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Meta Data Handling

From: Yaron Goland <yarong@microsoft.com>
Date: Tue, 18 Mar 1997 01:04:50 -0800
Message-ID: <11352BDEEB92CF119F3F00805F14F485025666AC@RED-44-MSG.dns.microsoft.com>
To: w3c-dist-auth@w3.org
1	Meta Data Handling

The proper handling of meta-data has been an issue for this group since
its inception. To date the spec has used links to handle meta-data. The
power of links is that they allow meta-data to be resources. However it
is often the case that useful meta-data does not need to be contained in
resources, witness the existence of HTTP headers.
It would seem logical to simply continue adding entity headers to HTTP
in order to provide for new meta data. The following is a list of
reasons why I believe it is not feasible to put non-request/response
related meta-data in entity headers.

1.	Putting arbitrary meta-data into the header will cause namespace
confusions and increase the processing load for proxies and other
intermediaries who need to sniff headers in order to ensure proper
handling of messages.
2.	It is a waste of bandwidth to return all headers at once.
3.	Violates the principal that one request should perform one
action. If a request sets headers as well as executing some arbitrary
method, it is necessary to provide error information for all of the
associated actions.
4.	There is no way to delete a header.
5.	There is no way to modify just one part of a header. So, for
example, if a header consists of "name: blah12, blah13, blah14" there is
no way to say that "blah13" should be replaced with "blah15".

While links solve all of these problems, they have their own problem,
for example, they make all meta data into resources. This is often
overkill. Status information such as modification dates, ownership
information, etc. can all be usefully recorded in small pieces of data
rather than requiring the overhead of a resource.

In order to keep the meta-data model "light", I further propose that
meta-data take the form of a header. In addition I recommend that
meta-data behave like headers in that multiple pieces of meta-data with
the same header name must be unambiguously combinable using commas.

In order to handle the assignment, modification, discovery, and removal
of meta-data I propose adding three new methods - METAPOST, METAGET, and
METADELETE. These methods, defined below, accept meta-data that is
structured like HTTP headers for assignment to a resource through
METAPOST. Allow for the discovery of meta-data through METAGET and
finally allow for the removal of meta-data through METADELETE.

METAPOST is meant to be a more generic form of LINK, METADELETE a more
generic form of UNLINK, and METAGET is the equivalent of GETLINKVAL for
this type of meta-data.

What this proposal will not address is the question of where meta-data
is stored. This issue is intentionally left open in order to allow
implementers maximum flexibility and power.

For example, let us say a server has the ability to accept arbitrary
meta-data. A user then comes along and puts a MS Word document on the
server. The server understands the Word document's format and is able to
retrieve from it author, creation date, and revision information. The
server is now free to make that information available to users who know
nothing about Word or its format through the META* methods.

For ease of conversation I would like to define the following terms as
used below:
Attribute - A piece of meta-data consisting of an attribute name and
entries.
Entry - One of a comma delimited list of values assigned to an attribute
name.

2	METAPOST Method

2.1	Attribute-Specify Header

AttributeSpecify = "Attribute-Specify" ":" #("name" "=" AttributeName
"," #("value" "=" AttributeValue)) CRLF
AttributeName = field-name
AttributeValue = [field-value without commas or quoted string]

2.2	Server-Supported Header

ServerSupported = "Server-Supported" ":" 1#field-name CRLF

2.3	Atomic Header

AtomicHeader = "Atomic" ":" CRLF

2.4	Explanation

The METAPOST method is used to add an entry to an attribute. If the
attribute does not exist then it will be created. If an
Attribute-Specify header does not contain any entries and the attribute
does not exist, and the attribute's definition allows for entryless
attributes or if the server does not enforce the attribute's semantics,
then the attribute is created with no entries. As specified for HTTP 1.1
headers, the attribute name is case insensitive.

The server-supported header specifies that the request should only go
through if the server supports the syntax and semantics of the specified
attributes, which may or may not be listed in an accompanying
Attribute-Specify.

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.

NOTE: The Server-Supported header seems to be a bit wasteful. It might
be worth defining attribute names to have the form: ["req"] "\"
field-name. The idea being that all attribute names would be prefixed by
a "\". If a "req" is listed before the "\" then users of the attribute
expect the server to provide some sort of behavior above and beyond what
is provided by the META* methods. If the server does not recognize the
attribute then it would refuse to METAPOST it.

2.5	Example

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

This message requests that the entries "Jim Whitehead" and "Roy
Fielding" be added to the 29348x attribute and that the entry Ooooga! Be
added to the Xyz attribute. However, the request should only go through
if the server supports the 29348x and LKHXZ attributes and only if all
the entries can be successfully assigned.

2.6	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.

3	METADELETE Method

The METADELETE method is used to remove one or more entries from an
attribute. If an attribute-specify header is used without any entries
then this is interpreted as meaning the header should be deleted. Note
that this is a different interpretation for the same structure as used
in METAPOST.

3.1	Example

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

The previous will delete the x98 value from the blah attribute.

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

Will remove the blah attribute, but only if the server enforces the blah
attribute syntax and semantics.

3.2	Response Codes

Same as METAPOST.

4	METAGET Method

The METAGET method is used to retrieve all the entries associated with
an attribute.  If entries are specified then the response will only
include entries that match those listed. This mechanism is useful for
quickly checking if an entry has already been set.

The atomic header may not be used with the METAGET method.

4.1	Example

METAGET http:\\foo HTTP/1.1
Attribute-Specify: name = ydfh, name = blah, value = foobar
Server-Maintained: ydfh, 29348x

The server is being asked to return all entries associated with the ydfh
attribute and to return the foobar entry of blah if it exists. However,
these values should only be returned if the server supports the ydfh and
29348x attributes.

4.2	Response Codes

Same as METAPOST with the exception of atomic related errors.

5	Link Attribute

If META* is adopted then links become attributes. I propose they be
defined as follows:

Name = "Link"
Entry = #(Source Dest Type *(";" link-extension))
Source = Specifier SP
Dest =  "Dest" "=" Specifier SP
Specifier = <"> URI <"> | "SET"
Type = "Type" "=" Token SP

Links would be created, retrieved, and deleted using the META* commands.
All previous descriptions of link behavior still hold. The current spec
thus prevents behavior such as setting both URIs to SET. However I also
propose removing the rule that either the source or destination must be
equal to the Request-URI. This will enable annotation facilities.

I have left the "Source" label off the initial URI in order to maintain
the same format as the Link header given in the HTTP 1.1 spec.

		Yaron
Received on Tuesday, 18 March 1997 04:04:48 GMT

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