Metadata and Metamodeling

Data Representation

The ways we present data are very crucial because they are directly related to human comprehension. The clearer we perceive objects, the more effective we can think and imagine. The time software developers spent to develop effective presentation tools to perform such tasks is not negligible. If we can separate data processing phase of a number-crunching scientific application from its data presentation or visualization, for example, then each phase can be managable and improved separately.

On the other hand, if the output of the application is not meant for human beings, but rather for other computer applications, then the presentation format of the data must be simpler or follow a strict specification.

One of the main reasons behind XML specifications was to offer solutions to simplify both of the above issues, one for data publishing and the other for data exchange.

Style Languages

HTML includes language elements for both document structure and presentation. As HTML documents became more data oriented and a medium of data exchange, the separation of data from presentation elements became of paramount importance. Because of this, XML documents include only data and structural information. There is absolutely no information about styling. This separation resulted in new XML specifications for presentation such as eXtensible Style Language [3], XSL Transformations [125], and XPath [115].

XSL defines how individual XML elements will be rendered into a presentationvlanguage. It has many advantages over other stylesheet languages such as being,
  • an industry standard,
  • widely used by many tools,
  • deployed on different computer platforms,
  • used as a common style information exchange language between applications, removing dependency on specific vendors.

XSL also offers all the advantages of XML [21].

Transforming XML

XSLT is a transformation language that transforms XML documents into any other form of text. Including other two specifications, XSL and XPath for styles and element accesses respectively, XSLT is a very powerful tool that is used for:
  • XML data rendering for publishing, i.e. production of HTML, PDF, PS, LaTeX, and Scalable Vector Graphics [114]. If documents are written in XML, document type conversions can be simplified.
  • Document transformations from one schema to another for compatibility between specifications. Any document exchange may require document filtering for unnecessary or confidential information as well as data defaulting for missing elements in schemas.
  • Data extraction and search. XSLT comes with the powerful XPath path language that standardizes element and attribute accessing and processing. Any tool that implements the XPath specification can be integrated to transformation processes seamlessly.
  • XSLT itself is specified in XML. This allows XSLT documents to be handled, stored, and searched just like other XML documents, which simplifies automation of rendering and conversions of documents.

Knox [71] also points out that being written in XML allows simple XSLT documents to be parts of more complex XSLT transformations by being aggregated or pipelined one after another.

XHTML, WML, and Formatting Objects

XHTML [123] is an XML specification for HTML. It was developed to bring some of the advantages of XML specifications to HTML such as widely available tools for editing, processing, and validating XML. In addition to these advantages, it was also aimed to bring modularity to HTML. New XML specifications canbe easily embedded into HTML documents, which in turn Web pages richer incontent. Examples of such content are SVG documents [114]. Scalable and more accurate graphics can replace bitmap images within HTML pages. Also, other markup languages for scientific notations and data representations, e.g., MathML[77], ChemML [26], and GML [46] can be seamlessly embedded into XHTML pages.XML metadata specifications such as RDF [113] and Dublin Core [34] can enrichXHTML pages even further, allowing users to make more precise searches on the Internet.

Wireless Markup Language [118] is also one of the XML rendering specifications with fewer style options. WML is meant to be simple and processed by handheld devices or cellular phones. XML data can easily be converted to WML pages through XSLT processors. XSL Formatting Objects [124] is another data presentation specification for document publishing. XSL FO provides a generalized framework for fine-grained paper specifications for quality printing where HTML and XSL lack the capability. Due to its nature, XSL FO documents can also embed other XML and style specifications.

XHTML, WML, and XSL-FO are all presentation specifications in XML fordifferent media such as large screens, smaller screens, and paper respectively. Collaboration systems can utilize such technologies to serve a wide range of userand environment types.