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

From: Greg Stein <gstein@lyra.org>
Date: Fri, 22 Dec 2000 22:18:21 -0800
To: ietf-dav-versioning@w3.org
Message-ID: <20001222221821.M22947@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).

eek.

[looking, looking...]

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.

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.

Ah. Actually, the VERSION-CONTROL would be executed within a working
collection. When the working collection is checked in, a new pair of version
resources are created (which internally reference the same content). For
example:

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

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

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

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.

(++) 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.

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.

>...
> 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?! :-)

>...
> Well, if they want to "slash through" the names of the labeled copy,
> they pretty much have to instantiate it somewhere in the namespace.

Yup.

> 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 :-)

> 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.

Cheers,
-g

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

-- 
Greg Stein, http://www.lyra.org/
Received on Saturday, 23 December 2000 01:14:32 GMT

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