Context
Get
the PBCore XML Schema Definition (XSD)
PBCore Diagram -- At-a-Glance
XML
Schema Primers
CONTEXT:
Knowledge
Representation: A metadata schema, as well as the actual descriptions of media
items that may use the schema, need to be presented in some logical,
clearly expressed framework so that the information can be understood, either in a human-readable form or machine or computer-readable.
Interchange
Structure: More
importantly, using well-formed methods and frameworks to express metadata schemas
and descriptions allows different parties to share data; they
are communicating using the same language, the same grammar, and the same structuring of data.
A language that is often used in files to express well-formed
data is XML, Extensible Markup Language. Unless the
party offering and the party accepting the well-formed data interpret, validate, and implement that data in the same way, information is likely to be mangled. Four conditions have been itemized in describing common data interpretation, validation, and implementation between systems sharing information:
- common metadata meanings (semantics),
- common grammar and rules for expressing data (syntax),
- commonly defined metadata dictionary element properties (attributes), and
- shared information architectures concentrating on entities with their hierarchical structures and interdependencies (data models)
Items 1 through 3 (semantics, syntax, and attributes) are usually referred to as the Metadata Dictionary. Item 4 is the expression of the Metadata Dictionary within a data model's framework. A DTD (Document Type Definition)
and an XML Schema are used to define the grammar and validate the
data being shared. Some have stated that DTDs and XML Schemas function
as blueprints for describing the structure of the XML language
in a document. These blueprints supply the...
- Sequence in which elements appear in an XML document
- Interrelationships between different elements (interdependencies, parent-child
associations or nested relationships)
- Types of data that are used to express elements and attributes
(text string, number, date, timestamp, etc.)
DTDs have been around longer than XML Schemas, and are very widely
used. However, they have some limitations in their capacities,
such as using non-XML syntax to compose a DTD, support for limited
data types, inability to identify namespaces (see What
is a Namespace?), and no support for extensibility or inheritance.
XML Schemas, however, do not have these limitations, and also allow
users to craft their own data types.
Typically, more complex data structures, with multiple data types,
require the use of an XML Schema over that of a DTD.
PBCore is offering an XML Schema Definition (XSD) document to define the framework of the PBCore elements and their inter-relationships and interdependencies (see the download links below). For further discussion in element hierarchies and interdependencies, see our web page Information about Hierarchical Relationships & Interdependencies Between Metadata Elements.
TWO SCENARIOS FOR DATA SHARING...
When sharing metadata and facilitating interoperability, data that is exported from an information system (one metadata island) is transformed into a PBCore compliant XML document, using the framework of the PBCore XSD. The receiving information system (another metadata island) imports the PBCore compliant XML document and transforms that data to match its own internal structures and metadata schemas.
The following discussion highlights two different scenarios in which data is being exchanged between information systems.
SCENARIO 1:
Station A uses an In-House tool to create and save descriptive records for its media assets. This tool can export the records as XML compliant with the PBCore XSD.
Station B uses the PBCore Project's Cataloging Tool to create and save descriptive records of its media assets. This tool can import and export metadata records as XML compliant with the PBCore XSD.
SCENARIO 2
Station X purchased a tool (System X) to create and save descriptive records for its media assets. This tool cannot export the records as XML (in Format X) that are compliant with the PBCore XSD.
Station Y purchased a different tool (System Y) to create and save descriptive records of its media assets. This tool cannot import or export the records as XML (in Format Y) that are compliant with either the PBCore XSD or Format X.
Because the respective stations use tools that utilize records in different formats, they cannot exchange records without first transforming them into a new format that can be understood by both tools.
The PBCore XSD can be used to move data between the two incompatible systems.
Station X uses a transformation method (such as XSLT--Extensible Stylesheet Language Transformations; W3C or Wikipedia definition) to translate records from Format X into PBCore XSD-compliant data.
Station Y transforms the PBCore XSD-compliant data into Format Y before importing the records into System Y.
Within an individual Public Broadcasting station, there are many departments, each maintaining their own data islands that facilitate their individual departmental functions and missions. PBCore compliant XML documents could act as the data transformation framework that allows the different systems to share data. Here is an example of a typical Public Broadcasting TV station's metadata islands.
There are many metadata schemes available for use by
various industries and communities, each with their own set of elements
and definitions.
