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further baseline-related comments on deltav-10.1

From: <Tim_Ellison@uk.ibm.com>
Date: Thu, 19 Oct 2000 09:25:43 +0100
To: ietf-dav-versioning@w3.org
Message-ID: <8025697D.002E57CF.00@d06mta07.portsmouth.uk.ibm.com>


Geoff's reply brings up a general problem with baselines and unversioned
collections.

As currently specified, a baseline captures the targets of version
selectors at, or below, the baselined collection. If there are unversioned
collections at, or below, the collection being baselined, there is
insufficient information to make the baseline useful for restoring that
configuration.

Here's a simple scenario that shows the problem:

1. Create a new workspace in the server-blessed collection
     MKWORKSPACE http://example.org/ws/ws1

2. Create file1 in that workspace
     PUT http://example.org/ws/ws1/file1

3. Put the file under version control
     VERSION-CONTROL http://example.org/ws/ws1/file1
     => http://example.org/vers/vh1/v1

4. Put the workspace root collection under baseline control. This should
create a new baseline history, baseline selector, and baseline version. It
returns the URL of the baseline version.
     BASELINE-CONTROL http://example.org/ws/ws1
     ==> http://example.org/bh1/b1

This baseline version captures the current targets of the version selectors
in the workspace, namely the one for file1. It's DAV:version-set contains:
     http://example.org/vers/vh1/v1

6. Some time later, create another workspace
     MKWORKSPACE http://example.org/ws/ws2

7. Put the workspace root collection under baseline control specifying the
previously-created baseline version
     BASELINE-CONTROL http://example.org/ws/ws1

<baseline-control><baseline><href>http://example.org/bh1/b1</href></baseline></baseline-control>

     ==> http://example.org/bh1/b1

The hope would be that this would create a version selector at
http://example.org/ws/ws1/file1 with target http://example.org/vers/vh1/v1,
making the new workspace isomorphic to the workspace when it was baselined.
However, this would only be possible if the baseline remembered more
information than just the DAV:version-set; it would also need to remember
that the collection had a "file1" internal member, and the associated
target version is http://example.org/vers/vh1/v1.

While the above scenario shows that there's a hole in the story for the
root collection of a baseline, the same problem exists for unversioned
collections anywhere below the root.

The main requirement is that a useful baseline is one that can be restored.
The issue to decide is what constitutes being "restored". Two general
approaches:

(A) Only allow baselining of versioned collections. Effectively, "baseline"
becomes an optional add-on to "versioned-collection", and baselines cannot
be offered by a server that is not prepared to offer versioned collections.
This supports a fairly hi-fi notion of "restore". It is not perfectly
fidelity, since non-versioned members are not captured; but this is
compatible with the general philosophy for versioning collections.

(B) Allow baselining of arbitrary collections, but capture relevant
namespace information along with the version selector.
For example, the DAV:version-set might instead contain (path, version)
pairs:
     <version-set>
          <item>
               <href>file1</href>
               <href>http://example.org/vers/vh1/v1</href>
          </item>
This would supports a weaker notion of "restore" for non-versioned
collections with versioned members; it would allow these non-versioned
collections to be re-created when the baseline is restored, although they
would not be restored with any of their dead properties.

Approach (B) feels more general, and allows baselines to be offered by a
server that does not offer versioned collections (e.g., something like
CVS). Moreover, the additional information captured in the baseline should
not represent a significant burden for servers planning to offer both
baseline and versioned collections.

Other baseline-related problems with draft 10.1:

section 15.10 CHECKOUT [p. 45-6]

- Needs to specify the semantics of checking-out a baseline selector. In
particular, what is the initial value of the DAV:version-set of the
checked-out baseline selector. Presumably it contains the targets of all
check-in version selectors at or below the collection to which the baseline
selector is attached.
It also needs to be specified whether the DAV:version-set property of a
checked-out baseline selector is updated live as the baselined collection
is modified; i.e., can I query the DAV:version-set property of a
checked-out baseline selector at any time to discover a perfectly accurate
picture of the baseline collection?

- Needs to specify the semantics of checking-out a baseline version. It's
not clear that that this is a particularly useful thing to do. Since the
baseline version is divorced from the baseline selector, there wouldn't
been to be any way to affect its DAV:version-set between checkout and
check-in.

section 15.11 CHECKIN [p. 45-6]

- Needs to specify the semantics of checking-in a checked-out baseline
selector. In particular, what is the value of the DAV:version-set of the
newly created baseline version, and how does it relate to the value of that
property on the baseline selector, and the configuration of the baselined
collection, at check-in time. What happens to version selectors below the
baselined collection that are currently checked-out? They should eithe be
ignored, or the operation should fail in their presence.  What happens to
non-versioned resources below the baselined collection? They should be
ignored when capturing the baseline.

- Needs to specify the semantics of checking-in a baseline working
resource.
Received on Thursday, 19 October 2000 04:29:51 GMT

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