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Re: Review of draft-ietf-deltav-versioning-10.4/5

From: Geoffrey M. Clemm <geoffrey.clemm@rational.com>
Date: Tue, 5 Dec 2000 14:35:49 -0500 (EST)
Message-Id: <200012051935.OAA13216@tantalum.atria.com>
To: ietf-dav-versioning@w3.org
       From: Juergen Reuter [mailto:reuterj@ira.uka.de]

   I am not very happy with the current structure of the document
   which divides all features into core WebDAV versioning and optional
   WebDAV versioning.  Currently, the core part is getting smaller and
   smaller, while the optional part continuously grows.  The problem I
   see is that server implementors will either implement the core part
   only, or, in practice, they will have to implement also the full
   optional part.

Note that the options are divided 6 packages, which are identified in
the DAV header response from an OPTIONS request.  These are "label",
"workspace", "baseline", "activity", "merge", and
"versioned-collection".  Clients are expected to look for these option
packages via the OPTIONS request.

We could restructure the document into one section per option package.
What do others think? 

   I think the original idea of core/optional versioning was to
   separate core versioning semantics from advanced semantics (mostly
   configuration management).  So, maybe you could reintroduce this
   idea into the protocol by defining three levels:

   - core versioning: minimally required features

   - versioning: fully fledged versiong including labels and branches
   (and maybe merging)

   - advanced versioning: configuration management and all remaining
   optional features

What is in the "versioning" level varies widely from server to server.
I believe the current division into orthogonal option sets is what is
required to achieve consensus.

   Section 1.3
   - ---------

   Version Selector/Working Resource:

   The definition of the term "Working Resource" as opposed to that of
   "Version Selector" seems confusing to me.  The pure fact that the
   CHECKOUT method is defined to be applicable on both types of
   resources, does not make them suitable for a direct comparison.

Note that the things that can be checked out are version-controlled
resources and versions.  The result of the checkout is either a
checked-out version-controlled resource, or a working resource,

   I think it would be better to characterize the term "working
   resource" independently from "checking out a version selector".

I agree.  I'll see what I can work up (this is connected to some
points you make below).

   Maybe when defining the term "version selector" you might want to
   add a note about checking out version selectors.  To summarize, I
   would like to suggest the following wording:

   Version Selector:
   When an existing resource is put under version control, it becomes a
   "version selector" resource.  A new version history resource is
   allocated, whose initial version contains the content and dead
   properties of the existing resource.
   A checkout may be applied to version selector.  This will make a copy
   of an existing version of the version history accessible for editing.

This might be a bit misleading since a checkout applied to a version
selector does not make a copy of anything ...  it just changes the
state of the version selector from checked-in to checked-out.
Checking out a version does make a copy (the working resource).

   A subsequent checkin will create a new version in the version history.
   Checking out a version selector is useful for environments where only
   a single user accesses the version selector.

Note though that multiple users can access a shared checked out
version selector by using the locking protocol.  Also, you need to be
careful to not mislead the reader into thinking that checking out a
version selector is useful only when a single user is doing
development (when workspaces are supported, multiple users each check
out their own version selector for a shared version history).

   Working resource:
   If multiple users concurrently try to check out the same version
   selector, they will interfere each other.  To avoid this, a checkout
   can also be directly applied to a version.  This will create a new
   resource, called "working resource".  The working resource can be
   modified and is automatically deleted when it is checked in.  The
   server allocates a distinct new URL for each new working resource.

Note that in general, we try not to go into this much detail in the
"terminology" section, but rather do so in the "semantics" or
"concepts" section.

   Section 2: WebDAV Versioning Semantics
   - ------------------------------------
   This section discusses rather concepts than semantics, right?

I'm happy with either word ... just for interest's sake, why don't you
think "semantics" are being described in this section?

   Section 3: New WebDAV XML Element Attributes
   - ------------------------------------------
   As both, the headline itself and the introductory statement to this
   section suggest, this section should be moved to (a new revision of)
   the WebDAV protocol.

I agree, but unless we defer the versioning protocol until such a new
revision is created, we are stuck with defining WebDAV extensions in
the versioning protocol when we find they are needed.  So as soon as
the WebDAV extension is defined, I'm happy to strip this kind of thing
from the versioning protocol.

I've recently received a suggestion from Yaron that we replace this with
tokens for the If header.  I think he's right, so it's likely that we'll
be killing this section anyway.

