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Re: Mixed model metadata proposal

From: Judith Slein <slein@wrc.xerox.com>
Date: Sat, 3 May 1997 07:37:12 PDT
Message-Id: <2.2.32.19970503143712.01690c38@pop-server.wrc.xerox.com>
To: Jim Whitehead <ejw@rome.ics.uci.edu>
Cc: w3c-dist-auth@w3.org
I like this proposal very much.  It insures that small chunk metadata can be
managed easily.  It provides a way to provide definitions of metadata
elements that don't have to be centrally managed.  It makes it possible for
Web Crawlers to learn to search metadata and relate it to the right
resource.  I like using the DAV:/Link metadata type much better than a LINK
header, because you can tell that the destination resource is metadata.

My comments are all ones of detail.

I think (and hope) you intend the DAV:/Link attributes to be part of the new
small-chunk metadata space you propose.  If this is the case, a couple of
things need to change.  

1. The BNF for text/tab-separated-values needs to allow Name to be DAV:/Link.
2. It might be useful to define an additional application/dav-link-search
media type more tailored to searching for links with a given TYPE, for example.

I want to be sure that it's possible to search for resources via their
metadata.  So I should be able to do GETMETA over a whole site or some
subtree of the site or some collection, and the response would tell me which
resources have matching metadata, and list the matching metadata for each
such resource.

If we want the server to be able to figure out whether an attribute value is
consistent with the attribute definition, we will have to standardize
attribute definitions so that servers can parse and understand them.

The definition of Value in the BNF for text/tab-separated-values doesn't
make sense to me.

You say that a server must not record two links which have the same source,
dest, and type.  Do you really mean this, or only that it must not record ON
THE SAME RESOURCE two links which have the same source, dest, and type?  I
think for purposes of managing link integrity it might be desirable to be
able to record the same link on its source resource and on its destination
resource.  It might even be a good thing for a server to have a policy of
doing this.

At 05:21 PM 4/29/97 PDT, Jim Whitehead wrote:
>
>A Proposal for Web Metadata Operations
>
>Draft v0.1, April 29, 1997
>
>Abstract
>
>This document provides rationale for why metadata support for Web resources
>is desirable, gives a model for separating existing metadata into small
>chunk and large chunk metadata, lists requirements for how to manipulate Web
>metadata, and provides a proposal which meets these requirements for how
>metadata can be created, deleted, and queried on Web resources using a set
>of extensions to the HTTP (version 1.1) protocol.
>
>Introduction
>
>In its most abstract form, metadata is "information about information."
>Information on the Web, known as Web resources, have many pieces of
>associated descriptive information which is often not explicitly represented
>in the resource itself. Examples of metadata include the creator of a
>resource, its subject, length, publisher, creation date, etc. Such
>descriptive metadata can be used to make information easier to locate by
>improving Web searches [Weibel, 1995], rate information to protect children
>from indecent content (e.g. the Platform for Internet Content Selection
>(PICS) [Miller et al., 1996]), capture copyright information, contain a
>digital signature, or store cataloging data. Many other uses are also
>possible.
>
>Another type of metadata is the relationship. A relationship captures an
>association between two or more resources, and can be one to one, one to
>many, or many to many. Relationships can be used to capture navigational
>relationships, such as "go to this resource next," or a table of content,
>and can also express hierarchies (parent/child, successor/predecessor)
>[Maloney, 1996] Relationships have many domain-specific uses, such as a
>piece of software which has many "implements" relationships with a
>requirements document. Annotations are another use of relationships in which
>the relationship points to commentary material on the resource. The use of
>relationships to capture associations between data items is an old idea,
>stemming from semantic data modeling [Abrial, 1974][Hull & King, 1987], and
>early hypertext work on the NLS [Engelbart, 1968] and Xanadu [Nelson, 1981]
>systems.
>
>Characteristics of Metadata
>
>To date, there have been many techniques for describing metadata
>information. On the Web there have been many mechanisms and proposals for
>metadata, including PICS [Miller et al., 1996], PICS-NG, the Rel/Rev draft
>[Maloney, 1996], Web Collections, XML linking, several proposals on
>representing relationships within HTML, digital signature manifests (DCMF),
>and a position paper on Web metadata architecture [Berners-Lee, 1997].
