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RE: live props on a VCR

From: Lisa Dusseault <lisa@xythos.com>
Date: Mon, 18 Dec 2000 14:51:14 -0800
To: "Geoffrey M. Clemm" <geoffrey.clemm@rational.com>, <ietf-dav-versioning@w3.org>
I hadn't considered this before, but now that I am, I'm not sure it's wise
to send depth headers on the REPORT method.  My three (somewhat related)

1)  Lack of clear model: Although depth makes sense when we're dealing with
well-modelled collections and resources, the model here is less clear.  A
REPORT on a "baseline" may well give a bunch of information on the baseline
and the things the baseline is involved in.  Is that depth infinity?  Does
depth 1 make sense?  A REPORT request could target the whole server (e.g. a
report asking for all orphaned resources).  A REPORT request could target
something more ephemeral like "all baselines in this collection" which is
not the same thing as depth infinity on the collection.  It's not like a GET
or a MKCOL where there's a clear model of the resource being targetted.

2)  Multiple targets of REPORT:  E.g. "merge-preview-report" in deltav draft
10.  What would depth header mean on this request?  The "target" of the
report is not a single resource or collection, but a combination of some
things being merged into other things.  When a combination of things is
specified in the request, the depth header is ambiguous.  It could apply to
any, all, or none.

2)  REPORT on specific kind of resource:  it seems that even when a report
targest only one resource, it's often a resource of a very particular kind.
For example, one could have a report which only made sense for a baseline,
or a report which only made sense for a versioned resource.  If depth: 1 or
depth: infinity were specified, what about items within that depth that
aren't the right kind of resource for this kind of report?

Thus, I'd suggest a good justification for requiring "depth" on REPORT.

If it turns out a depth modifier is required, we should consider whether:
 - it might be better just to define appropriate reports as always being
depth infinity (e.g. "report to me on all orphaned resources" would always
cover an entire server or at least a major section of it)
 - it might be better to include a "depth" modifier _inside_ the report
request body, so that the requester can be very clear about what resources
the depth modifier applies to.

-- Lisa

> -----Original Message-----
> From: ietf-dav-versioning-request@w3.org
> [mailto:ietf-dav-versioning-request@w3.org]On Behalf Of Geoffrey M.
> Clemm
> Sent: Monday, December 18, 2000 12:24 PM
> To: ietf-dav-versioning@w3.org
> Subject: Re: live props on a VCR
>    From: Greg Stein <gstein@lyra.org>
>    On Mon, Dec 18, 2000 at 12:07:39AM -0500, Geoffrey M. Clemm wrote:
>    >    From: Greg Stein <gstein@lyra.org>
>    >    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...
> Actually, REPORT takes a Depth header, so you could get all the
> properties you need in one REPORT request.
> It occurs to me that the current spec needs some more verbiage around
> REPORT with Depth.  It currently just says "if a Depth header is
> specified, a 207 multistatus MUST be returned".  It should say that
> the report results should be marshalled in the "prop" elements of the
> 207 multistatus.  We could define our own 2xx "multi-report" return
> value, but just re-using the 207 seems simpler.
> In fact, if people care, we could extend the REPORT method to allow
> you to request multiple reports at the same time.  Anyone want to
> do that while I'm in there making changes?
>    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
>     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... ]
> Except for the filtering out the collection itself,
> this is what you can do with a Depth header on the REPORT.
> And yes, it is pretty silly that there is no Depth value that says
> "just my immediate children".  That's probably what Depth=1 *should*
> have meant.  Oh well ...
>    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.
> If using the DAV:property report (and Depth) does what you need
> (i.e. get version info in a single simple query),
> it would be simpler to leave all this out of the protocol.
> Cheers,
> Geoff
Received on Monday, 18 December 2000 17:51:42 UTC

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