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Meta Data Redux

From: Yaron Goland <yarong@microsoft.com>
Date: Tue, 18 Mar 1997 21:49:47 -0800
Message-ID: <11352BDEEB92CF119F3F00805F14F485025666C9@RED-44-MSG.dns.microsoft.com>
To: w3c-dist-auth@w3.org
Ora - I erred in creating a system more general than I had intended. I
have rewritten my meta-data proposal. I wrote the introduction to
specifically address your points.

Mr. Mealling - Do you honest believe that insulting this group by
asserting that we are not dealing with "REAL" meta-data is going to
incline anyone to consider your proposals?

Ron - Generic object model based meta-data handling is the holy grail of
distributed systems. I propose we adopt the Pythonian approach to holy
grail related matters and RUN AWAY!!!! As such my rewritten proposal now
suggests that we only provide for the absolute minimum interoperability.
I feel that questions of packages, structures, and frameworks is best
left to other groups.

Terry Allen - I am actually a big believer in XML. However, while I do
believe that leveraging XML would make for a great meta-data system, I
do not believe that this group should deal with that issue. There are a
lot of arguments on both sides and I do not believe that the win for
this group is sufficiently large to justify the group's investment of
time. Many groups have attempted to solve the generic meta-data problem,
the current proposals I am familiar with include: XML, Web Collections
(sub-setting XML), Warwick Framework, MIMEDIR, and PICS. I am sure you
can imagine what life will be like if this group tries to pick a winner.
I say we do the minimum needed for interoperability, declare victory,
and publish the standard. =)
1	Why Meta Data

I propose that DAV's goals in regard to meta-data should be two fold:
1.	To provide for a mechanism for clients to place, delete and
discover relationships between resources.
2.	To provide for a mechanism for clients to place, delete and
discover simple meta-data consisting of a name and a short non-content
negotiable byte field.

I believe that the first goal is important because the ability to link
resources has been consistently proven to be of critical importance in
versioning and of high utility for distributed authoring. As such I feel
the group should provide for such a mechanism. I further believe that
the current LINK proposal, if modified with the removal of the
restriction requiring the source or destination to be equal to the
resource the link is recorded on, meets those needs in full and as such
the LINK and UNLINK methods should be maintained.

The second goal is meant only to provide for the absolute minimum,
useful, meta-data interoperability. I believe that this minimum is the
ability of a client to 
1.	Submit an opaque token and receive back a string of bytes that
are not subject to content negotiation.
2.	Assign a string of bytes of non-content negotiated bytes to an
opaque token.
3.	Remove the assignment of an opaque token and its value from a
resource.

From this point on I will use the term attribute name to refer to the
opaque token.

This goal is not to provide for a meta-data object model. The goal is
not to try to compete with or adopt PICS, Web Collections, MIMEDIR, the
Warwick framework, or any of the other proposals for generic meta data
handling.

I additionally propose that we intentionally leave undefined where
attribute names and associated values are recorded. This leaves the
implementer free to choose whichever meta-data management scheme they
desire. In fact, it leaves the implementer free to choose multiple
meta-data management schemes.

2	Attribute Name Space and Value

I propose that the attribute name space be opaque to a DAV client.
However, in order to prevent name collisions and to ensure the proper
management of the name space, I propose that attribute names be URIs.

I further propose that servers not be allowed to accept attributes whose
syntax or semantics they do not support. One can, of course, always
define a URI namespace whose semantics is "The server should accept any
attribute from this namespace and blindly record its value."

Finally, I propose that the value of an attribute be limited to a quoted
string or token. This restriction is to allow for the inclusion of
multiple attribute-specify headers in a request.

3	METAPOST Method

3.1	Attribute-Specify Header

AttributeSpecify = "Attribute-Specify" ":" #("name" "=" AttributeName
"," "value" "=" AttributeValue) CRLF
AttributeName = URI
AttributeValue = quoted-string | token

3.2	Atomic Header

AtomicHeader = "Atomic" ":" CRLF

3.3	Explanation

The METAPOST method is used to define a value for an attribute. If the
attribute does not exist then it will be created. If the attribute does
exist then the attribute value will be replaced. Of course this behavior
is subject to the semantic and syntactic rules of the attribute.

The Atomic header specifies the request should only go through if all
the entries can be assigned as requested. If not, the entire request
should fail.

3.4	Example

METAPOST http:\\foo HTTP/1.1
Attribute-Specify: Name = 29348x, Value = "Jim Whitehead", Name = Xyz,
Value = Ooooga!
Atomic:

This message requests that the quoted string "Jim Whitehead" be assigned
to the attribute 29348x and that the token Ooooga! be assigned to the
Xyz attribute. However, the request should only go through if all the
entries can be successfully assigned.

3.5	Response Codes

A server may reject entries because they are not consistent with the
definition of the attribute. In that case a 406 Not Acceptable should be
returned.
If the atomic header is included then all indicated assignments must
succeed or all must be rejected with a 412 Precondition Failed. If the
atomic header is not included then a 200 may be returned if at least one
entry was successfully assigned. A web collection returned in the
response will indicate which entries where successfully assigned.
I recommend that the atomic header not be required by a server in order
to be considered DAV compliant. Implementing atomic is extremely
difficult and not absolutely necessary for interoperability.

4	METADELETE Method

The METADELETE method is used to remove an attribute from a resource. If
a value is included in the attribute-specify header then the attribute
should only be removed if its value equals that specified in the header.
The atomic header may be used with METADELETE and has the same semantics
as with METAPOST.

4.1	Example

METADELETE http://foo HTTP/1.1
Attribute-Specify: Name = blah, Value = x98

The previous will delete the attribute blah if its value equals x98.

METADELETE http://foo HTTP/1.1
Attribute-specify: blah

The previous will remove the attribute blah, regardless of value.

4.2	Response Codes

Same as METAPOST.

5	METAGET Method

The METAGET method is used to retrieve the value of an attribute. If a
value is included in the attribute-specify header then the attribute
will only be returned if its value is equal to that given in the
attribute-specify header. The atomic header may be used with the METAGET
method.

5.1	Example

METAGET http:\\foo HTTP/1.1
Attribute-Specify: name = ydfh, name = blah, value = foobar

The server is being asked to return the value associated with the ydfh
attribute and to return the blah attribute's value if it equals foobar. 

5.2	Response Codes

Same as METAPOST with the exception of atomic related errors.
Received on Wednesday, 19 March 1997 05:03:04 GMT

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