>Related to the Web, but coming from a digital library perspective, are the
>Dublin Core [Weibel et al., 1995] metadata set and the Warwick Framework
>[Lagoze, 1996], a container architecture for different metadata schemas. The
>literature on metadata includes many examples of metadata, including MARC
>[MARC, 1994], a bibliographic metadata format, RFC 1807 [Lasher, Cohen,
>1995], a technical report bibliographic format employed by the Dienst
>system, and the proceedings from the first IEEE Metadata conference describe
>many community-specific metadata sets.
>
>Participants of the 1996 Metadata II Workshop in Warwick, UK [Lagoze, 1996],
>noted that, "new metadata sets will develop as the networked infrastructure
>matures" and "different communities will propose, design, and be responsible
>for different types of metadata." These observations can be corroborated by
>noting that many community-specific sets of metadata already exist, and
>there is significant motivation for the development of new forms of metadata
>as many communities increasingly make their data available in digital form,
>requiring a metadata format to assist data location and cataloging.
>
>Based on an examination of many Web metadata proposals, it appears that Web
>metadata can be broadly characterized into two categories, termed small
>chunk and large chunk. These are described below.
>
>Small chunk metadata
>
>Small chunk metadata includes data items such as:
>
>   * HTTP headers
>   * short attribute-value pairs
>   * typed links (e.g. HTTP links, or binary relationships)
>
>While developing a stringent definition of "small" is most likely
>impossible, since the definition is arbitrary, and seems to be based on
>unstated assumptions about retrieval performance (e.g., retrieval of small
>chunk metadata should be "trivially" or "unnoticeably" fast), much metadata
>has a small chunk flavor to it.
>
>Characteristics of small chunk metadata include: fast retrieval speeds, no
>need for content negotiation, no requirements on ordering, no need for
>"trust" information (e.g., digital signature, author information, hash of
>contents, date of creation), and relatively simple value information.
>
>Large chunk metadata
>
>Large chunk metadata includes data items such as instances of:
>
>   * PICS, PICS-NG collections
>   * Warwick collections
>   * MARC records
>   * Dublin Core records
>   * discipline-specific metadata records
>   * Web pages (e.g., an annotation page)
>
>Like the smallness of small chunk metadata, the largeness of large chunk
>metadata is similarly difficult to define. Characteristics of large chunk
>metadata include: requirements on the ordering of fields, encoded trust
>information, pointers to metadata schema descriptions, complex data models,
>and multiple levels of containment. Large chunk metadata often contains
>several instances of small chunk metadata. Typically large chunk metadata is
>larger than small chunk metadata, although it is easy to develop classes of
>both for which this assertion does not hold. As a result, there is an
>assumption that large chunk metadata takes longer to transmit than small
>chunk metadata. Large chunk metadata, when stored as a separate resource,
>has the advantage that several different representations of the information
>can be stored, such as translations into different natural languages, and
>then used in content negotiation.
>
>Mapping of metadata to the Web data model
>
>The mapping of metadata to the various data containers (resources, headers)
>in the Web data model varies depending on whether the metadata is stored on,
>in, or as a resource.
>
>On resource. In this case, the metadata is stored with the resource, but is
>not a part of the resource itself. Examples include HTTP links, HTTP
>headers, PICS labels (using the PICS-Label header). On resource storage is
>typically used for small chunk metadata, and on resource metadata is
>retrievable after 1 network request (a HEAD or GET).
>
>Within resource. The metadata is embedded within the resource, and is a
>defined part of the document type description. Examples include HTML REL/REV
>links, the HTML META tag, various HTML metadata proposals, Microsoft Word
>.DOC documents, and Web Collections. Within resource metadata is retrievable
>in 1 network request (GET). Within resource metadata has the advantage of
>being independent of access protocol, and is portable (when the resource
>moves, it does too). Within resource metadata tends to be small chunk.
>
>Is (whole) resource. The metadata is itself an entire resource. When the
>metadata is an entire resource, there usually exists a relationship (link)
>between the described and metadata (describing) resources. Examples include
>Web Collections, Warwick containers, Web pages. Typically large-chunk
>metadata ends up as whole resource metadata, such as the MIME encoding of
>Warwick containers described in [Knight, Hamilton, 1996]. Typically
>retrieval of whole resource metadata requires 2 network requests (one to get
>the links, one to get the metadata).
