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Re: collection version resources

From: Geoffrey M. Clemm <geoffrey.clemm@rational.com>
Date: Sun, 31 Dec 2000 20:40:12 -0500 (EST)
Message-Id: <200101010140.UAA22596@tantalum.atria.com>
To: ietf-dav-versioning@w3.org

   From: Greg Stein <gstein@lyra.org>

   Basic issue: needing a way to add a member to a working collection that
   refers to an existing version resource, then possibly allowing further
   changes to that added member before [activity] checkin.

Sounds reasonable.

   Background/details/etc:

   *) current situation: collection versions contain version histories. Working
      collections contain version histories or non-versioned resources.

Yes.

   *) VERSION-CONTROL creates a new VCR, which implies it cannot be used in a
      collection version or a working collection

Yes.

   *) BASELINE-CONTROL was suggested by Geoff for some purpose (not entirely
      clear), but it puts existing collections under baseline control. We have
      only one "public" collection, which is the repository root's VCR.

Your server would automatically have the "root collection" of your
repository under BASELINE-CONTROL, so a client would not need to
explicitly make a BASELINE-CONTROL request (until we get to subversion
"labels", when they would :-).

   *) (model-wise) Subversion's collection versions contain version resources.

What you are calling a "Subversion collection version" is what the
protocol calls a "collection baseline".  If there's any flexibility in the
subversion terminology choice, it would be good if Subversion could
also use the term "baseline", or perhaps "deep version" for this concept.

      New collection versions are created if the collection must refer to a
      different version resource of a member ("bubble up" occurs).

This is also true for the protocol ... if we use the protocol
terminology, this says "new collection baselines are created if the
collection must refer to a different version of a member ("bubble up
occurs"). 

      [ I say "model-wise" because this is the SVN model, but the DeltaV terms
	don't strictly apply because of the definition of a collection
	version's member resources. ]

The DeltaV concepts apply ... it's just that we need to get the
terminology alligned (or at least, understand the terminology mapping).


   *) Commits can be made atomic via merging an activity. This is a rather
      heavyweight operation in the SVN server:
      - for all non-versioned resources in the activity, create new version
	histories and initial versions
      - check in all working resources
      - update all VCRs to refer to the new version resources
      - the VCRs are baseline-controlled, so the baseline selector is checked
	out, a new baseline is created, and then the baseline is checked in.
	(per S9.11, DAV:auto-baseline)

      The resulting baseline would refer to the proper set of version
      resources, so we're fine so far.

All OK here.

   *) While SVN collections refer to specific versions, it could be possible to
      leave that only to the baselines and to somehow construct a "standard"
      collection version resource.

Yes, that would make things interoperate.

      [ this might be difficult because in DeltaV, a new collection version is
	created when the *set* of members is changed; in SVN, a new collection
	version is created when the set changes *or* a member gets a new
	version resource. The hard part is "creating" collection versions on
	set-changes, but not member version changes. Now that I have thought
	about it, it might not be possible to do this; if it *is* then it will
	probably be quite expensive and it doesn't fit the model at all ]

      [ hmm. an alternative would be to go ahead and create new collection
	version resources even when "no change" has occurred in the set of
	members. to a client, it simply sees spurious creation of collection
	versions, but shouldn't have a real issue with it. ]

Yes, this alternative would be fine.

      Given the second model in the "aside" above, it would seem that we could
      model a DeltaV collection version. A true SVN collection version would
      only be exposed via a baseline.

Yes, an "SVN collection version" is what DeltaV calls a "collection baseline".

   *) The problem: given a working collection, I need a way to specify that a
      member is a specific version resource.

It's true that a client needs to know what version it is looking at,
but I don't see that it needs to get this information from a working
collection (the client just keeps a list of the versions that it has).

      The member may be furthered
      modified before it is checked in.

That's fine, but there is no need to check it out "in the context of
the working collection" ... just check out that version directly.

      When the new (auto) baseline is
      constructed, it will refer to the original version resource or to the
      modified successor of that version resource.

That happens because the activity contains the working resource ...
no need for the working collection to contain the working resource.

