Extensible Markup Language XML

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Extensible Markup Language (XML)
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XML

• Extensible Markup Language has become the
  universal standard for representing data
  
• XML started out as a standard data exchange
  format for the Web
  
• Yet, it has quickly become the fundamental
  instrument in the development of Web-based online
  information services and electronic commerce
  applications
• Almost all recent electronic commerce standards
  are based on XML

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XML

• A subset of SGML (Standard Generalized Markup
  Language) it is defined by World Wide Web
  Consortium (http//www.w3.org)
  
• It is a fee-free open standard.
• HTML enables a universal method of displaying
  data XML provides a universal method of
  describing data
  
• Provides the ability to describe data in an open
  text-based format and deliver it using standard
  http protocol

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XML

• At present, many applications on the Web use XML
  for hosting large amounts of structured and
  semi-structured data
  
• Representation of information in XML documents
  has been increasing at an astonishing pace
  
• According to Meta Group, by 2003, about 65 of
  corporate data will be stored in an XML format

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XML The Unifying Technology
XML Messaging
Internet
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Maturity of Web Infrastructure
Technology
Standard
Innovation
Browse the Web
Program the Web
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XML helps address the challenge

• The data is self-describing
• e.g. the meaning of the data is included
  identifiers surround every bit of data,
  indicating what it means
  
• Far more flexible method of representing
  transmitted information
  
• e.g. batched orders sent together can have
  different fields and format without breaking apps
  on each end
  
• Open, standard technologies for moving,
  processing and validating the data
  
• e.g. the XML parser can automatically parse,
  validate, and feed the information to an
  application, instead of every application having
  to include this functionality

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XML An Example
Data stream in a typical interface
Electronic Commerce, 100, Turban, 25,
Addison-Wesley
Same data stream in XML
Electronic Commerce UANTITY 100 Turban
25 Addison-WesleyPUBLISHER
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Markup (or Tagging)

• XML uses textual markups to define data
• An XML document is comprised of a collection of
  tagged elements each containing a start tag
  (), an end tag (), and the
  content between the two tags
• Example
• 1234ABCD

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Tagging Data in XML

• 1234ABCD
• Considering the content only, it is not possible
  to understand what 1234ABCD stands for
  
• The tag name PONumber intuitively tells that the
  content is a purchase order number
  
• Similarly, an XML element might be tagged as
  name, gender, birth date, salary, price,
  
• XML is extensible in the sense that users can
  create their own vocabularies, the tag names are
  neither predefined nor limited

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Adding Structure to data

• Tagged elements may be nested to any depth to
  provide structured data, or may be repeated to
  represent a list of values
  
• A valid XML document contains a single root
  element, which constitutes the top-level of
  nesting
  
• In other words, a valid XML document represents a
  tree of elements

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Giving Meaning and Structure to Data
Start Tag
Start Tag

• 1234ABCD
• 20030601
  
  
• 
  
  
• 16
  
  
• 95

An Element
Another Element
An Attribute
Data
End Tag
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Giving Structure to Data
PurchaseOrderRequest
PurchaseOrderDate
LineItem
PONumber
ItemEAN_Identification
QuantityOrdered
UnitPrice
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Well-formed and valid XML documents

• There are two levels of correctness of an XML
  document
  
• Well-formed. A well-formed document conforms to
  all of XML's syntax rules. For example, if an
  element has an opening tag with no closing tag
  and is not self-closing, it is not well-formed.
• Valid. A valid document additionally conforms to
  some semantic rules. These rules are either
  user-defined, or included as an XML schema or
  DTD.

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Well-formed documents XML syntax

• The only indispensable syntactical requirement is
  that the document has exactly one root element
  (alternatively called the document element).
  
• The root element can be preceded by an optional
  XML declaration.
  
• version of XML
• character encoding and external dependencies.
• The specification requires that processors of XML
  support the pan-Unicode character encodings UTF-8
  and UTF-16

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Well-formed documents XML syntax

• XML comments start with .
• The text enclosed by the root tags may contain an
  arbitrary number of XML elements. The basic
  syntax for one element is
  
• content
• Here, content is some text which may again
  contain XML elements.

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Another example
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Well-formed documents XML syntax

• Attribute values must always be quoted, using
  single or double quotes ( OR )
  
• Each attribute name should appear only once in
  any element.
  
• Proper nesting elements may never overlap
• Normal
  emphasized strong
  emphasized strong
  
• Empty element tag, it has three equivalent
  forms
  
• author"John" genre"science-fiction"
  date"2009-Jan-01" /
  

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Entity references

• An entity in XML is a named body of data, usually
  text, such as an unusual character.
  
• An entity reference is a placeholder that
  represents that entity
  
• It consists of the entity's name preceded by an
  ampersand ("") and followed by a semicolon
  ("").
  
• XML has five predeclared entities
• amp ampersand
• lt
• gt greater than
• apos apostrophe
• quot quotation mark
• More entities are declared in the document's
  (DTD). (will see)

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Well-formed documents

• The document complies with its declared character
  encoding.
  