>
>Consistency maintenance
>
>Many sources have noted that metadata can be viewed as an assertion about
>the described data. In this view of metadata, an author attribute is viewed
>as an assertion that a particular person is the author of the information
>being described. Since the Web is a client-server system, there are two
>points of control over these assertions. With client controlled (or user)
>metadata, the consistency of the assertion is maintained by the user.
>Typically the server is unable to perform any validation of client
>controlled (or maintained) metadata. Alternately, the server can control the
>consistency of metadata assertions; one example is the last modified date of
>a resource.
>
>When metadata can be set by many different principals, as is the case on the
>Web, it is desirable to have some way of determining whether a particular
>assertion should be trusted. Trust information is a prominent aspect of the
>PICS container format, which contains a digital signature, contents hash,
>author information, and a valid date range which can be used to assess the
>trustworthiness of the assertions contained in the package.
>
>Requirements for Operations on Web Metadata
>
>The following are the relevant requirements for operations on Web metadata
>as specified in "Requirements for Distributed Authoring and Versioning"
>[Slein et al., 1997].
>
>[5.1.1] It must be possible to create, modify, query, read and delete
>arbitrary attributes on resources of any media type.
>
>[5.2.1] It must be possible to create, modify, query, read and delete typed
>relationships between resources of any media type.
>
>Proposal for Metadata Operations
>
>In early WebDAV proposals [Goland et. al, 1996] all metadata was whole
>resource metadata, with the exception of the links used to hold the
>relationship between the described resource and the metadata resource. While
>this approach handles large-chunk metadata well, it does have significant
>drawbacks for maintaining the referential integrity between metadata and the
>resource(s) it describes, especially when they are controlled by different
>principals. To ensure that metadata could be created and retrieved in one
>method invocation, several convenience functions were proposed which created
>a link and the metadata resource in one action. However, this led to
>difficulties in specifying the operations due to atomicity problems, and
>would be difficult to implement since a partial failure (e.g. link created
>OK, but metadata resource creation failed) would require rollback capability
>in the server. Another significant drawback to this approach is the
>difficulty of providing searches on the value of the metadata. While it was
>easy to propose a full-featured search on the type space of the links to the
>metadata, searches of the metadata itself quickly led to a consideration of
>the full resource searching problem, and difficult issues such as handling
>the wide range of natural languages and media types of resources being
>searched.
>
>Another early draft, the Netscape proposal [Cunningham & Faizi, 1996], gives
>operations for setting and retrieving attribute-value pairs stored in an
>attribute sheet associated with the resource. While this approach provides
>basic support for small chunk metadata, it lacks an attribute search
>mechanism, placing the burden of attribute searching on the client. It also
>has no support for large chunk metadata, although this could be provided in
>a limited way by storing a URI pointer to large chunk metadata in the value
>of an attribute.
>
>Neither a pure whole resource metadata approach, nor a pure on-resource
>approach is able to handle the range of current and proposed Web metadata.
>The whole resource approach has referential integrity problems, and the
>on-resource approach cannot handle the many large chunk metadata formats. As
>a result, the proposal in this document uses a mixed approach for handling
>metadata, providing support for both on-resource, small chunk metadata and
>whole resource, large chunk metadata. This mixed proposal provides
>operations for creating, deleting, and querying attribute-value pairs stored
>on Web resources. Simple binary relationships are stored in "Link" metadata,
>which can point to large chunk metadata resources.
>
>The mixed proposal requires a modification to the object model for HTTP/1.1
>resources to provide a repository for metadata state information in addition
>to the current repositories of state within an HTTP resource: the body and
>headers. This new state information consists of attribute-value pairs, in
>which the attribute's name is a URI, and the attribute's value is an untyped
>octet stream. URIs are used for attribute names to provide a distributed,
>extensible name space for attribute names. URIs also have the capability, if
>dereferenced, of providing descriptive information on the syntax, semantics,
>and use of the attribute.
