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RE: Deleting versions

From: Lisa Dusseault <lisa@xythos.com>
Date: Thu, 31 May 2001 19:50:10 -0700
To: "Clemm, Geoff" <gclemm@rational.com>, "DeltaV" <ietf-dav-versioning@w3.org>

Geoff said:
> I agree with Tim's responses.  To make sure that nobody misunderstands
> the semantics of the method preconditions and postconditions, I propose
> to modify section 1.6 to read as follows:
> Lisa: Does this address your concern?

It's an improvement in the definition of post/pre-conditions, certainly.
There are still a couple of issues remaining:

1.  By having (DAV:must-be-root-version) as a postcondition, you're
preventing an implementation from deleting the last remaining version from a
version history.  I assume this is your intent?  Like so many other things,
it must be inferred rather than finding it stated in the text.  *sigh*

2.  I don't think the issue of a version-controlled-resource with zero
versions is sufficiently addressed.  For your convenience, here's the
section from 2518:

   A write locked null resource, referred to as a lock-null resource,
   MUST respond with a 404 (Not Found) or 405 (Method Not Allowed) to
   any HTTP/1.1 or DAV methods except for PUT, MKCOL, OPTIONS, PROPFIND,
   LOCK, and UNLOCK.  A lock-null resource MUST appear as a member of
   its parent collection.  Additionally the lock-null resource MUST have
   defined on it all mandatory DAV properties.  Most of these
   properties, such as all the get* properties, will have no value as a
   lock-null resource does not support the GET method.  Lock-Null
   resources MUST have defined values for lockdiscovery and
   supportedlock properties.

   Until a method such as PUT or MKCOL is successfully executed on the
   lock-null resource the resource MUST stay in the lock-null state.
   However, once a PUT or MKCOL is successfully executed on a lock-null
   resource the resource ceases to be in the lock-null state.

A direct read of this paragraph combined with deltaV draft 15 (and your
implication) would indicate that you can't issue a VERSION-CONTROL method on
a lock-null resource.  That's bogus.  A "write locked null resource" is
there so that the creator can do all the write operations and state changes
they need before making the resource visible to everybody.

So, I suggest that VERSION-CONTROL (and perhaps other methods like CHECKOUT,
MKWORKSPACE...) should be explicitly allowed by DeltaV to be done on write
locked null resources, and that the spec define a precondition that can be
returned if the server decides not to allow operations on write-lock


> -----Original Message-----
> From: Tim Ellison [mailto:tim@peir.com]
> Sent: Friday, May 25, 2001 5:43 PM
> To: DeltaV
> Subject: RE: Deleting versions
> > -----Original Message-----
> > From: ietf-dav-versioning-request@w3.org
> > [mailto:ietf-dav-versioning-request@w3.org]On Behalf Of Lisa Dusseault
> > Sent: 25 May 2001 17:58
> > To: DeltaV
> > Subject: Deleting versions
> >
> >
> >
> > I know not all implementations of DeltaV will allow deleting
> old versions,
> > but the specifications specifically allows it.  I've been looking
> > into that
> > functionality and encountered some issues and questions.
> >
> > #1)  Is there some way of finding out, before trying the delete, if it's
> > possible to delete a version?  Before you say
> > "supported-method-set", allow
> > me to point out that this property is not shown to exist on versions.
> Sure it is.  All resource properteis described in versioning-15 sec 3.1
> states "The version-control feature introduces the following REQUIRED
> properties for any WebDAV resource.", and since "The
> version-control feature
> MUST be supported if any other versioning feature is supported."
> it follows
> that you can always ask a versioning resource for its supported methods.
> > #2) It looks like there's a set of error msgs the server can
> return if it
> > decides to prevent the user from deleting the referenced
> version, the root
> > version, or all versions.  What error msg should the server return if it
> > decides to prevent the user from deleting the "current" version
> > (though one
> > that is not checked out)?
> Please clarify what you mean by the "current" version?
> > Or what if the server decides to prevent the only
> > remaining version from being deleted?
> This case is not called out by the spec., so I would expect 403 Forbidden
> with no interoperable extended error info.  Just as there will be other
> cases that servers (say for implementation reasons) refuse
> methods and have
> interop way of explaining why.
> > #3) I don't understand the following text from 3.12:
> > "(DAV:update-predecessor-set): If a version is deleted, any reference to
> > that version in a DAV:predecessor-set MUST be replaced by a copy of the
> > DAV:predecessor-set of the deleted version."  Does that mean that before
> > deleting a version, the client must munge the predecessor-set
> > properties of
> > a bunch of other versions?
> Yes, potentially.  The postcondition is required to fix up the history to
> show a continuous ancestry for the remaining versions.
> > But the predecessor-set is protected!
> ... from clients.  There are many examples of protected properties that a
> server can/will modify.
> > Or does
> > it mean that the server must update the predecessor-set before
> performing
> > the action, and if it cannot, it returns the error msg?  Please
> > explain this
> > better in the draft & to the list...
> It is a postcondition of the DELETE method for a version, therefore if it
> cannot be made true it MUST be as though the method was never sent.  No
> partial results will be left by the server, whether the fix up is done
> before/after the action is up to the implementation -- the spec states
> effectively that by the time 'the response is dispatched' the
> postcondition
> is true.  This is the same for all methods (not just DeltaV).
> > #4) Can situations arise where resources can have no versions?
> No.  Take a look at the 3.12 postcondition
> 	(DAV:must-be-root-version): If the root version of a version history
> is
> 	deleted, there MUST be another version that is the new root version,
> 	i.e. that is the ancestor of all other versions in the version
> history.
> > I can
> > actually think of two ways this might be achieved:
> >  - A client deletes all the versions (but not the version-controlled
> > resource)
> See above.
> >  - A null resource gets versioning turned on but no body is added
> A what? <g>
> > Must clients be able to deal with VCRs with zero versions?  If
> > not, then we
> > need to make a requirement on servers that they must not ever
> create a VCR
> > with zero versions.
> The way to create a version-controlled resource is with VERSION-CONTROL
> which always creates a version-controlled resource based on a version.  If
> the target resource was versionable the new version is created, or the
> VERSION-CONTROL can reference an existing version in the request body.
> Tim
Received on Thursday, 31 May 2001 22:51:52 UTC

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