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

From: Juergen Reuter <reuterj@ira.uka.de>
Date: Fri, 01 Dec 2000 19:26:18 +0100
To: ietf-dav-versioning <ietf-dav-versioning@w3.org>
cc: reuterj@ira.uka.de, jjh@ira.uka.de
Message-ID: <"iraun1.ira.0088601:001201.182348"@ira.uka.de>
Hello, all!

This is a review of the core versioning part and label option of the
draft-ietf-deltav-versioning-10.4 (with updates for 10.5).

General Comments
- --------------

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.
If, instead, a server implements the core part and only some features
of the optional part, the client will have to figure out the server's
capabilities through all those DAV:supported-*-set properties.  I
think only very few clients will be so smart to adapt to the server's
individual capabilities.  Most clients probably will either rely only
on the core part or will require the full optional part.  Or, even
worse, implementors will start to create client/server pairs with
built-in features of the optional part as needed in the specific
pair.  In either case, interoperability will tremendously suffer.

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

This would lower the gap between the currently defined two levels und
thus help to preserve interoperability.

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

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.

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

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.

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.

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

In general, this does not work.

RFC 2616 says:

       Method         = "OPTIONS"                ; Section 9.2
                      | "GET"                    ; Section 9.3
                      | "HEAD"                   ; Section 9.4
                      | "POST"                   ; Section 9.5
                      | "PUT"                    ; Section 9.6
                      | "DELETE"                 ; Section 9.7
                      | "TRACE"                  ; Section 9.8
                      | "CONNECT"                ; Section 9.9
                      | extension-method
       extension-method = token
       token          = 1*<any CHAR except CTLs or separators>
       separators     = "(" | ")" | "<" | ">" | "@"
                      | "," | ";" | ":" | "\" | <">
                      | "/" | "[" | "]" | "?" | "="
                      | "{" | "}" | SP | HT

Hence, the token %XYZ& is a valid HTTP method name (even if it is
unlikely to be ever used; but anyway...).  However,
http://www.w3.org/TR/2000/REC-xml-20001006 says:

[4]   NameChar     ::=  Letter | Digit | '.' | '-' | '_' | ':' | CombiningChar | Extender
[5]   Name         ::=  (Letter | '_' | ':') (NameChar)*
[44]  EmptyElemTag ::= '<' Name (S Attribute)* S? '/>' [WFC: Unique Att Spec]

Thus, the Name M:%XYZ& is not a valid name for an XML tag.

Personally, I would prefer a syntax like:

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

This would also contribute to a clean DTD.  I think, only if you have
a very small, definitely fixed set of tokens, it may be appropriate to
create tag names from the elements of this set.  But the set of HTTP
method names is neither small, nor fixed.

Another important point here is that the above syntax leaves it open
to you to add new method related features in future versions of DeltaV
or WebDAV, e.g. something like this:

<D:method-property xmlns:D="DAV:">

Section 4.5: DAV:supported-live-property-set
- ------------------------------------------
This is another example for the anomaly in the DAV:prop element usage
Though, to correct this, WebDAV itself needs to be revised. :-(

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:">

The syntax for the request body and repsonse body for the REPORT
method should be properly adapted.

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.

To make the wording even clearer, I would like to suggest to change it
as follows:

  This property is an ordered set that contains a URL for each
  predecessor of this version.

(And the same for the DAV:successor-set property.)

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.

Section 5.1.5: DAV:checkin-date (protected)
- -----------------------------------------
I still vote for giving a rationale for the term "reasonable
approximation".  What is the idea here?  Do you fear trouble when a
client checks in a revision, then reads the date, compares it to its
local time and detects a clock skew?  But, anyway, why should a client
do that?  Or are you thinking of a distributed repository of versioned
resources that is made accessible through an HTTP port of a single
host/domainname?  Checking if revision X of resource A was checked in
later than revision Y of resource B might become a problem if A and B
are hosted on different machines.  Anaway, what is the rationale here?

Section 5.2.3: DAV:predecessor-set
- --------------------------------
Same as section 5.1.2.

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".

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

> 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.

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?

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.

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

Section 8: New WebDAV Methods
- ---------------------------
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.

- --------------------------
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.

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?

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).

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).

> 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."

> 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.

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

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.

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

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.

> 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.

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/>

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.

> <!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.

> <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

> <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

- ------------------------------------------------------------------------

Received on Friday, 1 December 2000 13:26:43 UTC

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