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Re: COPY and retain history (was: Re: Subversion support)

From: Geoffrey M. Clemm <geoffrey.clemm@rational.com>
Date: Sat, 23 Dec 2000 10:36:44 -0500 (EST)
Message-Id: <200012231536.KAA10146@tantalum.atria.com>
To: ietf-dav-versioning@w3.org

   From: Greg Stein <gstein@lyra.org>

   On Sat, Dec 23, 2000 at 12:01:15AM -0500, Geoffrey M. Clemm wrote:
   >    From: Greg Stein <gstein@lyra.org>
   > 
   >    SVN can do a copy-by-reference, so they're cheap as hell. I can label
   >    something simply by doing:
   > 
   >      COPY http://www.lyra.org/svn/project
   >      Destination: http://www.lyra.org/svn/labels/1.0a1
   > 
   > According to the protocol, that would create a bunch of new
   > non-version-controlled resources.  They could be automatically put
   > under version control, but then as far as the protocol is concerned,
   > they would be completely unrelated to the resources that they came
   > from (i.e. changes couldn't be merged back to /svn/project).

   Well, I need to figure out exactly where I want to go with this. To create a
   true copy-by-ref, I'd want to use VERSION-CONTROL on a null resource to
   point(*) at the appropriate version (and an existing version history). Then
   I'd want to fork a new line of descent from that version.

Actually, I believe you need to do a BASELINE-CONTROL on an empty
collection.  While everything in that new collection is "checked-in"
it can't be changed, which makes your "copy-by-ref" a very good
implementation for a "baseline-controlled collection".  When you MERGE
activities into that new collection, you have "forked a new line of
descent".  But because you have told the server that these collections
are based on the same "baseline history", it knows to do the
book-keeping needed to allow you to merge from one "line of descent"
to another (and you just use the MERGE method to do so).

   The fork would probably happen in some kind of lazy fashion. Hmm... yes, in
   SVN, the two VCRs would point at the same version resource. As soon as you
   go to modify one or the other, a fork is created.

A collection "version" doesn't give you what you need (it just says
what version histories are its members), but you need to know what
*versions* are its members.  That's what a baseline gives you ...
it knows what versions are needed for all of the members of the
collection.

   Ah. Actually, the VERSION-CONTROL would be executed within a working
   collection.

A VERSION-CONTROL requests creates version-controlled resources,
but a working collection member must be either a non-version-controlled
resource or a version history, not a version-controlled resource.
It doesn't do you any good to have a version-controlled resource
in a working collection, because when you check in the working
collection, the resulting collection version can only have version
history members so the "selected version" information in the
version-controlled resource would be lost.

   When the working collection is checked in, a new pair of version
   resources are created (which internally reference the same content).

Note that even if we did allow version-controlled resources in
a working collection, a VERSION-CONTROL request with an existing
version does not create any new versions (it just uses the
version specified in the VERSION-CONTROL request).

   For  example:

       CHECKOUT /repos/$svn/ver/67/somedir

The CHECKOUT would need an "activity" parameter (since you're
doing all your changes in the context of an activity), but
you probably just omitted that for brevity.

       VERSION-CONTROL /repos/$svn/wrk/activity-name-here/67/somedir/foo.c
       (pointing at /repos/$svn/ver/67/otherdir/foo.c)

I asume that "/repos/$svn/wrk/activity-name-here/67/somedir"
is the working collection that results from the CHECKOUT?
As indicated above, it wouldn't do any good to create a
version-controlled resource referring to a particular version,
in the working collection, because when you checked in the
working collection, the collection version would just have
a reference to the version history, and the particular version
information (/ver/67) would be lost.

       CHECKIN (activity)

   Now, we'd have two new version resources(++):
       /repos/$svn/ver/68/somedir/foo.c
       /repos/$svn/ver/68/otherdir/foo.c

Actually, the only new "version" resource you'd get is the
one you checked out (i.e. the working collection .../somedir).

Now in the new *baseline* that was created when you did the checkin
(i.e. baseline 68), there would be a *reference* to every version but
the only *new* version would be that new collection version for
"somedir".