   Section 4: New WebDAV Properties
   - ------------------------------
   Again, move this to WebDAV.  Note that 4.1.3 and 4.1.4 do *not*
   introduce new WebDAV properties, as the headline claims.

Yes, 4.1.3 and 4.1.4 just got put there because there seemed to be no
place better to put them.

   Section 4.4: DAV:supported-method-set
   - -----------------------------------
   > method-name type: a type in the DAV://METHODS/ namespace

   In general, this does not work. ...
   Personally, I would prefer a syntax like:

     <D:supported-method-set xmlns:D="DAV:">

This would be fine with me.  I'll make that change if nobody objects.

   Section 4.6: DAV:supported-report-set
   - -----------------------------------
   Same problem as in section 4.5.  However, this is currently
   independent from WebDAV and hence can and should be fixed, e.g.:

   <D:supported-report-set xmlns:D="DAV:">

One reason DeltaV (and WebDAV) use this approach is that it
allows for consistent use of namespaces, i.e. so you can have
your XML parser to namespace resolution of these report names.
If one made the change you suggest, it would have to be
something like:

   <D:supported-report-set xmlns:D="DAV:">

   Section 5.1.2: DAV:predecessor-set (protected),
   Section 5.1.3: DAV:successor-set (protected)
   - ---------------------------------------------
   It is very important that the server really maintains the order of the
   DAV:href elements, as noted for the predecessor set.  But the
   successor set should also be maintained.  Think for example of
   graphically displaying a revision graph.  You really do not want the
   branches of the revision graph to be aligned differently each time the
   graph is re-displayed.  Moreover, an application might want to
   designate the first DAV:href element of the successor set to continue
   the main development branch.

Unlike the DAV:predecessor-set (which is set by the client),
the DAV:successor-set is a query (the inverse of DAV:predecessor-set)
computed by the server, and constraining
the order of the results of this computation would place an (in my view)
unacceptable constraint on server implementations.

If a server wants to interoperate with multiple clients, it should
use an activity to capture the "main branch", and not depend upon
some server dependent predecessor set ordering.

   Section 5.1.4: DAV:checkout-set (protected)
   - -------------------------------------------
   This should also be an ordered set maintained by the server.  For
   example, think of a client that chooses to graphically display working
   resources and checked-out version selectors as part of a revision
   graph.  Or, to give another example, depending on the underlying
   revision control system, the first element in the set may be a
   designated candidate to continue the main development branch.

Same response as for DAV:successor-set.  This is not a client specified
property, it is a server computation/query.

   Section 6: WebDAV Versioning and Existing Headers
   - -----------------------------------------------
   I think this headline does not express what is following.  Personally,
   I would prefer a wording like:

   "Versioning Vis-a-Vis WevDAV Headers" or
   "Impact of Versioning on WebDAV Headers".

The new versioning header is still a WebDAV header, so I personally
prefer the current title.  Anyone else have a preference?

   Section 7.1.1: Example - GET request with DAV:must-not-be-checked-out
   The example contains a CHECKOUT request, not a GET request.

Oops!  Thanks for catching that.

   > In this example, the request to CHECKOUT /foo.html fails because
   > /foo.html is already checked out

   ... and /foo.html is a version selector, or because the server chooses
   not to branch off a new version.

If it were those other things, it would have returned a different
token in the response body.

   Section 7.2: OPTIONS
   - ------------------
   > If the server supports checking out a version selector

   i.e. checking out a version selector is an optional feature and should
   be moved to Optional WebDAV Versioning?

We're currently addressing this in a separate thread.  My proposal
is that support for checking out a version-controlled resource be
mandantory (since it is easy for both client-workspace and server-workspace
servers to implement).

   Section 7.3: PUT
   - --------------
   In the preconditions section, you use the format
   "<DAV:some-error-tag/>: Error condition".  To be kind,
   you should introduce the reader to this format or use a more
   self-explanatory format.

OK, what would the more self-explanatory format be?  (:-).

But more seriously, I agree this needs explanation.  I'll add it to
the "notational conventions" section.

   Section 7.4: PROPPATCH
   - --------------------
   > <DAV:cannot-modify-protected-property/>: An attempt to use PROPPATCH
   > to modify a property (either core or optional) defined by this
   > document as being protected MUST fail.

   > <DAV:cannot-modify-unsupported-property/>: An attempt to modify a
   > property defined by this document (either core or optional) whose
   > semantics are not enforced by the server MUST fail.  This helps ensure
   > that a client will be notified when it is trying to use a property
   > whose semantics are not supported by the server.

