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Re: svn commit: r1153493 - /subversion/branches/1.7.x/STATUS

From: Daniel Shahaf <d.s_at_daniel.shahaf.name>
Date: Thu, 4 Aug 2011 23:04:29 +0300

C. Michael Pilato wrote on Thu, Aug 04, 2011 at 10:57:46 -0400:
> On 08/03/2011 10:40 AM, Daniel Shahaf wrote:
> > cmpilato_at_apache.org wrote on Wed, Aug 03, 2011 at 14:00:39 -0000:
> >> + (As an aside, Serf's potential as a platform for future
> >> + improvement remains unproven and doubtful. For example, HTTPv2
> >> + removes canonical resource URLs, which works against the caching
> >> + proxy concept that seems to be the strongest argument in favor of
> >> + Serf's approach. But that's not strictly germane here.)
> >
> > That sounds odd for caching not to be taken into consideration in
> > HTTPv2's design. And glancing at the httpv2 design notes suggests that
> > it was explicitly a goal.
> >
> > Are you saying that somehow HTTPv2 actually made the cacheability
> > situation worse in some cases? Or just that it doesn't make the
> > situation as as good as it promised to?
>
> As I dig into this a bit, I realize that the situation isn't quite as bad as
> I originally thought. But that's only because of a coding oversight on my
> part. (Happy accident?) I'll explain below. Note that I'm assuming that
> cacheability works best when the RA layers use a single canonical URL to
> fetch a given resource.
>
> Let's first talk about the cost of addressing any particular server
> node_at_revision resource. In HTTPv1, clients couldn't just calculate the URL
> of a resource -- they had to negotiate with the server using WebDAV/DeltaV
> abstractions. Multiple roundtrips per calculation ... performance shutdown
> ... you get the picture. mod_dav_svn helps here by transmitting in its
> update-style REPORT responses a "version resource URL", which the client
> caches via the davprops store in the working copy to avoid future costly
> lookups. HTTPv2 facilitates client-side construction of resource URLs
> without server negotiation, therefore has no need of the davprops persistent
> cache mechanism, and as such the code doesn't use the davprops stuff at all
> when HTTPv2 is active.
>
> The second factor of interest here is the canonical URL issue. If I have a
> file that was created in revision 1 and remains unchanged henceforward, a
> client can address that file via any number of URLs. After all, file_at_1 ==
> file_at_2 == file_at_3 == ..., right? mod_dav_svn again tries to help here by
> normalizing the version resource URL that it sends to the client for a given
> resource based on the created-path and created-rev of the resource. So no
> matter which version of our file we're talking about, mod_dav_svn will
> report its versioned resource URL as:
>
> .../!svn/ver/<CREATED-REV>/<CREATED-PATH>
>
> Here's where I think the current code falls short. While the update process
> still pays attention to the canonical version resource URL transmitted by
> the server (that was the happy accident ... ra_serf *could* be ignoring that
> today in favor of self-constructed URLs), that URL isn't cached in the WC
> any longer. This means that future (non-update-style) operations performed
> by the client will be addressing the resources by some self-constructed,
> probably-non-canonical URL. Stuff still works, of course, but this eats at
> the cache-friendliness.
>
> Does that help to explain things better?
>

Yes; the problem is that some client-initiated requests do not
canonicalize a node's identification to its created-path_at_created-rev,
and consequently some[1] opportunities for caching are missed.

<handwaving>I suppose that could be remedied --- eg, if the wc started
caching those created-* coordinates.</handwaving>

Thanks for the explanation,

Daniel

[1] Not sure what fraction of opportunities that 'some' is.

> -- C-Mike
>
> [SIDEBAR: It just occurred to me that the server is still transmitting
> HTTPv1-style version resource URLs, not HTTPv2-style URLs ... I guess that's
> a separate issue, though.]
>
> --
> C. Michael Pilato <cmpilato_at_collab.net>
> CollabNet <> www.collab.net <> Distributed Development On Demand
>
Received on 2011-08-04 22:05:10 CEST

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