• The encoding may be declared either externally
  ("Content-Type" header of HTTP) or internally.
  
• Element names are case-sensitive.
• ...
• Choosing meaningful names implies the semantics
  of elements and attributes to a human reader

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Valid documents XML semantics

• By leaving the names, allowable hierarchy, and
  meanings of the elements and attributes open and
  definable by a customizable schema or DTD, XML
  provides a syntactic foundation for the creation
  of purpose specific, XML-based markup languages.
  
• The schema merely supplements the syntax rules
  with a set of constraints.
  
• Schemas typically restrict element and attribute
  names and their allowable containment hierarchies
  
  
• Such as, an element named 'birthday' contains 3
  elements year, 'month' and 'day. Each is only
  character data.

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Valid documents XML semantics

• An XML document that complies with a particular
  schema/DTD, in addition to being well-formed, is
  said to be valid.
  
• An XML schema expressed in terms of constraints
  on the structure and content of documents
  
• Before SGML and XML, software designers had to
  define special file formats and special-purpose
  parsers and writers.
  
• XML's regular structure and strict parsing rules
  allow software designers to leave parsing to
  standard tools
  
• Well-tested tools exist to validate an XML
  document "against" a schema

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Document Type Definition (DTD)

• The principle purpose of the DTD is to declare
  the hierarchy of document elements
  
• A document type definition defines
• The name of the elements,
• The content model of each element,
• How often and in which order elements may
  appear,
  
• If the end-tags can be shortcut,
• The possible presence of attributes and their
  default values,
  
• The names of the entities

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An Example DTD

• PurchaseOrderDate, LineItem)
  
• QuantityOrdered, UnitPrice)
  
• other elements are skipped -- ...

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DTDs

• A DTD specifies the structure of an XML element
  by specifying the names of its sub-elements and
  attributes
  
• Sub-element structure is specified using the
  operators
  
• set with zero or more elements
• set with one or more elements
• ? optional
• or
• All values are assumed to be string values,
  unless the type is ANY in which case the value
  can be an arbitrary XML fragment

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DTDs

• There is a special attribute id which can occur
  once for each element
  
• EMPTY- the element has no content
• Empty elements usually have attributes that give
  them useful properties
  
• There is no concept of a root of a document an
  XML document conforming to a DTD can be rooted at
  any element specified in the DTD

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Element Identity, Ids, and ID References

• To support element sharing, XML reserves an
  attribute of type ID, which allows a unique key
  to be associated with an element
  
• An attribute of type IDREF allows an element to
  refer to another element with the designated key
  and IDREFS may refer to multiple elements
  
• John
  Smith
  
• ...
• ....
• 1995

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Entities

• Entities represent the physical structure of an
  XML document
  
• Two types of entities
• General entities apply within the top level
  element and in attribute values
  
• Parameter entities apply within the internal and
  external DTD subsets
  
• Entity reference in a document
• This contract is between receipent
  and contractor and the award is
  payment.
  
• Entity reference expanded
• This contract is between METU and EC
  and the award is 1 EURO.
  
• By changing the entity declarations you can
  create any contract.

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General Entities

• General entity declaration
• Entity reference in a document
• The xml is derived from ISO 8879, an
  International Standard. labelxml/
  
• Entity reference expanded
• The Extensible Markup Language is derived
  from ISO 8879, an International Standard.
  Language/

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Parameter Entities

• is for use only in DTDs
• Parameter entities carry information for use in
  the markup declaration, often a set of common
  attributes shared by several elements or a link
  to an outside DTD.
• Parameter entities whose references are purely
  within DTD are known as internal entities,
  whereas references that draw information from
  outside files are external entities
• Parameter entities use a sign both in their
  declaration and in their references to
  distinguish themselves from general entities

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Parameter Entities

• Parameter entity declaration
• Parameter entity reference in DTD
• Parameter entity reference expanded

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DTDs

• The oldest schema format for XML
• Disadvantages
• It has no support for newer features of XML, most
  importantly namespaces.
  
• It lacks expressiveness. Certain formal aspects
  of an XML document cannot be captured in a DTD.
  
• It uses a custom non-XML syntax, inherited from
  SGML, to describe the schema.
  
• Still used in many applications because it is
  considered the easiest to read and write.