   What about moving these features to WebDAV?  They are probably of
   generic interest and not versioning specific (assuming that the
   definition of the term "protected property" also could be moved to

When there is a new version of the WebDAV protocol open for editing, I
agree we should move it there.

   Section 9.1: VERSION_CONTROL
   - --------------------------
   The logical structure of the rules in this section seems rather weird
   to me.

   The section contains the following precondition:

   > <DAV:must-be-versionable/>: The request-URL MUST identify a
   > versionable resource, a null resource, or a version selector.

   Accordingly, I would expect three paragraphs in the beginning of this

   "If the request-URL identifies a versionable resource, then ..."
   "If the request-URL identifies a null resource, then ..."
   "If the request-URL identifies a version selector, then ..."

   A classification into disjunctive sets of cases also is probably
   less error prone.

Some of these conditions apply to more than one of these cases,
so with a disjunctive set of cases, we would have to repeat preconditions
appearing in other cases, which seems error prone.

On the other hand, there are two preconditions that only apply to
version controlling a null resource, and those should be grouped
together (and currently are not).  I'll fix that.

   Actually, in section 9.1 currently there is only a paragraph about
   versionable resources.  The second paragraph talks about the case when
   the request *body* identifies a version.

   The postconditions section, on the other hand, classifies cases
   thoroughly.  Still, I wonder which rules apply when the request-URL
   identifies a version selector.  Section 9.1 only says that

   > the DAV:version-control request
   > body element MUST NOT contain a DAV:version element."


   > If the request-URL identified a version selector at the time of the
   > request, the VERSION-CONTROL request MUST NOT change the state of that
   > version selector.

   But then, what does a VERSION-CONTROL do at all if the request-URL is
   a version selector?

Nothing.  This is here to let a client use the same protocol sequence
to put a resource under version control, whether or not the server
automatically puts all new resources under version control (which a
server is allowed to do).  I'll add a sentence to this effect, since
I've had that question before.

   Section 9.2: CHECKOUT
   - -------------------
   > A versioning server MUST support either version selector CHECKOUT or
   > version CHECKOUT, and MAY support both.

   These two concepts should be explained somewhere (e.g. in the terms
   section or in the introductory section).  As far as I understand,
   version selector CHECKOUT is appropriate only when a single user
   accesses a versioned resource?  And as soon as two or more users want
   to access it, you need working resources and hence have to apply
   version CHECKOUT?  If so, this should be somewhere explained (e.g. as
   rationale or concepts, for example in the introduction).

I agree that these variants need to be better explained and contrasted.

I believe that we should break this up into three categories:
public checkouts (in place checkouts, must be supported)
client workspaces (working resources, optional)
server workspaces (server side workspaces, optional).

   Possible you might choose leaving version selector CHECKOUT as
   core versioning feature, but moving version CHECKOUT to optional
   versioning (preferably as part of the fully fledged versioning as
   proposed in the beginning of this mail).

I agree.  This would give us required functionality (public checkouts)
and the two options for private checkouts: client workspaces (working
resources) and server workspaces

   > The content and dead properties of the working resource MUST be the
   > same as the content and dead properties of the DAV:checked-out
   > version.

   I would prefer the following wording:

   "The content and dead properties of the working resource MUST be
   initialized with the content and dead properties of the
   DAV:checked-out version."

One could phrase postconditions in the form of "this is the state
following the successful execution of the request" or "this is what
the server must do to successfully execute this request.  Either one
works, but we should be consistent, and currently the postconditions
are in the first form.  I also believe the first form is simpler.

   > If DAV:apply-to-target is specified in the request body, the CHECKOUT
   > is applied to the version identified by the DAV:target of the version
   > selector, and not the version selector itself.

   For better understanding, I would suggest the following wording:

   > If the request-URL contains a version selector and DAV:apply-to-target
   > is specified in the request body, the CHECKOUT is applied to the
   > version identified by the DAV:target of the version selector, and not
   > the version selector itself.

Fine by me.  I'll add that in.

   Section 9.3: CHECKIN
   - ------------------
   The meaning of DAV:keep-checked-out is not just to keep a checked out
   resource when applying a CHECKIN, but also to perform a subsequent
   update of the checked out resource's DAV:checked-out property.  As far
   as I understand, this results in a behaviour that is the same as that
   of a CHECKIN immediately followed by a CHECKOUT (except that in the
   case of a working resource its URL is recycled).  Hence, I would
   suggest to name this XML element e.g. "DAV:checkpoint" rather than

The term "checkpoint" is used by several CM systems to mean something
like a "baseline", so I think DAV:keep-checked-out is less likely to
confuse people.