>
>Disadvantages of storing metadata in the existing HTTP object model lead to
>the desire to modify it. While HTTP headers can and are used for small chunk
>metadata, they have drawbacks for distributed authoring. Since users may
>potentially create the name of an attribute, this raises the possibility of
>name collisions with existing headers. More importantly, since there could
>be potentially many attributes stored on a resource, it is important for
>network efficiency that these attributes not be returned with every GET or
>HEAD request. There are many proposals for placing metadata inside a Web
>resource (e.g., placing a PICS record inside a resource), however, there is
>no general way to define metadata in the body of resources of any media
>type. As a consequence, placing metadata in the body would reduce metadata
>use to just a few specific resource media types, limiting the general use of
>metadata. Since metadata in headers leads to network inefficiency, and
>metadata in bodies is impossible to generalize across all media types, it is
>necessary to add new state for attribute-value metadata to the HTTP/1.1
>object model.
>
>The sections below describe in detail new HTTP methods which can be used to
>create (ADDMETA), delete (DELMETA), search and retrieve (GETMETA)
>attribute-value metadata, including simple bidirectional links. All of these
>methods may return a message body that contains a listing of attribute
>name/value pairs, however, the syntax for how to package these name/value
>pairs has intentionally not yet been specified. It is hoped that one of the
>Web metadata packaging proposals currently being discussed (e.g., Web
>Collections or PICS-NG) will be useable as the return format for WebDAV
>metadata methods. Until these specifications have settled, it is premature
>to use them in a specification.
>
>ADDMETA
>
>Body
>
>Body = *Pair
>Pair = Name HT *Value CRLF
>Name = URI
>Value = Octet-CRLF | (CRLF HT)
>Octet-CRLF = <Octet excluding CRLF>
>
>Explanation:
>
>The ADDMETA method is used to create one or more new attribute-value pairs
>on the resource specified by the Request-URI. The body of the message MUST
>be of content type text/tab-separated-values, containing a sequence of
>attribute name/value pairs. Each name/value pair consists of a URI attribute
>name, followed by a TAB, followed by a stream of octets which specify the
>attribute's value. The value of the attribute may extend over several lines
>in the body, each extension line beginning with a TAB. The name and value
>uniquely define a metadata item; there may be multiple instances of the same
>attribute name with different values, but only one instance of a particular
>name/value pair. When used as the name of an attribute, the octets which
>comprise the URI are used to determine its uniqueness; if two (or more) URIs
>have different octet values, but are equivalent names for the same network
>resource (e.g., http://foo.com/bar.html and ftp://foo.com/bar.html), they
>are still considered to be different attribute names.
>
>The server MUST attempt to create all the included name/value pairs. The
>return message body (TBD) will indicate which creation attempts failed.
>
>Example:
>
>ADDMETA /foo.html HTTP/1.1
>Host: ics.uci.edu
>Content-Type: text/tab-separated-values
>
>http://www.purl.org/W3C/Dublin/Author<TAB>Jim Whitehead
>DAV:/LINK<TAB>Type = "DAV:/VERSIONING/HISTORY"
><TAB> Source = "http://ics.uci.edu/foo.html"
><TAB> Dest = "http://ics.uci.edu/foo.html/version_history"
>
>Response Codes
>
>200 OK indicates the server successfully created all of the name/value pairs
>described in the request body.
>
>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.
>
>Error conditions: empty body? Partial success/failure -- could not create
>one of the name/value pairs.
>
>TBD: A response message body indicating which name/value pairs the server
>was unable to create.
>
>DELMETA
>
>Body
>
>The body may either be of content type text/tab-separated-values, using the
>syntax defined for the ADDMETA body, or of content type
>application/dav-meta-search, using the syntax defined for the GETMETA body.
>
>Explanation
>
>The DELMETA method is used to remove a name/value pair from the resource
>specified by the Request-URI. When the message body is of content type
>text/tab-separated-values, the server MUST remove any attribute name/value
>pair defined on the resource which exactly matches a name/value pair
>specified in the message body.
>
>When the message body is of content type application/dav-meta-search, the
>server MUST remove any attribute name/value pair defined on the resource
>which satisfies the search specification in the message body. If a server
>implements the GETMETA and the DELMETA methods, it MUST provide support for
>search specifications of content type application/dav-meta-search, and MAY
>accept search specifications in other formats and/or content types for the
>DELMETA method. All search formats accepted by GETMETA SHOULD be accepted by
>DELMETA.
