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Re: PROPFIND, VCRs, Label, Depth, href ... oh my!

From: Geoffrey M. Clemm <geoffrey.clemm@rational.com>
Date: Fri, 2 Feb 2001 00:03:04 -0500 (EST)
Message-Id: <200102020503.AAA05902@tantalum.atria.com>
To: ietf-dav-versioning@w3.org

   From: Greg Stein <gstein@lyra.org>

   I've been thinking a bit about PROPFIND and the Label: header. Let's say
   that I do a PROPFIND against a VCR using a Label header:

   PROPFIND /some/vcr
   Label: foo

   response...
   <D:multistatus ...>
     <D:response>
       <D:href>/the/version/resource</D:href>
   ...

   The DAV:href element presumably has the version resource URI because you did
   a PROPFIND against that resource (effectively).

Sounds right.

   Now, let's throw the Depth: header into the mix. What does it apply to? The
   VCR or the Version? It is murky with the Label: header in there.

It wouldn't make any sense to apply it to the version, because a
version is never a collection, but we should make that clear.

   Depth applies to the VCR:
       The PROPFIND will return DAV:response elements for version resources
       corresponding to those selected by the label, from each of the VCRs
       according to the Depth.

Yes.

       DAV:href returns Version Resource URIs. How do we match them up against
       the hierarchy defined by the VCRs?

Well, if your server supports the version history option (which it
should :-), you can do a Depth PROPFIND (without the label header) for the
DAV:version-history property, which would pretty much get you what you need.

   Depth applies to the Version Resource:
       Presumably, you're selecting a collection version. Its children are
       version histories, so you're going to get back whacky properties or
       nothing at all for the children.

Actually, collection versions are no longer collections (they just
have the DAV:version-controlled-binding-set property), so you'd
get back nothing at all, which makes this a not very useful
interpretation.

       The DAV:href probably would exist here, too: which DAV:response is for
       the collection version, and which for the members?
       [ you could tell by looking for errors :-) ]

Yeah, don't go there.

   Okay... let's try to fix the DAV:href thingy by saying you apply the Depth:
   header to the VCRs (i.e. apply Depth to the VCR before linking thru to the
   version resources) and the DAV:href returns the VCRs' URIs. That makes some
   sense, but now we have a problem: how do we get the version resource URI? We
   can't use DAV:checked-in because that is a VCR prop. And, oh darn! We just
   nuked DAV:version.

I think returning the VCR URL in the href would be very misleading.
Those aren't the properties of the VCR, they are the properties
of some totally different version resource.  I'd much rather say
that servers that care about this should implement the DAV:version-history
option.

   My suggestion:

   *) define Depth: on a PROPFIND of a VCR to apply to the VCR hierarchy rather
      than the version resource (lack of) hierarchy

Definitely.

   *) the DAV:href returns the VCR's URI
      [ I'm not sure that I like this, though :-( ]

I'm *sure* I don't like it (:-).

   *) bring back DAV:version

I think the fact that we would need to bring it back is an
indication that putting the VCR's URI in the href was a
bad/misleading thing to do.

   Hmm. I'm thinking that the use of a Label: header is rather spurious to the
   above issue. It does make it a bit harder to figure out the version resource
   URI (without the Label:, you could ask for the VCR's DAV:checked-in; there
   isn't an easy equivalent to find the V.R. when you're trying to use a label)

Note that using the Label header with Depth is basically bogus anyway
when you have versioned collections, because you won't be traversing
the collections that are selected by the label, but rather the
collections that correspond to the current target of the
version-controlled collections.  So I wouldn't torque the protocol
around at all for this.

Bottom line: Labels are for giving a version a human meaningful
name.  Baselines are for capturing a set of versions.  A server can
implement baselines with labels, but a client can't, and shouldn't
try to.

Cheers,
Geoff
Received on Friday, 2 February 2001 00:03:59 GMT

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