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proposal for ordered collections

From: Jim Davis <jdavis@parc.xerox.com>
Date: Wed, 3 Dec 1997 23:04:44 PST
Message-Id: <3.0.3.32.19971204000444.007bfea0@mailback.parc.xerox.com>
To: w3c-dist-auth@w3.org
This is a proposal to modify WebDAV to include support for ordered
collections.

I have already stated the case for this feature elsewhere, and Jim
Whitehead has neatly summarized it.  Moreover, I believe I have
refuted all objections thus far raised to it without being (as yet)
further rebutted.  I agreed to define the proposal more formally, and
here I do so, although the credit for this definition more properly
goes to Judy Slein, who did all the work, but as she was too busy to
write it up, I am submitting it. Of course she should not be blamed
for the errors I introduce here.

The idea in a nutshell is to allow servers to optionally support
collection resources whose stats includes not only the list of members
but also the which order of members in the list.  It also includes
headers that specify the position of a newly added member in an
ordered collection.

The proposed modifications are in order of textual occurrence in draft V5.

Change 4.1 (Collection Resources) as follows:

Replace the first sentence:

   A collection is a resource whose state consists of an unordered list
   of internal members, an unordered list of external members, and a
   set of properties. 

with:

   A collection is a resource whose state consists of a list
   of members and a set of properties.  Members MAY be internal or
   external.  Servers MAY also extend the state to include the order
   of the members in the list.  Such collections are known as
   "ordered" collections.  

and add after the next two paragraphs:

   Servers MAY support ordered collections. If they do, they MAY allow
   both ordered and unordered collections.

8.1 PROPFIND

Modify this paragraph by adding the sentence marked with change bars

   The response is a text/xml message body that contains a  multistatus
   XML element that describes the results of the attempts to retrieve
   the various properties.  If a property was successfully retrieved
   then its value MUST be returned in a prop XML element.  If the scope
   of PROPFIND covers more than a single resource, as is the case with
   Depth values of "1" and "infinity", each response XML element MUST
   contain an href XML element which identifies the resource on which
   the properties in the prop XML element are defined. 
|  If the resource is a collection, and if the collection is ordered,
|  then the response XML elements must be returned in the order in
|  which they appear in the collection.
   In the case of
   allprop and propname, if a principal does not have the right to know
   if a particular property exists, an error MUST NOT be returned.  The
   results of this method SHOULD NOT be cached.

8.3 MKCOL

Add after the third paragraph of 8.3.1

   If the Is-Ordered header is present, then if the server supports
   ordered collections, then the value of the is-ordered property of
   the newly created collection MUST be the same as the value in the
   header.  If the server does not support ordered collections, then
   if the value in the header is T, then the request should fail with
   a 501 Not Implemented status code.  If the Is-Ordered header is not
   present, then if the server supported ordered collections,
   it MAY create an ordered collection.

8.4.1 (INDEX) modify the paragraph, adding the sentence:

   Collections that respond to an INDEX method with a text/xml entity
   MUST contain a single multistatus XML element which contains a
   response XML element for each member.
|  If the collection is ordered, then the elements must occur in the
|  same order they occur in the collection.


8.5.1 ADDREF

Add after paragraph two:

  If the collection is ordered, then if an Insert-Position header is
  provided, then it specifies the position of the new member.  The
  Insert-Position header is defined below.  If no such header is
  provided, it is RECOMMENDED that the member be added last.  If the
  member can not be inserted at the specified position then the ADDREF
  method MUST fail with a 412 Precondition Failed status code.  If the
  collection is not ordered and an Insert-Position header is provided,
  the ADDREF method MUST fail with a 412 Precondition Failed status
  code.

8.10.1 PUT for Non-Collection Resources

Add after the second paragraph

  If the resource is being added to an ordered collection, then if an
  Insert-Position header is provided, then it specifies the position
  of the new member.  The Insert-Position header is defined below.  If
  no such header is provided, it is RECOMMENDED that the member be
  added last.  If the member can not be inserted at the specified
  position then the ADDREF method MUST fail with a 412 Precondition
  Failed status code.  If the collection is not ordered and an
  Insert-Position header is provided, the ADDREF method MUST fail with
  a 412 Precondition Failed status code.

[Perhaps this paragraph should also be added after the second
paragraph in 8.11.1.1 COPY for HTTP/1.1 resources.  I don't think I
need it for MOVE since it is a COPY plus a DELETE.]

Add to section 9 (HTTP Headers)

9.9 Insert-Position Header

    InsertPosition = "Insert-Position" ":" AbsPosition | RelPosition
    AbsPosition = "First" | "Last"
    RelPosition = ("Before" | "After") URI 
     ; URI is defined in section 3.2.1 of [Fielding et al., 1997]

  The Insert-Position header is used with methods that add resources
  to collections.  It specifies the position of the resource being
  added, either absolutely or relative to an already existing member.

  If the relative form is used, then the resource named by the URI
  MUST already exist as a member of the collection.  If not, the
  method MUST fail with a 412 Precondition Failed status code, except
  that if the client does not have sufficient access to obtain a list
  of the members of the collection, a different status code MAY be returned.

[Perhaps the syntax should reserve some means for future extension? Or the
writeup should have some language about what the server does if it does not
understand the content of the header? ]

9.10 Is-Ordered Header

    IsOrdered = "Is-Ordered" ":" T | F

  The Is-Ordered header is used with MKCOL, and causes the is-ordered
  property of the newly created collection to be set to the same value
  as the header.
	
Add to section 13 (DAV Properties) 

13.X is-ordered property

  Name: is-ordered
  Namespace: http://www.ietf.org/standards/dav/
  Purpose: Tells whether the collection supports ordering
  Value: T | F
  Description:  This is a live property.  All collections MUST have this
  property.  On servers that do not support ordering, its value MUST always
  be F.  
Received on Thursday, 4 December 1997 03:05:34 GMT

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