      Note that the member may be a collection or a non-collection resource.
      (I state this because I think Geoff said to use a BASELINE-CONTROL for
       this copy-by-ref thing, but that would only apply to a collection)

As long as it is a working resource, it will be in the checkout-set of
the activity, the new version will be captured by the new baseline and
because of the semantics of the MERGE operations, will become the
DAV:checked-in version of the appropriate version selector.

   My ideal solution would be to allow collection versions, and thusly working
   collections, to contain version resources *instead* of version histories.

No need for this change.  Activities contain versions, and baselines
contain versions, which should be all the version containment that
you need.

   This would solve the model disparity for collection versions. However,
   putting a version resource into a working collection isn't quite right...
   That would prevent a change of the copy-by-ref'd resource before it was
   checked in. It almost seems that the needed model would be a working
   resource where the DAV:checked-out property is the copied version resource.
   But that might spawn a new version resource even when we don't make any
   changes...

You are about to get lost in a maze of twisty passages, all of which
look alike ... don't go there (:-).

   sigh. Here would be an ideal sequence of semantic operations:

   1) CHECKOUT a collection version (within an activity)

Yes.

   2) "copy" a specific version resource into the working collection; the copy
      should also be a version resource, tied to the same version history. In
      fact, using the BIND method is pretty close: construct a new URL for a
      given version resource.

You would need to "bind" the appropriate version history into the
working collection, not "copy" a version into it (which would create
a new version history, not link you to an old one).

BIND would be the most direct way (the DeltaV spec is my highest
priority, then the ACL spec, and then BIND ... I'm hoping JimW will get
the BIND editing done, but having your first baby does tend to absorb
quite a bit of time (:-), so I may end up getting to it before he does.

Alternatively, you can just CHECKOUT a collection version that
contains the desired version history (creating a second working
collection), MOVE the desired version history from the second working
collection into the first working collection, and then DELETE the
second working collection.  This achieves the result of a BIND
without actually using the BIND method.

   3) somehow "checkout" the "new" version resource to create a working
      resource

That's the easy part.  You just checkout that version resource, to get
a new working resource.

You don't need anything more than the implict connection between the
working collection and the working resource (i.e.  the working
resource checks out a version whose version history is now a member of
the working collection).

   4) PUT/PROPPATCH changes to that working resource

Yup.

   5) MERGE the activity

Yup.

   Yes... steps (2) and (3) are the hard part.

Step (3) is easy, so that just leaves step (2) to think about.
If you just use the BIND protocol (it's going to happen, so you won't
be *that* far in front of the crowd), step (2) is simple as well.
Or if you don't want to have a BIND dependency, just use the
CHECKOUT, MOVE approach, which isn't too bad.

   I'm all for fitting into the existing DeltaV model(s), but I can't see how.
   Hmm. What if I copy/bind/whatever a version resource ("VR") into the working
   collection ("WC").

BIND the version history in, and you've got the collection change done.

   If changes are needed, then I do a CHECKOUT on VR and a
   working resource ("WR") is returned.

Yes (but there's no need for the working collection to be involved in this
version checkout).

   PUT/PROPPATCH request(s) are performed on WR.

Yes.

   At checkin/merge time, the server understands that WR corresponds to
   VR.

It's true that the DAV:checked-out property of a WR identifies the VR
that was checked out.

   The new collection version (or baseline; the actual new "thing" seems to
   be moot here) is created to refer to VR, or to VR' (resulting from the
   checkin of WR).

Well, it's actually very important to get the "thing" right to get
interoperability, so let's go with "baseline".  Then, yes, the new
collection baseline will refer to VR' (because the MERGE updates
the version-controlled resource to have VR' as its DAV:checked-in
version, which means that VR' is added to the DAV:version-set of
the new baseline.

   It may also be that a version-controlled resource would go into WC which
   initially points at the original version resource. However, this creates
   problems when we go to check out that version resource to make further
   changes. That would lose the association between WC and the "new" member.
   But the nice part about the VCR is that it can point to VR'. Otherwise,
   there is a need to "replace" VR (within WC) with VR'.

The MERGE takes care of getting VR' into the new baseline, so there's
no need to get it into the working collection.

   Obviously... I haven't figured out the best way to model this (within the
   existing DeltaV or figuring out how it needs to change). Any thoughts?

Let me know if any of this doesn't work for you.

Cheers,
Geoff
Received on Sunday, 31 December 2000 20:41:00 GMT

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