>
>Response Codes
>
>TBD -- need to reuse the response format from ADDMETA to return the
>name/value pairs which were removed.
>
>Error conditions: Syntax error in search syntax.
>
>GETMETA Method
>
>Body
>
>Search = "(" "OR" *And-Expr")"
>And-Expr = "(" "AND" Name Value ")"
>Name = "(" "name" search-pattern ")"
>Value = "(" "value" search-pattern ")"
>search-pattern = <">*("*" | "?"
>         | SpecialOctet | escaped-octet) <">
>SpecialOctet = <OCTET without <"> or "*"
>        or "?"  or "\">
>escaped-octet  = "\" OCTET
>
>Explanation
>
>The GETMETA method returns all attribute name/value pairs defined on the
>resource specified by the Request-URI which match the search syntax
>specified in the message body. If a server implements the GETMETA method, it
>MUST provide support for search specifications of content type
>application/dav-meta-search, and MAY accept search specifications in other
>formats and/or content types.
>
>application/dav-meta-search media type
>
>The application/dav-meta-search media type uses a subset of the s-expression
>syntax to specify an attribute search syntax. Searches are a logical or of
>limited regular expression matching of attribute name/value pairs. Each
>name/value pair search is a logical and of regular expression matching on
>the name and the value of the attribute. The "*" operator, which matches any
>sequence of zero or more octets, and the "?" operator, which matches a
>single octet, are the only regular expression operators allowed. If a search
>needs to specify a literal "*" or "?", these characters are escaped using
>the slash "/" convention, hence literal "*" is represented as "/*" and
>literal "?" is represented as "/?".
>
>Examples
>
>GETMETA /foo.html HTTP/1.1
>Host: www.ics.uci.edu
>Content-Type: application/DAV-meta-search
>
>(OR (AND (name "http://ydfh") (value "*"))
>    (AND (name "foo:blah")(value "*")))
>
>GETMETA /foo.html HTTP/1.1
>Host: www.ics.uci.edu
>Content-Type: application/DAV-meta-search
>
>(OR (AND (name "*y?f*")(value "*"))
>    (AND (name "f*?h")(value "*"))
>
>Assuming that the metadata available on http://www.ics.uci.edu/foo.html did
>not change between the requests, the response to the second GETMETA request
>should, at a minimum, include all the responses from the first GETMETA
>request.
>
>GETMETA /index.html HTTP/1.1
>Host: www.ics.uci.edu
>Content-Type: application/DAV-meta-search
>
>(OR (AND (name "*")(value "*")))
>
>The server will return a list of all attribute name/value pairs defined on
>the resource http://www.ics.uci.edu/index.html.
>
>GETMETA /index.html HTTP/1.1
>Host: www.ics.uci.edu
>Content-Type: application/DAV-meta-search
>
>(OR (AND (name "DAV:/LINK")(value "*")))
>
>The server will return a list of all links defined on the resource
>http://www.ics.uci.edu/index.html.
>
>Response Codes
>
>The response format for matching name/value pairs is TBD.
>
>Error conditions: syntax error in search syntax. No matching name/value
>pairs?
>
>Link Metadata Type
>
>Link := linkname HT linkvalue
>linkname := "DAV:/Link"
>linkvalue := Type SP Source SP Destination *(SP link-extension)
>Source := "Source" "=" <"> URI <">
>Destination := "Dest" "=" <"> URI <">
>Type := "Type" "=" <"> URI <">
>
>link-extension = token ["=" (token | quoted-string)]
>
>A link can be viewed as a piece of metadata stored on a resource, which can
>be stored in an attribute name/value pair. The Link predefined metadata type
>provides a standard syntax for expressing typed links with two endpoints. By
>definition, the name of a link attribute is "DAV:/Link" and the value of the
>attribute is a triple consisting describes the link's type, source, and
>destination, and potentially some extra descriptive information.
>
>When recoding a DAV:/Link attribute, a server is only required to record the
>Source, Destination, and Type. It may drop all other information in the
>attribute value field if it so chooses. In addition a server MUST not record
>two links which have the same source, destination, and type but differ on
>other attributes. A link is uniquely identified by the
>source/destination/type triple.
