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RE: error condition for delete of VHR when VCR is in checked-in c ollection

From: Julian Reschke <julian.reschke@greenbytes.de>
Date: Thu, 11 Jul 2002 09:26:59 +0200
To: "Clemm, Geoff" <gclemm@rational.com>, <ietf-dav-versioning@w3.org>
Message-ID: <JIEGINCHMLABHJBIGKBCKENPEOAA.julian.reschke@greenbytes.de>

Geoff,

again thanks for taking the time. Comments inline.

>    let me try to summarize.
>    In the presence of BINDs, DELETE is defined as
>
>    1) removing the binding specified in the request URI,
>
> Yes.
>
>    2) optional deletion of the resource identified by the URI if there
>    are no references left to it. Note that references may be in other
>    namespaces, on other servers and even in different URI schemes.
>
> If there are no "bindings" to it.  A reference (e.g. a redirect
> reference, or a DAV:href reference) is not a binding.

I agree that a reference in the sense of a DAV:href or a redirect reference
does not *require* the server to keep the resource. But my point is that
with bindings, there's no way a client can definitively know, and a server
can decide to keep a resource as long as it wants. The BIND spec defines how
long a resource MUST be kept, but not when it should be cleaned up after all
client-visible bindings are gone.

Thus, a specific RFC3253 implementation is absolutely free to *treat* a
"soft" reference (like a DAV:href in the DAV:version-controlled-binding-set)
as internal reference to the resource, and thus keep the resource. In this
specific case, this allows the server to resurrect a bind to the VHR at the
original URI although the client has done a DELETE on it in the meantime.

Are you saying that this behaviour breaks the spec in some way?

>    Any spec that wants to cooperate with bindings can NOT change this
>    semantics of DELETE
>
> Correct, but it can supplement the definition with additional
> explicit semantics (RFC3253 does so in a number of places).

My point is that these semantics need to distinguish between:

- removing the bind (something the BIND spec defines -- the name is gone,
that's all) and
- removing the resource (and here's the catch --  spec can define what
removing the resource means, but it can't define *when* this happens or
whether it happens at all)

>    (or does RFC3253 explicitly say that a VHR can have only a single
>    binding???).
>
> No (in fact, it explicitly introduces other bindings in working
> collections).
>
>    So we have a VCR that refers to a VHR through the
>    DAV:version-history property. The client issues a DELETE on the VHR
>    URI. Possible results:
>
>    a) the server may refuse to perform the Delete, either because
>    deletion of VHRs is not supported, or because a VCR still refers to
>    the VHR.
>
> Or pretty much for any reason that it wants.
>
>    b) the server deletes the VHR and un-version-controls the VCR (to avoid
>    breaking the live versioning properties).
>
>    c) the server deletes the VHR and keeps the VCR version-controlled - in
>    which case the spec should say what happens to the
>    DAV:checked-in/DAV:checked-out and DAV:version-history.
>
>    I think RFC3253 needs to either forbid b) and c) or properly define the
>    semantics.
>
> Well, (c) is probably what an interoperable client has to assume,
> given what is currently stated in the spec (where the semantics is
> just that you get a "404" when you try to use the URL in the
> DAV:checked-in/DAV:checked-out and DAV:version-history properties).

OK with me.

> The problem is that we could not achieve consensus on any semantics
> like "b", even though some variant of "b" is what some existing
> versioning servers do (where each kind of server has its own variant).
>
>    More below...
>
>    > From:  Clemm, Geoff
>    >
>    > ... checking in a VCC never creates a binding to a VHR, but
>    > rather just creates a reference to the VHR in its
>    > DAV:version-controlled-binding-set property (i.e. it doesn't
>    > create a new name for that VHR, but just uses an existing name to
>    > create a reference).
>
>    I don't think it's really relevant whether the collection version
>    has a reference or a binding.
>
> A binding is very different semantically from a reference (this is
> emphasized in the binding spec).  If you delete a binding to a

Agreed. I wasn't using the term "reference" in the sense of "redirect
reference", but in the sense of "any internal reference that the server may
have to the resource".

> resource, any other binding to that resource can still be used to
> locate (and apply methods to) that resource.  But a reference just
> contains a URL, which means that if you delete any of the bindings
> used by that URL, that reference can no longer be used to locate the

Agreed. In POSIX speak, a "redirect reference" is like a symbolic link,
while a bind is like a hard link...

> resource.  Similarly, if you MOVE a resource (give it a different
> binding), that has no effect on other bindings to that resource, while
> references will always have the same URL, which means that they will
> identify whatever resource is currently identified by that URL
> (which changes if you MOVE a different resource to that URL).

In the particular case (a DAV:href pointing to a VHR) this is no issue
because RFC3253 clearly states that the URL for a VHR may never be reused
for a different resource. May point is that is MAY be reused for the same
resource, if the server is able to do that after a DELETE.

>    Servers may delay the de-allocation of a resource until there is no
>    reference left, whether it's visible or not.
>
> Servers must delay the deallocation of a resource until there are no
> bindings to that resource, but a reference to a resource just contains
> a URL, and if you issue a DELETE on that URL, the reference will no
> longer be usable to locate that resource.  So as far as a reference
> is concerned, the resource is gone as soon as a successful DELETE
> has been performed on the URL contained by that reference.

Yes.

>    If we allow clients to permanently destroy VHRs that are
>    referenced in a collection version, there's no way to satisfy the
>    postconditions for an UNCHECKOUT of the VCC, right? That's why our
>    implementation keeps the reference, and therefore is able to
>    resurrect the VHR when needed.
>
> For UNCHECKOUT, it creates a new binding in the VCCl "when a version
> history is identified by the DAV:version-controlled-binding-set".  But
> if the VHR has been deleted, then that VHR is not longer identified by
> the DAV:version-controlled-binding-set (i.e. the VHR URL no longer
> identifies a resource), so no "restoration" semantics is implied.

I think that would come as a surprise to the user. It means that the result
of a UNCHECKOUT may be imcomplete in the sense that the server silently
drops a version-controlled member. If this really is the intention (and
required), this should be explained.

Again the question: if a server is able to reconstruct the VHR at it's
original location, is it *allowed* to do so?

>    > - a subsequent delete on the VHR URL just deletes the original
>    > binding to the VHR, but the version history resource (and the
>    > versions inside the version history) is not affected
>    >
>    > Since the DAV:version-controlled-binding-set just has a reference to
>    > that VHR using its server-assigned URL, if you DELETE that URL, you
>    > break all references to that VHR from the DAV:v-c-b-s properties of
>    > collection versions.
>
>    So if the VHR can not be re-created, should the server forbid
> the delete?
>
> That is certainly one option.  The other is to allow the delete and
> effectively treat every version-controlled-binding to that VHR as no
> longer existing.
>
>    >    - thus, upon UNCHECKOUT of the VCC, I can reconstruct the VHR
>    >    (re-creating the original binding)
>    >
>    > The members of the VCC also refer to the VHR via the
>    > server-assigned URL, so even if UNCHECKOUT could restore the
>    > version-controlled member of the VCC, the DAV:version-history
>    > property of that member would contain the VHR URL that was
>    > deleted.
>
>    But that's not a problem if the '*same* VHR is reconstructed at the
>    original URL.
>
> Yes, but nothing in the semantics of UNCHECKOUT imply such a
> reconstruction.

OK, I can live with servers *not* doing that -- what I'd like to know is
whether this behaviour (reconstructing the VHR) actually is RFC3253
compliant.
Received on Thursday, 11 July 2002 03:27:23 GMT

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