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Re: BASELINE option

From: Greg Stein <gstein@lyra.org>
Date: Sat, 17 Feb 2001 13:57:25 -0800
To: ietf-dav-versioning@w3.org
Message-ID: <20010217135725.N27443@lyra.org>
On Sat, Feb 17, 2001 at 03:05:14PM -0500, Edgar@edgarschwarz.de wrote:
> When asked a while ago about the difference between version controlled
> collections and baselines you told me that baselines are a depth 1 thing.

A version-controlled collection (VCC) is a VCR that refers to a baseline
from its DAV:checked-in value.

Conceptually, a baseline is simply an unordered, unstructured *set* of hrefs
to version resources. (depth doesn't really apply: baselines are member
resources rather than collections)

> If I understood that correctly this would mean that a baseline doesn't
> capture states of collections and their contents recursively.

If your collections are versioned, then it certainly does capture their
state. Even though the baseline itself is unstructured, you can "reassemble"
the collections by examination of the DAV:version-controlled-binding-set
properties of the collection versions found in the baseline.

Let's say that your baseline has { CV1, V2, V3, V4 } in it. CV1 is a
collection version; the others are regular version resources. To reassemble
these into a tree, you would fetch the binding-set and the version histories
from the resources:

    Resource  History  Binding-set
    --------  -------  --------------
    CV1       H1       { H2, H3, H4 }
    V2        H2       n/a
    V3        H3       n/a
    V4        H4       n/a

As you can see, the members of CV1 are V2, V3, and V4, by matching up the
version histories.

The above is just the conceptual model. It is actually much easier to see
the relationship by simply going to the DAV:baseline-collection of the
baseline. That is a set of VCRs that are already arranged for you.

> BTW, I use baselines which don't catch collection members. If I want them in my
> baseline I add them as a subbaseline. Also baselines only exist in the context of
> a workspace in my system. So a subbaseline can be identified by it's relative path
> from workspace root.

This is one way to do it. A lot of baselines, but acceptable.

> As a remark, I'm still not sure why we need the terms "configuration" and "baseline".
> Isn't a baseline just a version of a configuration ?

I'm not a terminology guy. I'll leave this to somebody else.

> You argue correctly that a baseline is efficient by containing just version
> and path of the resources it contains.

Nope. Just versions. No paths.

> The same is valid for subbaselines. Imagine a medium sized baseline containing
> 1000 resources.
> Now you need a bugfix in one resource. You create a new baseline of the configuration
> duplicating 999 entries from the predecessor.

Untrue. That is an *implementation* detail. Nobody said that you must store
the set of versions this way. As a matter of fact, there is no way to
*retrieve* the set of versions, thus no reason to ever truly keep a copy of
them around.

To query the set of versions, you must walk over the DAV:baseline-collection
and fetch the DAV:checked-in values.

[ as a counter-point: Subversion doesn't store anything when you "create" a
  new baseline. baselines are synthesized at request time from information
  within the repository. even then, we only synthesize a portion of the
  DAV:baseline-collection. even better: it is a very lightweight operation
  since the repository is logically organized as a set of baselines.
  concretely, each baseline is merely a diff against the previous baseline ]

> In 12.2.1 (DAV:subbaseline-set) you write: "A server MAY reject attempts to modify
> the DAV:subbaseline-set of a checked out configuration"

This is because a server may not support subbaselines, so this is how it
prevents clients from using the feature.

> My interpretation of this statement is, that a server understanding BASELINE at least
> must know about checking out subbaselines.

If you can't set the subbaseline-set, then the client can never create
subbaselines. Thus, any subbaselines that exist are due entirely to the
server's operation. If it is creating them, then yes: it must fully support
them, including checking them out.

> This sounds plausible. Because if not
> it couldn't work with any baseline containing subbaselines.
> If that's right some comments:
> It MAY reject modifying the subbaseline-set. But probably it can change some
> resources and create a new baseline containing the old subbaselines ?
> So it already has to know a lot about subaselines, but can't do all with them.
> But not much is missing here. This can be confusing.
> I think make the whole subbaseline stuff optional or remove the MAY sentence.

Splitting subbaselines out into another option would be too complicated. It
is much easier to cut it all off with a simple "MAY" sentence in 12.2.1.


Greg Stein, http://www.lyra.org/
Received on Saturday, 17 February 2001 16:55:01 UTC

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