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FW: DeltaV Passthrough issues

From: Lisa Dusseault <lisa@xythos.com>
Date: Fri, 2 Feb 2001 15:42:32 -0800
To: "Ietf-Dav-Versioning@W3. Org" <ietf-dav-versioning@w3.org>
Message-ID: <CNEEJCPIOLHKIOFNFJDPCEOOCCAA.lisa@xythos.com>
In an earlier mail, Geoff said:
"   From: "Lisa Dusseault" <lisa@xythos.com>

   What about getting the DAV:version property for the base resource,
which
   if pass-through is working to the current target, should result in
   getting the DAV:version for the current target?

I'm not sure what you mean by "if pass-through is working to the
current target",  but in any case, as Greg pointed out, if you
want the version URL for the DAV:checked-in or DAV:checked-out
version of a version-controlled resource, you get that from the
DAV:checked-in and DAV:checked-out property value.  There's no
need to have a second way of getting the same information."

What I meant by "passthrough" is what has been talked about before in
the context of redirect resources, and now seems to be a feature of
versioning.  To recap:  When new kinds of resources that have targets,
like "Direct Reference Resources" or "Version controlled Resources"
(DRR/VCR), are exposed to clients that are not familiar with the new
kinds of resources, it's often beneficial to allow some of the methods
the client may send to "pass through" to the target of DRR/VCR.  Also,
when RR/VCRs exist in collections along with ordinary resources,
PROPFIND can usefully show properties of the target resources, along
with properties of the DRR/VCR itself. So when I do a PROPFIND depth 1
on a directory asking for the "DAV:content-length" property, it would be
useful for me to see the content-length of ordinary resources, the
content-length of the targets of links, and the content-length of the
target version of each VCR.  I thought this was called "passthrough"
when bindings were being discussed, but apparently is no longer refered
to in this way.

Anyway, the idea for bindings at one time was to allow some kind of
reference resource to exist, which when queried, would respond "as if"
it was the target resource.  However, sometimes the client wants to
address the reference resource directly.  The server needs to know if
the client is addressing the reference resource, or the target of the
reference resource.  Thus, the "Apply-To-Redirect-Ref" header was
defined.  If the client includes this header, e.g. with a DELETE, the
server can understand this to mean that the client wishes to delete the
redirect reference itself, rather than get redirected to the target
reference.

All of this applies to versioning, because with the current state of the
draft, it's clear that a VCR is a kind of reference resource.  I quote:

"A PUT or PROPPATCH to a version-controlled resource with the
DAV:auto-version property set will automatically check out that resource
prior to executing the PUT or PROPPATCH.  If that resource is
write-locked, the resource remains checked-out until the resource is
unlocked, at which time the resource is checked in and a new version is
created in the version history of that resource."

"For certain methods (e.g. GET, PROPFIND), if the request-URL identifies
a version-controlled resource, a label can be specified in a Label
request header to cause the method to be applied to the version selected
by that label from the version history of that version-controlled
resource."

This means that PUT and PROPPATCH are ALWAYS parsed as applying to the
version (passthrough), even though the URL was to the VCR.   It also
means that GET and PROPFIND _sometimes_ apply to the version
(passthrough).

It's not as clear what other methods are applied to.  It seems MOVE
source is defined as the VCR (no passthrough), but the COPY source is
the defined as the default version.  The language all seems to imply
that a COPY when the source URL is a VCR should be interpreted as
copying the latest version from the source, to the destination.

The consequence of a model that supports "pass through" to the target of
a reference resource is that the model must specify clearly when
passthrough occurs, and when it doesn't.  In addition:
 - What methods the Label: header can apply to (and what it _can't_)
 - When the label header is absent, for each method, what does it apply
to -- the VCR or the target version?
 - Whether there is a way of overriding the default assumption of what
the method applies to

Some of the specific questions that came up when I was reading the spec:
 - If any PROPPATCH to a VCR actually creates a new version; how does
one proppatch the properties actually on a VCR, such as
DAV:auto-version, DAV:predecessor-set and DAV:precursor-set, which
aren't protected?
 - If a PROPFIND with a Label: header is applied to a non
version-controlled resource, what does "no effect" mean?  What happens
if the specified label wasn't found -- does PROPFIND then return the
properties of the target version?  Or the VCR?
 - Does a PROPFIND to a VCR result ever result in a merge of the
properties of the VCR and its target version?
 - Since COPY seems to always get its content from a single version, I
would say that "passthrough" always seems to be on for COPY.  Is there
any way of NOT doing passthrough with COPY?  I.e, is there any way of
copying a VCR along with its VHR and VRs?  This would result in two
VCRs, two VHs and 2*n VRs, which would be the same initially, but could
diverge after.

A related issue is one I brought up in a separate mail, but I'll explain
the relationship here too:  how does one define properties which apply
not to an individual version, but to the whole shebang -- to the VCR,
VHR and all VRs?  An example is "owner", which in its meaning of "the
person responsible for this resource", can be taken to mean that when
ownership of a VCR is transferred the new owner becomes responsible for
all old versions related to the VCR.  No?  But we can't go about
changing properties of old versions...

Lisa
Received on Friday, 2 February 2001 18:43:26 GMT

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