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Re: Label header vs PROPFIND depth 1

From: Stefan Eissing <stefan.eissing@greenbytes.de>
Date: Wed, 24 Apr 2002 16:58:51 +0200
Cc: "'Deltav WG'" <ietf-dav-versioning@w3.org>
To: <tim@ellison.name>
Message-Id: <CA085454-5793-11D6-9959-00039384827E@greenbytes.de>

Am Dienstag den, 23. April 2002, um 12:16, schrieb Tim Ellison:

> I agree that there is no point in getting into the details unless 
> we agree
> whether a version is a variant of a checked-in version-controlled 
> resource.
> Once we get agreement on that we can go through the remainder of 
> the issues
> and use that as a guidepost.
>
> So, you think it is a variant, and I think it isn't.

The result of a GET has to be cacheable by HTTP proxies. For the
LABEL header to be compliant with GET, it has to select a variant
(as variant in rfc2616) of the resource and declare so in the Vary
header.

I think there is no way around it without breaking GET and I hear
that Roy Fielding has got a big knife and is after the SOAP guys for
related matters...

I think LABEL has to be rethought.

//Stefan

> Anybody else like to chip-in?  (anybody else following this or has 
> everyone
> else gone home<g>?)
>
> Regards,
> Tim
>
>> -----Original Message-----
>> From: Julian Reschke [mailto:julian.reschke@greenbytes.de]
>> Sent: 23 April 2002 10:42
>> To: tim@ellison.name; 'Deltav WG'
>> Subject: RE: Label header vs PROPFIND depth 1
>>
>>
>> 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 resourceís 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 serverís
>> 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 Wednesday, 24 April 2002 10:58:55 GMT

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