Week 5: XML Galore!

"Introducing the Extensible Markup Language"

XML is extensible: it can be altered and added to indefinitely to tweak the language to suit the needs of the user. This makes it a robust language to use for digital libraries. As things change, XML can accommodate the changes without requiring a total overhaul of the system. Libraries like things that work that way, because it doesn't require them to reinvent the wheel. It is also useful for metadata, because the tags can be used for labeling different types of metadata.

"A Survey of XML Standards" is a good reference source for the different versions of XML because it provides other resources to look at for further instruction. The sheer number of versions illustrates the extensibility of XML.

"Extending your Markup" is an interesting and short overview of how XML works. Again, it is a good resource for a novice to look at to get started in this new world.

Major definitions:
DTD: document type definitions. This tags a given field as including a given type of information, such as author. They define the structure of the XML document.
DTD elements:
Nonterminal: they have a series of other choices or sequeneces. A DTD defining a book has sequences following it such as author.
Terminal: They do not have choices. They may include things like PC data, or are empty, or labeled as 'any'.

DTD attributes: do not prescribe order on the DTD, but include further information

Namespaces: to prevent conflict between two fields that use the same tag but in different contexts (email address vs. postal address) namespaces define the two as distinct. Do not play well with DTDs.

Linking: Goes beyond HTML to describe different types of linking
Xlink: describes how 2 documents can be linked
Xpointer: links 2 parts of the same document.
XPath: (used by Xpointer) describes the linking path

XSLT: Extensible Style Sheet Language Transformer: goes from XSL to HTML.

XML Schema: Overcome the limitations of DTDs (expression limited and non XML syntax)

Document definition markup language (DDML): define datatypes
Document content description (DCD)
Schema for object-oriented XML (SOX)
XML-Data (replaced by DCD)

"Introduction to XML schema"

Schema replace DTDs! They do the same things like define the element, define child elements, define the order of the elements, and other similar things. However, they are more extensible, richer and powerful, they support data types and namespaces and they are still XML. Essentially, they perform the same function as DTD's only better.