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Valid documents XML semantics

• Other schema languages
• XML Schema (XSD) (will see)
• RELAX NG (specified by OASIS, now an ISO standard
  as part of DSDL)
  
• ISO DSDL (Document Schema Description Languages)
  
  
• Schematron

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XML Namespaces

• Namespaces are a simple and straightforward way
  to distinguish names used in XML documents, no
  matter where they come from
  
• The only reason namespaces exist, is to give
  elements and attributes programmer-friendly names
  that will be unique across the whole Internet

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Example

• xmlnsh"http//www.w3.org/HTML/1998/html4"
• Book Review
• XML A
  Primer
  
• AuthorPrice
• PagesDate
• Simon St. Laurenthtd
  
• 31.98
• 352
• 1998/01

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XML Namespaces

• The prefixes are linked to the full names using
  the attributes on the top element whose names
  begin xmlns
  
• The prefixes are just shorthand placeholders for
  the full names
  
• Those full names are URIs, i.e. Web addresses

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Extensibility in XML

• Anyone can invent new tags and attach a meaning
  to those tags
  
• But if every user creates its own XML definition
  for describing his data, it is not possible to
  achieve interoperability
  
• For example, one may prefer to use the tag name
  POR, while another prefers using the tag name
  PurchaseOrderReq
  
• In other words, a tagged document is not very
  useful without some kind of agreement on the tags
  among inter-operating applications

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Extensibility in XML

• Anyone can invent new tags and attach a meaning
  to those tags
  
• For example
• This device
• This device
• But if every user creates its own XML definition
  for describing his data, it is not possible to
  achieve interoperability
  

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Agreement on tags is necessary

• In other words, a tagged document is not very
  useful without some kind of agreement on the tags
  among inter-operating applications
  

Mobile Device
Hand Held Device
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Many Efforts for Standardized Tags

• HL7 for healthcare
• RosettaNet for supply chain integration in
  Information Technology and Electronic Components
  domain
  
• GS1 again in supply chain
• ebXML for eBusiness
• Common Business Library (CBL) for electronic
  catalogs, purchase orders, etc.
  

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XML Parsers

• A parser takes an XML document and makes its
  structure and content available to an application
  through an API
  
• There are two main Application Programming
  Interfaces (APIs) for writing parsers
  
• Document Object Model (DOM) and
• Simple API for XML (SAX)
• Today, many parsers are both DOM and SAX compliant

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XML DOM Parser
A parser validates and makes the data
contained in an XML document available
to the application
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XSLT Processor

• Converts an XML document to another form
• An XSL style sheet is a set of transformation
  instructions for converting a source XML document
  to a target document

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Why XML?
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XML vs EDI
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XML vs EDI
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XML vs EDI
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Critique of XML Advantages

• It is text-based.
• It supports Unicode, allowing almost any
  information in any written human language to be
  communicated.
  
• It can represent the most general computer
  science data structures records, lists and
  trees.
  
• Its self-documenting format describes structure
  and field names as well as specific values.
  
• The strict syntax and parsing requirements make
  the necessary parsing algorithms extremely
  simple, efficient, and consistent.
  
• XML is heavily used as a format for document
  storage and processing, both online and offline.
  
  
• It is based on international standards.

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Critique of XML Advantages

• It allows validation using schema languages such
  as XSD and Schematron, which makes effective
  unit-testing, firewalls, acceptance testing,
  contractual specification and software
  construction easier.
• The hierarchical structure is suitable for most
  (but not all) types of documents.
  
• It manifests as plain text files, which are less
  restrictive than other proprietary document
  formats.
  
• It is platform-independent
• Forward and backward compatibility are relatively
  easy to maintain
  
• Its predecessor, SGML, has been in use since
  1986, so there is extensive experience and
  software available.
  
• An element fragment of a well-formed XML document
  is also a well-formed XML document.
  

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Critique of XML Disadvantages

• XML syntax is redundant or large relative to
  binary representations of similar data.
  
• The redundancy may affect application efficiency
  through higher storage, transmission and
  processing costs.
  
• XML syntax is verbose relative to other
  alternative 'text-based' data transmission
  formats.
  
• No intrinsic data type support XML provides no
  specific notion of "integer", "string",
  "boolean", "date", and so on

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Critique of XML Disadvantages

• The hierarchical model for representation is
  limited in comparison to the relational model or
  an object oriented graph.
  
• Expressing overlapping (non-hierarchical) node
  relationships requires extra effort.
  
• XML namespaces are problematic to use and
  namespace support can be difficult to correctly
  implement in an XML parser.
  
• XML is commonly depicted as "self-documenting"
  but this depiction ignores critical ambiguities.
  

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Some well-known XML based languages and
applications

• RSS Rich Site Summary
• Ajax
• SOAP Simple Object Access Protocol
• WSDL Web Services Description Language
• SVG Scalable Vector Graphics
• Regarding Office Apps OASIS, OpenOffice,
  Microsoft Office
  
• HL7 Clinical Document Architecture (CDA)
• ...

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HL7 Clinical Document Architecture (CDA)

• A specification for document exchange using
• XML,
• the HL7 Reference Information Model (RIM)
• Version 3 methodology
• and vocabulary (SNOMED, ICD, local,)
• CDA Header
• Metadata required for document discovery,
  management, retrieval
  
• CDA Body
• Clinical report
• Discharge Summary
• Referral

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Clinical Document Architecture
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HL7 CDA

• Level One
• The unconstraint CDA Specification
• Only the header is well structured
• Level Two
• Section Level Templates are applied with coded
  terms
  
• Level Three
• Entry Level Templates are applied
• Machine Processable!

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HL7 CDA Example