   Section 9.4: UNCHECKOUT
   - ---------------------
   > An UNCHECKOUT request can be applied to a checked-out version selector
   > to cancel the CHECKOUT.

   This probably should read:

   "An UNCHECKOUT request can be applied to a checked-out version selector
   or working resource to cancel the CHECKOUT."

   Or should I use DELETE to cancel a version CHECKOUT (though I
   personally would prefer UNCHECKOUT)?  If so, an appropriate note for
   the reader should be added.

I'll add that note.  UNCHECKOUT is there primarily to reset the
content and dead properties of a checked-out version-controlled
resource.  For a working resource, a simple DELETE will suffice (there
is no modified version-controlled state to reset in that case).

   Section 9.5: SET_TARGET
   - ---------------------
   Typo: "Preconditions:" has a boldface colon.  This occurs also in some
   other sections (e.g. 15.12, 16.1).

Thanks for noticing that!  I'll get rid of those.

   Section 10.1: DAV:version-tree-report
   - -----------------------------------

   > The DAV:version-tree-report describes all the versions of the version
   > history of a version in the form of a nested tree of versions.

   I would prefer the following wording:

   The DAV:version-tree-report describes all the descendant versions for a
   given version in a version history in the form of a nested tree
   of versions.

That would be a functional change, rather than a wording change.
Currently DAV:version-tree-report gives you the full report for
all versions in the history (i.e. a report rooted at the initial
version rather than one at the specified version).

Do folks think we want to make that change?  We could make
this report on version selector be the whole tree, while this
report on a version be rooted at that version.

   > ANY value: zero or more property-name elements
   > property-name type: a property type
   > property-name value: none

   This should be explained more verbosely.

   > <!ELEMENT prop (see [RFC2518], section 12.11)>

   Argh!  This is definitely not a valid element definition.

Since the spec is being read by humans and not XML parsers,
we should be OK (:-).  But I'm happy to replace the !ELEMENT
with English text.

   Section 10.1.1: Example - DAV:version-tree-report
   - -----------------------------------------------

   >     <D:creator-displayname/>
   >     <D:predecessor-set>
   >   </D:version-tree-report xmlns:D="DAV:">

   Typo: should be <D:predecessor-set/>

Thanks for catching that!

   Section: 16.1 LABEL
   - -----------------
   > When comparing two label names to decide if they match or not, a
   > server SHOULD use a case-sensitive octet-by-octet comparison of the
   > two label names.

   I propose to replace "SHOULD" with "MUST".  Otherwise, a client can
   not be sure, what the server actually does and has to assume the worst
   case, i.e. case-insensitive match.

Yeah, I'd prefer that too, but this was the compromise after a long
and 'vigorous' (:-) debate.  One hates to say "SHOULD" except as a
last resort, and this was one of those cases.

   > <!ELEMENT add (label-name)>
   > <!ELEMENT set (label-name)>

   I still think that the difference between these two elements and their
   semantics does not get clear.

Hmm.  Can you identify what mistake a reader could make based
on the current definitions?

   > <DAV:must-be-version-or-version-selector/>: The request-URL MUST
   > identify a version or a version selector.

   This probably should read:

   "<DAV:must-be-version-or-version-selector-or-collection/>: The
   request-URL MUST identify a version or a version selector or a

In general, we consider a Depth:n request to be just a macro for
a set of Depth:0 (or non-Depth) requests, whose responses appear
separately in a 207.  If one of the collections is not a version
selector, it MUST return this error token in the 207 response
for this particular collection.

   > <DAV:must-not-be-checked-out/>: If LABEL is being applied to a version
   > selector, it MUST NOT be checked out.

   The phrase "it MUST NOT be checked out" is ambigous
   (noun/verb/adjective phrase or passive voice).  For clearity, what
   about the following:

   "<DAV:must-not-be-checked-out/>: Applying LABEL to a version selector
   MUST fail, if the DAV:checked-out property appears on that version

I have tried to consistently use "checked-out" as an adjective and
"checked out" as a verb.  In this case, I erred (I should have said
"checked-out").  I'll fix that.

Thanks for the great detailed review!!

Received on Tuesday, 5 December 2000 14:36:39 UTC

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