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Re: Versioning spec review - 02.3

From: Geoffrey M. Clemm <geoffrey.clemm@rational.com>
Date: Mon, 25 Oct 1999 17:41:31 -0400
Message-Id: <9910252141.AA24961@tantalum>
To: w3c-dist-auth@w3.org

   From: jamsden@us.ibm.com

   Last paragraph: How about using components of versioning support instead of
      "levels"?

In the latest rev of the spec, the versioning options fell quite naturally
into 3 levels, so given the client-side advantages of having 3 levels of
support vs. multiple orthogonal versioning options, I'd like to go with
levels, if at all possible.

   <jra>
   Can a server support ONLY mutable resources, or does it have to have
   support for immutable resources too?

I assume by support for immutable resources, you mean providing a
client with some way of saying "never allow a DAV:overwrite on this
revision".  To me, that's an access control issue, not a versioning
issue, and I'd rather not mix in limited forms of access control
that will just end up conflicting with the general forms when they
are provided.  Since neither DMS nor CM systems provide this functionality
on a resource by resource basis, we gain no interoperability by 
specifying it.

   For example, DMS systems that
   don't currently support immutable revisions. In this case, the
   DAV:overwrite would always be T.

A DMS must let you do a checkin with DAV:overwrite=F (otherwise
you'd never have more than the initial version).  The only difference
in this regard between a DMS system and a standard CM system is that a
DMS system allows you have DAV:overwrite=T, which a standard CM system
would refuse.

   B.T.W., this isn't mentioned in the
   semantics of CHECKIN. Looks like mutable revisions are missing from
   CHECKIN although the postconditions mentions the concept.

Do you mean something like "If DAV:overwrite=T then the server MUST
support DAV:overwrite having the value T"? (:-) 

   <jra> What is the URL for the baseline? Is it something the server
   generated, or is it derived from the associated collection.

All baselines for a particular versioned collection can be found in
the DAV:baselines collection of that versioned collection.  The
server generates the URL of that collection, and generates the
name for a baseline in that collection. 

   The
   protocol doesn't say how a baseline is created, or how this URL is
   derived. Did I miss this? I don't see anything in CHECKIN.

I took that out, since I agree with your contention that it doesn't properly
belong in CHECKIN.  So you now just create baselines with the MKRESOURCE
command.  There's a ToDo note in the draft saying this should be explicitly
mentioned in the MKRESOURCE method description.

   In any case, any place where a specific revision is required, but
   revision id must be specified. For example, putting a configuration in
   the RSR. The same must be true of baselines.

Baselines and revisions are indicated in RSR's by their URL's.  We
could allow alternative compound notations as well, but is there
a compelling reason to do so (it does add complexity).

   How is constraining a "property collection" any harder than
   constraining a method?  You write down the constraint, and servers and
   clients must follow it.

   <jra> A method can define specific semantics that are supported. It
   doesn't have to list a bunch of things you can't specify.
   </jra>

I had no trouble specifying the appropriate constraints on the
appropriate properties in the protocol.  Live properties commonly have
their values constrained by the server, so this is nothing unusual for
WebDAV.

   having a downlevel lock check out the resource into the default
   workspace has the right semantics for down level clients working
   against auto-versioned resources ... the PUT that would auto- version
   fails because the resource is already checked out into the default
   workspace.

   <jra> Nice idea, but I think it overloads lock too much. Another
   way to look at it (from the point of view of a non-versioning aware
   client) is the lock prevents anyone else from checking out the
   revision. So the down-level client locks, does a bunch of PUTs knowing
   that no-one else can change the resource, and then does an
   unlock. Each PUT still does a CHECKOUT/PUT/CHECKIN, but only the
   down-level client can directly or indirectly checkout the
   revision. The effect is the same, but this keeps lock out of
   checkout/checkin which might be a good thing.
   </jra>

One of the bad things about creating a new revision on every downlevel
PUT is that it fills the history of a resource with "non-interesting"
revisions, but we have to do it to prevent lost updates.  In contrast,
a LOCK/UNLOCK request gives us exactly the information we need to
implement a more intelligent revision creation policy.  To not take
advantage of this information when designing the protocol strikes me
as remiss.