   Since we have two version resources, but the same version history, we
   effectively have the beginning of our fork.

   Note: S5.10 of the 10.11 draft should be moved. I'd say Section 3 offhand,
   but it would seem that you could do a VERSION-CONTROL on a null resource
   even without version histories. Sure, once you bring in version histories,
   the semantics (pre/post conditiosn) might change a bit. But S5.10 has no
   relation to server-side workspaces. I'd say the null resource part goes into
   core, and then behavior gets modified/refined by the options.

If we allowed version-controlled resources as members of working
collections, then I'd agree, but by the reasoning above, we don't.
 
   (++) hmm. One thought. Whenever a checkin anywhere occurs, a complete set of
   version resources are created, whether a specific version resource was
   actually involved in the checkin or not. While allowed by the spec (it would
   be as if some Invisible Bob was creating new version resources), there may
   be some implication that I'm missing. Need to think on this one.

Creating a new version of every resource in the repository as a side
effect of checking in a single file would make it unlikely that an SVN
server or client would interoperate with any other WebDAV server or
client.

On the other hand, creating a new *baseline* which refers to the new
versions you created plus all the existing versions that you didn't
change would work just fine.

   Oh. I might have the answer already :-). I can avoid creating new version
   resources if I simply jigger them a bit:

       /repos/$svn/ver/17.3/somedir/foo.c
       /repos/$svn/ver/17.3/otherdir/foo.c

   Here, the "17.3" refers to the internal, stored content. As new repository
   versions are creatd, the 17.3 remains stable, so we aren't implicitly
   creating new version resources any more.

   Now to think on this model some more, and see whether the repository can
   work using this model.

Yes, the stable version URL (where you only generate a new one when
the version content changes) will be essential for interoperability.

   >...
   > In other words, you "instantiate" a baseline in a baseline-controlled
   > collection whenever you want to expose that baseline as a namespace.

   For a label... yah. This would work great.

   If we do true labels, rather than just copies, then I'll probably go ahead
   and add some formalized baseline code.

   Darn... I just keep adding more and more of the DeltaV draft. What's next?
   Workspaces?! :-)

OK, well, I wasn't going to say anything, but since you brought it
up (:-), the only important characteristic of a workspace is that:

- it is a collection displaying version-controlled resources
- it has at most one version from any given version history

Your "label" collection happens to be exactly that.  So you might
as well call it a workspace, and be able to use the
"DAV:workspace-collection-set" to let clients know "where they
can find/put their labels" (which answers your earlier question
about "how will a client find where those label collections live".

   > But I agree that they probably don't want to instantiate all their
   > labels as copies in the namespace (even if subversion can do it
   > cheaply :-),

   It's an O(1) operation :-)

Yes, I wasn't suggesting they don't want to do it for efficiency
reasons, but rather to keep the namespace manageable,
they might want to just instantiate "important" labels as
collections in the namespace, and just use simple "string labels"
on baselines for all the ones that aren't that important.

You can then use a string label for everything, but only
instantiate "important" labels in the namespace (and then
clean them out of the namespace when they become no longer
important).   That way you can remember everything without
trashing your namespace as time goes on.

   > so I guess I'll withdraw my objection to labels, as long
   > as they are used on baselines (which I believe is the only place
   > subversion would place them?) and not on individual file versions.

   We would definitely be doing labels on baselines. I never investigated them
   thoroughly (as you can tell, we aren't even sure what kind of label
   functionality we want, let alone the modelling of them via DAV). But your
   label-a-baseline idea is exactly spot on :-)

   The copy functionality will always exist, so creating a new VCR from the
   ether is important.

Or creating a new baseline-controlled-collection from the ether is
important ...

   (*) if somebody says that a VCR doesn't "point", I'll kick 'em in the shins

Saying that "a VCR points" is fine, but at risk of getting
my shins kicked, I would have objected if you'd said that
a VCR *is* a pointer (:-).

Cheers,
Geoff
Received on Saturday, 23 December 2000 10:37:35 GMT

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