|
This technical overview provides an introduction to
the technical basics of the PRISM Specification. Complete
technical descriptions and examples are provided within
the specification and should be used used when implementing
PRISM.
Technical Decisions
A number of technical decisions were made when the
PRISM Specification was developed. The technical basis
for PRISM includes:
Dublin Core Metadata
The elements in the Dublin Core form the basis for PRISM’s
metadata vocabulary.
XML Compliance
PRISM descriptions are XML documents [W3C-XML], thus
they begin with the standard XML declaration: <?xml version=”1.0”?>.
A character encoding may be given if needed. As indicated
by the two attributes beginning with ‘xmlns:’, PRISM
documents use the XML Namespace mechanism [W3C-XML-NS].
This allows elements and attributes from different namespaces
to be combined. Namespaces are the primary extension
mechanism in PRISM.
RDF Compliance
PRISM descriptions are compliant with the RDF constraints
on the XML syntax. Thus, they begin with the rdf:RDF
element. PRISM follows the case convention adopted in
the RDF specification.
Metadata Language
PRISM recommends that the language of the metadata record,
which is potentially different than the language of
the resource it describes, be explicitly specified with
the xml:lang attribute.
Unique Identifiers
PRISM requires that resources have unique identifiers.
If, for example, a resource is identified by a URI,
it can be specified by the rdf:about
attribute of the rdf:Description
element. The dc:identifier
element can be used for other kinds of identifiers,
such as ISBN numbers or system-specific identifiers.
PRISM Metadata Elements
Metadata elements defined within the PRISM Specification
are from the prism:
namespace, PRISM Rights Language prl:
namespace and from Dublin Core (dublin:
namespace).
General Purpose Elements
These elements form the basis for PRISM’s descriptive
metadata. General Purpose Elements include dc:identifier,
dc:title, dc:creator, dc:type and prism:category
Provenance
These elements describe the supply chain for a resource
to indicate what the source material for a resource
was and through which organizations the resource has
passed. PRISM uses the dc:source property to identify
the original basis for the resource, the dc:publisher property to identify
the primary provider of the information (such as a major
wire service), and the prism:distributor
property to identify other members of the distribution
chain, if any.
Timestamps
There are several times that mark the major milestones
in the life of a news resource: The time the story is
published, the time it may be released (if not immediately),
the time it is received by a customer, and the time
that the story expires (if any). Timestamp elements
include prism:creationTime,
prism:releaseTime and prism:expirationTime.
Subject Description
These elements describe the subject matter of a resource.
There are many different kinds of subjects. Subject
elements include dc:subject, prism:person and prism:industry.
People, places, things, events, … are all possible
subcategories of ‘subject’. Best practice
is for subject description elements to reference controlled
vocabulary terms.
Resource Relationships
It is often necessary to describe how a number of resources
are related. For example, an image can be part of a
magazine article. PRISM defines a number of elements
to express relations between resources. Relationship
elements include prism:isPartOf,
prism:isBasedOn and prism:isReferencedBy.
Rights and Permissions
Licensing content for reuse is a major source of revenue
for many publishers. Conforming to licensing agreements
is a major cost – not only to the licensee of the content
but also to the licensor. For these reasons, PRISM provides
elements and controlled vocabularies for the purpose
of describing the rights and permissions granted to
the receiver of content. The PRISM specification provides
those elements in two namespaces. Basic, commonly used,
elements are defined as part of the prism:
namespace. A separate namespace is defined for the elements
in the PRISM Rights Language (prl:
namespace). Rights elements include dc:rights;
prism:copyright and prl:usage.
PRISM In-line Markup
Important information, such as dates and the names
of people, places, and things, occurs in the text of
an article. Some organizations prefer to mark that data
in-line rather than create a large set of subject description
elements. PRISM provides elements in the PRISM In-line
Markup pim: namespace
for inline markup. These can be mixed into DTDs that
specify the allowed structure of the document. In-line
markup elements include pim:location,
pim:person and pim:industry.
Controlled Vocabularies
The PRISM Controlled Vocabulary pvc:
namespace elements provide a mechanism for describing
and conveying all or a portion of a controlled vocabulary
or authority file. The pvc:
elements may be used to define entire new taxonomies.
Controlled vocabulary elements include pvc:descriptor,
pvc:code and pvc:definition.
Property values in PRISM may be strings, as shown above,
or may be terms from a controlled vocabulary.
Controlled vocabularies are an important extensibility
mechanism. They also enable significantly more sophisticated
applications of the metadata.PRISM specifies controlled
vocabularies of values for some elements such as dc:type
and prism:category.
Others elements will use controlled vocabularies created
and maintained by third parties, such as the International
Standards Organization (ISO). For example, PRISM recommends
the use of ISO 3166 (Codes for Countries) as the value
of elements like prism:location.
Other third-party controlled vocabularies, such as the
Getty Thesaurus of Geographic Names[TGN] may be used.
Site-specific controlled vocabularies, such as from
employee or customer databases, may also be used at
the risk of limiting interoperability. PRISM provides
a small namespace of XML elements so that new controlled
vocabularies can be defined.
PRISM Element Usage
Although the PRISM specification contains a large
number of elements and controlled vocabulary terms,
most of them are optional. A PRISM-compliant description
can be very simple, or quite elaborate. It is not necessary
to put forth a large amount of effort to apply metadata
to every resource, although it is possible to apply
very rich metadata to resources whose potential for
reuse justifies such an investment. Similarly, PRISM
implementations need not support every feature in the
specification. Simple implementations will probably
begin with the elements listed in Section 5, and only
add more capability as needed. Simple implementations
will still be able to correctly parse complex descriptions
because all PRISM descriptions obey the RDF constraints
for structuring XML.
Back to Top
|
