Forum for metadata schema implementers

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Forum for metadata schema implementers

• SCHEMAS and the
• Semantic Web
• SCHEMAS Workshop
• Budapest, 10 May 2001
• Thomas Baker, GMD

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Semantic Web vision
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What is it?

• Standards-making activity of World Wide Web
  Consortium (W3C)
• Making Web-accessible data easier to process
• Making machine-readable statements about all
  kinds of things (Web pages, organisations,
  people, concepts, products, etc) and the links
  between them
• Agreeing on explicit terms for characterizing
  relationships between things described on Web

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Core architectural principles

• Simple linked data model
• a Web link means "has something to do with"
• URIs everything has a unique address
• Both resources and the metadata terms used to
  describe them
• XML universal file format
• XML namespaces unique addresses for metadata
  vocabulary terms

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Why?

• Not only display Web data for people, but process
  automatically (by software)
• To share data between programs and resources
  designed independently
• Essential trait of a massively distributed Web
• Incorporate, reuse, re-purpose data for
  unanticipated objectives
• Allow diverse communities to communicate on the
  basis of partial (imperfect) understanding

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How?

• Create webs of information about related things
  using explicit statements
• Statements follow a common model and use
  machine-processable vocabularies
• URIs ensure that vocabulary terms are tied to
  unique definitions that everyone can find on the
  Web

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"RDF"

• Basic model for making statements about
• Resources anything named with a URI
• Description stating the properties of the
  resource using terms named by URIs
• Framework a common model (grammar) for
  statements using diverse vocabularies
• Statements most commonly written in XML

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Example
lt?xml version"1.0"?gt ltrdfRDF xmlnsrdf"http//
www.w3.org/1999/02/22-rdf-syntax-ns" xmlnsdc"ht
tp//purl.org/dc/elements/1.1/ gt
ltrdfDescription about"http//docs gt
ltdccreatorgt Joe Smith lt/dccreatorgt
lt/rdfDescriptiongt lt/rdfRDFgt
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URIs Everything defineduniquely on the Web
Tim Berners-Lee The most fundamental
specification of Web architecture, while one of
the simpler, is that of the Universal Resource
Identifier, or URI. The principle that anything,
absolutely anything, on the Web should be
identified distinctly is core.
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URIs as anchors for merging data

• URIs are fixed points on global Web for
• Metadata vocabulary terms, defined with
  "dictionary entries" in namespace schemas
• The resources described by those terms
• These points can be used to superimpose graphs,
  merging statements
• Creates market for aggregation, data merging,
  annotation, and filtering services

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A set of statements
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...merged with others
Node and arc model can use multiple
vocabularies, distinguished by URI
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URIs anchor conceptsindependently of language
dccreator
Server in Germany
DCMI Server
Server in Jakarta
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Elementary sentences

• In RDF, meaning is expressed by Subject,
  Predicate, and Object
• This simple structure can be used to describe
  most of the data processed by machines
• More complex will not interoperate in the diverse
  Web environment
• Instead of asking machines to understand people's
  language, ask people to make the extra effort
  Tim Berners-Lee

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Partial understanding

• As in real world, anyone on Web can assert (in
  RDF sense) anything about anything
• Assumption you will understand some of these
  statements but not all.
• Ignore the ones you don't understand
• Tolerance of inconsistency and errors
• Web itself "Error 404 File not found", but
  unchecked exponential growth

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Walk before we run

• W3C Semantic Web activity priorities
• Enable simple applications now
• Focus on general principles Web automation, data
  aggregation, identity...
• ...while planning for future complexity

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Pie-in-the-sky...
Some day... maybe...
Longer-term goal
SCHEMAS Registry
This workshop
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"Schemas"

• Declare, like a dictionary, the names and
  definitions ("semantics") of vocabulary terms
• From official standards to project-specific
  schemas
• Printed documentation or Web pages
• Or machine-processable schemas in XML or XML/RDF

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"Namespace schemas"

• One type of schema
• For officially declaring a unique set of elements
  and definitions
• Ideally, addressed on the Web with a URI
• May be an XML or XML/RDF schema
• In SCHEMAS Project, namespace schemas by
  definition only "declare" names and definitions
  of vocabulary terms

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"Application profiles"

• A type of schema
• Declare which vocabulary terms a particular
  application or project uses in its metadata
• May "mix and match" terms from different
  namespaces
• In SCHEMAS Project, a Profile by definition only
  reuses terms defined somewhere in a namespace

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What profiles say

• Simple statements about the semantics used in a
  project or application
• My project http//project.org uses vcardfn.
• My project uses vcardfn in describing people.
• My project uses dcqLCSH to qualify dcSubject.
• My profile relabels cmuconductor as "Dirigent"
  (in German).

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For what purpose

• Statements can be merged for exploratory queries
  of an entire landscape of schemas
• What elements do projects use to describe people?
• What controlled vocabularies do projects use for
  Subject elements?
• Which projects use elements from the vcard
  namespace?
• How do geospatial projects use elements for Date?

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What RDF profiles do not say

• Not for describing the tag structure of complex
  documents (i.e. theses, order forms), like XML
  Schema
• Useful for validating metadata records ("Must
  have at least one, but no more than four,
  authors.")
• But nested structures are hard to merge
• Software must be hand-coded for a particular
  document type "Unless you use my document
  schema, your software cannot use the data."

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Existing schemas today

• Project schemas are often designed as documents
• "These are the fields we want to see"
• But if the schema was not designed with a
  well-defined data model, it may not have one
• Information nested in 100 different ways
• Conceptually "messy" or complexly nested schemas
  cannot be merged

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Making schemas comparable

• Declaration in RDF "Here's what our schema says"
• Uniform model makes schemas comparable
• Simple model "walk before we run"
• Documentary and social purpose
• Define semantic coherence across applications
• Support schema design and harmonization

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Thomas.Baker_at_gmd.de