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Re: live props on a VCR

From: Greg Stein <gstein@lyra.org>
Date: Mon, 18 Dec 2000 06:48:25 -0800
To: ietf-dav-versioning@w3.org
Message-ID: <20001218064825.C30887@lyra.org>
On Mon, Dec 18, 2000 at 12:07:39AM -0500, Geoffrey M. Clemm wrote:
> 
>    From: Greg Stein <gstein@lyra.org>
> 
>    The spec is pretty clear in 2.1.3 that dead properties of a version resource
>    must also be exposed on the VCR. It also says that live props may differ.
>    But it would be good to know if a VR's live props must also be exposed on
>    the VCR.
> 
>    Specifically, I want to get the DAV:version-name from the VCR (rather than
>    issue a second PROPFIND against the DAV:target of the VCR).
> 
> You could just use the property report to avoid the extra roundtrip, i.e. 
> to get the version-name of the target of a vcr:
> 
> REPORT /whatever HTTP/1.1
> ... some headers ...
> <D:property-report xmlns:D="DAV:">
>   <D:target>
>     <D:version-name/>
>   </D:target>
> </D:property-report>

hehe... that will be even worse. The PROPFIND that I'm using is a Depth:1
request. I'm fetching the props for a whole bunch of resources. If I
switched to a REPORT, then I'd need to do a PROPFIND on a collection to get
all the children, then a PROPFIND/REPORT to link thru the target to get the
props off the version resource. Blarg...

What would be neat is a collection property providing the URLs of its
members. You could then do a property-report kind of like:

  REPORT /the-collection HTTP/1.1
  ... some headers ...
  <D:property-report xmlns:D="DAV:">
    <D:member-set>
      <D:target>
        <D:version-name/>
      </D:target>
    <D:/member-set>
  </D:property-report>


:-)

(this would also be nice in that I wouldn't have to filter out the PROPFIND
 result for the collection itself... I'd only get the internal members)

[ and I'm not truly suggesting this new property, but it is certainly an
  interesting item to ponder... ]

>    I don't see anything in the spec about exposing VR live props thru the VCR
>    (I could have easily missed it so far). I'd like to state that they *are*
>    exposed.
> 
> In general, the VCR live properties can differ from those of the
> version identified by its DAV:target (e.g. creationdate,
> getmodificationdate, etag, etc).  Currently, a server can implement a
> VCR PROPFIND without traversing over to the target (which in a
> distributed versioning system could exist on a different server).
> There are some live properties (such as DAV:version-name and
> DAV:version-history) that could be cached on the VCR because they only
> change when you change the DAV:target, but I am concerned that this
> blurring of the distinction between a VCR and the version identified
> by the DAV:target of the VCR is likely to confuse implementers into
> thinking that all the live properties of the VCR and the DAV:target
> version have the same value.

Well, the simple issue in my mind is "I've got this (VC) resource here, but
I don't know what it is [without the version-name]." There is a non-trivial
amount of work to go and fetch that version-name; if it had simply been
exposed via the VCR, then I'd be fine.

I'd think the following properties are "important" to have appear on the
VCR, and a statement that they MUST be the same:

  DAV:version-name
  DAV:checkin-date
  DAV:comment             (*)
  DAV:creator-displayname (*)
  DAV:version-history
  DAV:version             (+)

(*) actually, the VCR could define these itself, so these might be confusing
to have appear on the VCR.
(+) this is unchanging, but is just a copy of the DAV:target prop  (??)

There are some informational properties: checkin-fork, checkout-fork, and
mutable; I don't have any feelings about them. I'm also not going to suggest
the *-set properties, since they could be computed and will change from time
to time.

The properties above are all (typically) immutable. It would be nice to have
a MUST for these, and then a SHOULD for others (or even punt the others).
Punting all others would satisfy Geoff's concern about people blurring the
distinction between the resources. Avoiding the *-set properties will also
help out with the remote system issue.

Yet, *having* these can provide great benefits to the client. Consider the
version-history property: a client can then quickly link to the history and
gather information. The version-name and checkin-date provides useful
information about what you're looking at. (the comment/creator-displayname
are handy, but not terribly so -- they only talk about the last change
rather than anything intrinsic about the resource).

Cheers,
-g

-- 
Greg Stein, http://www.lyra.org/
Received on Monday, 18 December 2000 09:44:51 GMT

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