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RE: Label header vs PROPFIND depth 1

From: Julian Reschke <julian.reschke@greenbytes.de>
Date: Tue, 23 Apr 2002 11:41:50 +0200
To: <tim@ellison.name>, "'Deltav WG'" <ietf-dav-versioning@w3.org>
Message-ID: <JIEGINCHMLABHJBIGKBCEEFGEHAA.julian.reschke@greenbytes.de>
Tim,

before going into details, I'd like to repeat that the way the label header
is defined makes the selected revision a variant of the VCR. That's what
RFC2616 says, and IMHO there's no way to avoid that.

If you strongly feel that this is a problem (I may agree), than the label
header should be removed from the spec (deprecated in the issues list), and
we should go back looking at the original requirements it's supposed to
handle.

> From: ietf-dav-versioning-request@w3.org
> [mailto:ietf-dav-versioning-request@w3.org]On Behalf Of Tim Ellison
> Sent: Tuesday, April 23, 2002 10:58 AM
> To: 'Deltav WG'
> Subject: RE: Label header vs PROPFIND depth 1
>
>
> Juliam wrote:
>
> > > Right, so imagine what kind of answer you would get back in the
> > > multistatus -- some would be from the unversioned resource
> propfinds and
> > > some from version propfinds, and you wouldn't know which were
> versioned
> > > (without explictly looking for a live property to distinguishe
> > > them), _and_
> >
> > Yes -- I think this is what RFC3253 says.
> >
> > > it would contravene the 2518 depth result requirement I
> > mentioned earlier.
> >
> > I don't think so.
>
> From your example below I see that you envisage the hrefs being
> those of the
> version-controlled resource (even though the properties come from the
> labelled version).
>
> This is too confusing.  Al the existing code that people write to
> associate
> the href with the properties would have to be re-written as the reported
> properties are from a different resource to the href.

No, they aren't. They are from a specific variant of the VCR (as defined by
HTTP).

> > > a version-controlled resource for which the label doesn't exist?
> > > >
> > > > See section 8.6, the DAV:must-select-version-in-history
> precondition.
> > > > It is an error if you request a label that does not exist on a
> > > given VCR.
> >
> > So assume we have /a (collection, not-versioned), /a/b
> > (not-versioned), /a/c
> > (versioned, having a version labeled "x") and /a/d (versioned,
> > without that
> > label).
> >
> > I think we all agree that
> >
> > GET /a/c
> > Label: x
> >
> > Should return the content of the version selected by the label x
> > (including
> > a vary header and probably a content-location header).
>
> Agreed -- I think we specify a Location: header, but whatever
> (i.e. resource
> location, not content location).

I think content-location is correct:

"The Content-Location entity-header field MAY be used to supply the resource
location for the entity enclosed
in the message when that entity is accessible from a location separate from
the requested resource’s URI. A server
SHOULD provide a Content-Location for the variant corresponding to the
response entity; especially in the
case where a resource has multiple entities associated with it, and those
entities actually have separate locations by
which they might be individually accessed, the server SHOULD provide a
Content-Location for the particular
variant which is returned."

while the RFC says for location:

"The Location response-header field is used to redirect the recipient to a
location other than the Request-URI
for completion of the request or identification of a new resource. For 201
(Created) responses, the Location is that
of the new resource which was created by the request. For 3xx responses, the
location SHOULD indicate the server’s
preferred URI for automatic redirection to the resource. The field value
consists of a single absolute URI."

> > GET /a/d
> > Label: x
> >
> > will return 403 or 409 with an error element (precondition
> > DAV:must-select-version-in-history failed).
>
> Agreed.
>
> > GET /a/b
> > Label: x
> >
> > will just return b's content, and the vary header when present will NOT
> > include "Label".
>
> Can I hedge on the vary: header in the response?  Given that the GET cache
> may be invalidated by the resource /a/b coming under version control, and
> acquiring a label, I may be inclined to believe that we should
> return a vary
> header for every label: request for a version-controlled or versionable
> resource.

I think this is right. Which means that vary: label must be set upon GET on
*any* resource that is versionable (on a server supporting the LABEL
feature).

> > Similarily, similar results will be returned for a PROPFIND/depth
> > 0 on these
> > resources. Note that the response element will be "/a/c" in the
> > first case, even if the properties of a version with a different
> > URI were reported.
>
> No.  I think the href must be for the version resource selected by the
> label, not the version-control resource at the request-URI.

Nope. You can't do that. This breaks PROPFIND. HTTP offers you:

- variants (representations) of the resource, which will have the same URI,
or

- redirects (where the request is redirected to a different URI).

You can't have both.

> > Finally, we have PROPFIND/depth 1 on /a:
> >
> > - as /a isn't version-controlled, the label header is ignored for *this*
> > resource
> >
> > - Geoff and I claim that it *should* apply to the members then,
> > so we'd get:
>
> ...and I say it shouldn't apply to members, otherwise the result will
> contain properties from resources that are not in the same collection
> membership hierarchy, and thereby contravene 2518 (as well as
> complicate the
> result enourmously).

RFC2518:

"Consequently, the multistatus XML element for a collection resource with
member URIs MUST include a response XML element for each member URI of the
collection, to whatever depth was requested. Each response XML element MUST
contain an href XML element that gives the URI of the resource on which the
properties in the prop XML element are defined. Results for a PROPFIND on a
collection resource with internal member URIs are returned as a flat list
whose order of entries is not significant. "
Received on Tuesday, 23 April 2002 05:41:57 GMT

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