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Re: Why does MERGE automatically checkin resources related to act ivities?

From: Greg Stein <gstein@lyra.org>
Date: Wed, 3 Oct 2001 04:30:10 -0700
To: "Clemm, Geoff" <gclemm@rational.com>
Cc: ietf-dav-versioning@w3.org
Message-ID: <20011003043009.J24906@lyra.org>
On Tue, Oct 02, 2001 at 09:47:08AM -0400, Clemm, Geoff wrote:
>...
> I do believe Roy made a good case for making this behavior
> be under client control, so I'd like to modify the marshalling
> of the MERGE request so that there is a DAV:auto-activity-checkin
> flag to MERGE that indicates whether or not the client wants this
> auto-activity-checkin behavior.  Does anyone object to this change?

Not a problem here.

> (I'd like to make the default to not do the checkin, since this
> is more consistent with the non-activity semantics of MERGE, which
> does not merge checked-out resources.

Not a problem.

>...
>    From: Greg Stein [mailto:gstein@lyra.org]
> 
>    On Mon, Oct 01, 2001 at 09:50:50AM -0400, Clemm, Geoff wrote:
>    > Greg: If you split your "commit" into an activity "CHECKIN"
>    > followed by a baseline "MERGE", I believe you would have the
>    > right framework for doing branching (in particular,
> 
>    We use cheap copies, not branching.
> 
> A "cheap copy" is probably a better name for what I had in mind
> anyway, so I'll use that term here instead of "branching".
> 
> In particular, I noticed in the subversion "mapping to WebDAV" design

Careful with that doc :-)  it could stand some updating...

> notes, that you were moving towards using something other than "COPY"
> to marshal the creation of a "cheap copy".  I believe that
> BASELINE-CONTROL is the appropriate way to marshal this.  The
> DAV:baseline-collection property of a baseline exposes that "cheap
> copy" in an interoperable way.

Actually, I was planning to do a COPY from somewhere in a BC as the
marshalling. For example:

    COPY /repos/svn/$svn/bc/567/trunk HTTP/1.0
    Destination: /repos/svn/$svn/wrk/uuid/branches/gstein-work

This contains the revision/path to copy, and the destination in a working
collection to copy it to.

>    > a branch is just a CHECKIN that is not followed by a MERGE).
>    > This would have CHECKIN of an activity create a new
>    > subversion revision (aka a DeltaV baseline), but this revision
>    > wouldn't become the "current" one until you did the MERGE.
> 
>    Not supported.
> 
> Isn't this just what a "cheap copy" is (i.e. a new revision that does
> not replace the current one)?

The copy occurs within a set of working resources. To get a revision, you
must check in those resources. That revision is then "current".

A cheap copy means that copying a tree does not duplicate the entire tree in
the repository. Effectively, we create a symlink within the repository. A
copy is unrelated to the revisions -- the latter is a feature of the entire
repository, while copies of files/dirs are structural things *within* a
repository.

> And don't you provide a way to "merge"
> one "cheap copy" into another (i.e. a MERGE)?

We have code for merging two trees, but the user model for doing that (and
the resulting marshalling for the feature) is not yet decided.

[ we merge trees when you checkin an activity, and the changes get merged
  with the current revision ]

> I assume the reason you don't want to always use a CHECKIN/MERGE sequence
> is that your revisions, although cheap,

I believe you've led yourself down the wrong path :-)  Our cheap copies are
not on a revision basis, but they are about copying files/dirs *within* the
repository. A given checkin could have dozens of copy operations. The
resulting repository looks like a standard tree, but we happen to represent
it without a lot of duplication, and with history about the copy.

> are expensive enough
> you don't want to create them more often than is necessary
> (i.e. only do the CHECKIN if the MERGE will succeed)?

Revisions are cheap. There's no problem with spitting out hundreds of them
(technically; that many could blow up people's brains)

We can't do a CHECKIN/MERGE because the checkin operation automatically
merges. Thus, we could not support a client that looks at those operations
as a two step process. Our operation is a single "checkin and merge", and
the current MERGE semantics models that.

The basic issue here is that the model used by Subversion (and I suspect a
goodly number of other systems) does not provide for checking in a new
"head" revision and hiding it until somebody cares to make it visible.

>    > If so, I believe this would then remove your need for
>    > having CHECKIN be a side effect of MERGE.  But this of course
>    > works only if subversion allows a new revision to be created that 
>    > does not immediately become the current revision.  But don't
>    > you have to do that to support branching?
> 
>    Effectively, we do:
> 
>    $ svn cp /trunk /branches/gstein-work
> 
> Could that be marshalled as:
> 
> <create baseline for /trunk>
> BASELINE-CONTROL /branches/gstein-work 
>  <D:version> <the-trunk-baseline> </D:version>

No. We aren't copying baselines. We're copying the "/trunk" directory. The
destination is a working resource, not something that would go under
baseline control.

>    Hmm. I just realized that the original question made it seem like
>    some weird side effect. But in the case under discussion, you're
>    doing a MERGE on an *activity*. Thus, it makes perfect sense to
>    take all resources contained in that activity as the merge source.
> 
> Yes, if the activity is "done".  Roy's examples were for cases
> where the activity has an interesting state you want to merge
> into your workspace, even though the activity is not yet done.

Sure... Roy's examples are quite valid. I think the original question was a
misdirection.

Cheers,
-g

-- 
Greg Stein, http://www.lyra.org/
Received on Wednesday, 3 October 2001 07:26:07 GMT

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