   <jra>
   Lock is an access control mechanism. We've already got the method and
   need to have it mean something. This is just an attempt to do
   something reasonable. If we want to keep locking optional though,
   we'll have to make sure none of this functionality is required.
   </jra>

Current WebDAV locking (with lock tokens) are good for some things,
but any time it is not reasonable to expect a client to hold and
remember a lock token, WebDAV locks are not appropriate.  It is better
to just have the protocol treat the operation as undefined or an
error, rather than increase the complexity of the protocol (and of
servers that implement it) with functionality that is not useful.

   Marshalling, how is the Target-Selector overridden with a
   specific label or id?

   It isn't.  The Target-Selector with a label or id is just for folks with
   lightweight workspaces that don't have rsr's.  For folks with workspaces,
   overriding with a specific label is far less common, and can be done by
   accessing the DAV:revisions or DAV:revision-labels collections.

   <jra>
   Two problems: if the Target-Selector is just a label, what is used to
   select the collection revisions in the URL path?

Core and Activity versioning servers will not be generating versioned
collections, so there will be no collection revisions in the URL path.
Versioned-Collection servers will provide repositories, which are
required to be located in a non-versioned part of the namespace
(i.e. under no versioned collections).  In either case, the DAV:revision-labels
collection will have no collection revisions in its URL path, so no
collection revision selection is required.

   Second, folks with
   workspaces will override with a specific label LOTS of times to do
   compares, browse old versions, prepare for merging, look at what other
   people are doing, etc.

The DAV:revision-labels collection is there to give clients easy access
to any revision by using a label.  No need for a specific header, and
suitable for use in any context where a URL can be specified (as opposed
to the Target-Selector, which just works for a Request-URL).

   CHECKIN is doing PROPPATCH work. This is not a propertyupdate, its a
   set of parameters for the CHECKIN method. We should not reuse the
   propertyupdate, but rather create a new element, specific to the
   method. Otherwise we have to specify a whole bunch of restrictions
   about what can go in the propertyupdate.

Not if we place no restrictions on what can go in the property-update.
Note that I'm happy to say no specification of the DAV:checkin-policy
in a CHECKOUT request, but if we *do* allow it, I believe that the
most appropriate way is to allow CHECKOUT to do an arbitrary PROPPATCH
to "initialize" the new working resource.  Think of CHECKOUT as a
"MKRESOURCE" for working resources.

   I don't think we need two ways of updating properties on a resource,
   PROPPATCH and CHECKIN. You say later on that some of the properties
   are not actually saved. This is even worse. Method parameters should
   be coupled with the method, not the resource.

We already have two: PROPATCH and MKRESOURCE.  So if we add a third,
there is at least precedent.

   Having "uncheckout" as an option to CHECKIN is reasonable because
   they are logically very similar in basic intent (i.e. "I'm done
   with this resource").  We already have DAV:overwrite which doesn't create
   a new revision, and DAV:identical-uncheckout which only creates a new
   revision if it has changed, so DAV:uncheckout seems to fit in very smoothly.

   <jra> But I think the intent of the two methods is the exact
   opposite. The intent of CHECKIN is to create a new revision an
   preserve changes unless perhaps there's no need to because there were
   no changes. The intent of UNCHECKOUT is to ensure a new revision is
   not created, and the changes are lost. Coupling these two concepts in
   the same method seems wrong. This is taking control couples too far.

Both DAV:overwrite and DAV:indentical-uncheckout conflict with your
description of the intent of CHECKIN.  One never creates a new revision,
and the other only sometimes creates a new revision.

      Notice that most of these
      checkin policies could be marshalled in simple headers.

   So we could use up the single header namespace, instead of using XML
   which has user-defined namespaces ? (:-)

   <jra> Since we own headers, there is no namespace issue.

This "we" is a very diverse group (i.e. everyone proposing extensions
to HTTP) with overlapping needs/interests.  Using up a name that is
"global" to this group is, in my opinion, a very significant issue.
After all, we've already made life difficult for the archery extension
group by defining the semantics of "Target-Selector" (:-).

      7. The paragraph about Target-Selector specfies a revision id or label is
      incorrect. The selector "self" cannot be applied to collections on the path
      because its a revision of the collection that's needed, not the versioned
      collection as a whole.

   And if so, it will "correctly" return an error (you should be using a workspace
   to look at things in versioned collections).

   <jra>
   But down-level clients can't do this.
   </jra>

Down-level clients won't be issuing requests with Target-Selector headers.

Cheers,
Geoff
Received on Monday, 25 October 1999 17:41:33 GMT

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