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Re: shared version histories

From: Geoffrey M. Clemm <geoffrey.clemm@rational.com>
Date: Sun, 31 Dec 2000 20:58:38 -0500 (EST)
Message-Id: <200101010158.UAA22619@tantalum.atria.com>
To: ietf-dav-versioning@w3.org

   From: Greg Stein <gstein@lyra.org>

   I'm seeing some issues in the draft when a namespace has two VCRs that refer
   to versions from the same history.

   For example, let's say that I have a version history with:

       V1 -> V2 -> V3 -> V4

   In the VCR namespace:

     /coll/foo.c -> V2
     /coll/bar.c -> V4  [copied from foo.c, then extended]

If it really was "COPY"d, then foo.c and bar.c would be in different
version histories, by the semantics of COPY.  You'd have to use
BIND or the MOVE trick to get this situation.

Note though, that if /coll were a workspace, this could not happen,
because you cannot have two different version controlled resources
with the same version history in the same workspace.  (You could have
two bindings to the same version-controlled resource, but then they
would share the same DAV:checked-in version).

   I see two problems arising from this now:

   1) a MERGE request is going to have problems because it doesn't know which
      VCR to update when bar.c is moved from V4 to V5. foo.c also refers to the
      same version history, so the MERGE may want to update that to V5, too.

Yes, that is why this is not allowed to happen in a workspace.

      [ based on my understanding of MERGE which says "update all VCRs which
	refer to the version history of the merge version (V5)". in this case,
	two VCRs do. ]

Currently the protocol is not explicit about what happens in this
situation (i.e. it talks about "the" merge target).  This means
that currently, a server could call it an error, update just some
of the VCR's, or update all of the VCR's.

   2) I have a change set style of an activity. The resulting version history
      should look like:

       V1 -> V2 -> V3 -> V4 -> V5
	       \
		V2.1

      But the limitations imposed on activities prevents two versions in the
      same line of descent. (but the point is *after* the change set is
      applied, I want two lines of descent)

You can have multiple lines of descent, but V2.1 is not allowed to be
in the same activity as V3, V4, or V5.

      [ oh, wait a sec. DAV:unreserved fixes this ]

Depends what you mean by "fixes".  DAV:unreserved just lets thing
branch in the same activity at CHECKOUT time.  You cannot CHECKIN
something (whether or not it is reserved) if it would cause a fork
in an activity.

   I have a feeling that problem (1) could be lessened because V5 was created
   by checking in a change against V4, so we only update VCRs that refer to V4.

In general, you can update a VCR to any other version from the version
history of that VCR (skipping forward, backward, or sideways in the
history is all legal), so only updating a VCR if it currently has
the predecessor of the change would be a significant loss of functionality.

   But that isn't actually in the draft, and it might not be desirable (MERGE
   isn't always based on checking stuff in; it can also be used for workspace
   management).

Yes.

   Okay. (2) is moot. The introduction to activities is very misleading. It
   says that versions in an activity must be on a single line of descent.
   Twice. Well, that gets completely thrown out later when DAV:unreserved is
   described. The introduction should say something about this to prevent
   confusion (like I just did above).

The confusion must be in the definition of "unreserved", which needs to
make it very clear that the unreserved fork only applies to a checkout ...
the *versions* in an activity must be on a single line of descent, so the
checkout forking is only temporary.

   I'll look for other issues with shared version histories, but this is my
   start. Of course, there is also the problem of getting two references in
   there in the first place (in the working collection model; it would appear
   that the workspace model can just use VERSION-CONTROL to create the second
   reference).

Yes, but not in the same workspace.

Cheers,
Geoff
Received on Sunday, 31 December 2000 20:59:25 GMT

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