and control HTTP/2
> stream priority and dependencies.

Thanks for keeping up the good work on HTTP/2.

The suggestion looks good on first inspection. I had a questions without much time to think of answers sorry:

1) What about the "post-transfer" state? Should Section 6.3 of the spec says

"The PRIORITY frame can be sent for a stream in the "idle" or "closed" state. This allows for the reprioritization of a group of dependent streams by altering the priority of an unused or closed parent stream."

It seems a bit horrible to have to keep "old" easy handles laying around in order to support this possibility.

2) CURLOPT_STREAM_DEPENDS_ON validation could be tightened up to return error if attempting to depend on self. Maybe it already does, in which case it would be good to document.

"A stream cannot depend on itself. An endpoint MUST treat this as a stream error (Section 5.4.2) of type PROTOCOL_ERROR."

3) How about dependency exclusivity? If someone inserts an exclusive dependency into an existing tree it will change the parent stream of the existing dependents. Figure 4 in section 5.3.1 shows an example. Should there be a way for the user to interrogate the dependency parent of a handle, or to inform them of changes via a callback?

4) With the server push API using the term "parent" things could get a bit confusing. It is more like "push parent" and "dependency parent" but that sounds a bit ugly.

Kind Regards
Lucas

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Received on 2015-06-05