>
>Please note the use of ":=" in the BNF productions above. This means that
>white space is never implicit, simplifying link search specifications.
>
>References
>
>[Abrial, 1974] J. R. Abrial, "Data Semantics", in J. W. Klimbie and K. L.
>Koffeman eds., Data Base Management, Proceedings of the IFIP Working
>Conference on Data Base Management, Cargese, Corsica, France, April 1-5,
>1974, p. 1-60.
>
>[Berners-Lee, 1997] T. Berners-Lee, "Metadata Architecture." Unpublished
>white paper, January 1997.
>http://www.w3.org/pub/WWW/DesignIssues/Metadata.html
>
>[Cunningham & Faizi, 1996] J. Cunningham, A. Faizi, "Distributed Authoring
>and Versioning Protocol", version 0.1, unpublished manuscript, October,
>1996. http://www.ics.uci.edu/~ejw/authoring/ns_dav.html
>
>[Engelbart, 1968] D. C. Engelbart and W. K. English. "A Research Center for
>Augmenting Human Intellect" , AFIPS Proceedings of the Fall Joint Computer
>Conference , 1968. Vol. 33, Part 1, p. 395-420. Thompson Book Company,
>Washington, D.C. 1968.
>
>[Goland et. al, 1997] Y. Y. Goland, E. J. Whitehead,Jr., A. Faizi, S. R.
>Carter, D. Jensen, "Extensions for Distributed Authoring and Versioning on
>the World Wide Web" Internet draft, work-in-progress.
>draft-jensen-webdav-ext-01,
>ftp://ds.internic.net/internet-drafts/draft-jensen-webdav-ext-01.txt,
>
>[Hull & King, 1987] R. Hull and R. King, "Semantic Database Modeling:
>Survey, Applications, and Research Issues", ACM Computing Surveys, Vol. 19.,
>No. 3, September 1987, p. 201-260.
>
>[Lagoze, 1996] C. Lagoze, "The Warwick Framework, A Container Architecture
>for Diverse Sets of Metadata." D-Lib Magazine, July/August, 1996.
>http://www.dlib.org/dlib/july96/lagoze/07lagoze.html
>
>[Lasher, Cohen, 1995] R. Lasher, D. Cohen, "A Format for Bibliographic
>Records," RFC 1807. Stanford, Myricom. June, 1995.
>
>[Maloney, 1996] M. Maloney, "Hypertext Links in HTML." Internet draft
>(expired), work-in-progress, January, 1996.
>
>[MARC, 1994] Network Development and MARC Standards, Office, ed. 1994.
>"USMARC Format for Bibliographic Data", 1994. Washington, DC: Cataloging
>Distribution Service, Library of Congress.
>
>[Miller et.al., 1996] J. Miller, T. Krauskopf, P. Resnick, W. Treese, "PICS
>Label Distribution Label Syntax and Communication Protocols" Version 1.1,
>W3C Recommendation REC-PICS-labels-961031.
>http://www.w3.org/pub/WWW/TR/REC-PICS-labels-961031.html
>
>[Nelson, 1981] T. Nelson, "Literary Machines." Swarthmore, PA, 1981.
>
>[Slein et al., 1997] J. A. Slein, F. Vitali, E. J. Whitehead, Jr., D. G.
>Durand, "Requirements for Distributed Authoring and Versioning on the World
>Wide Web," Internet-draft, work-in-progress,
>draft-slein-www-dist-author-00.txt
>
>[Weibel, 1995] S. Weibel, "Metadata: The Foundations of Resource
>Description." D-Lib Magazine, July, 1995.
>http://www.cnri.reston.va.us/home/dlib/July95/07weibel.html
>
>[Weibel et al., 1995] S. Weibel, J. Godby, E. Miller, R. Daniel, "OCLC/NCSA
>Metadata Workshop Report." http://purl.oclc.org/metadata/dublin_core_report
>
>
>
>
>
Name:			Judith A. Slein
E-Mail:			slein@wrc.xerox.com
Internal Phone:  	8*222-5169
External Phone:		(716) 422-5169
Fax:			(716) 265-7133
MailStop:		128-29E
Received on Saturday, 3 May 1997 10:34:32 GMT

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