XML

XML

What is XML?

eXtensible Markup Language

A derivative of the lesser known SGML

Originally designed to be a flexible text format for electronic publishing

Plays an important role in data exchange on the Web and in other application areas

Designed for interoperability with SGML as well as HTML

Features of XML

A cross-platform, software and hardware independent means for transmitting information

Describes a class of data objects known as XML Documents

Partially describes the behavior of computer programs which process these documents

More expressive than HTML

XML uses a Document Type Definition (DTD) or an XML Schema to describe the data

Expressiveness of XML

HTML has nearly 100 different pre-defined elements with each element having several attributes

XML has no pre-defined elements (tags). The expressiveness of XML is attributed to its simplicity and not a plethora of elements

Tags have to be “invented” by the author of an XML document!

Self descriptive

XML and HTML

HTML was designed to display data and to focus on how data looks

XML was designed to describe data and to focus on what data is

XML was designed to carry data – data exchange

XML isn’t a replacement for HTML

XML is a complement to HTML

Data Exchange with XML

Data can be exchanged between incompatible systems 

Converting the data to XML can greatly reduce the complexity of data exchange between incompatible systems over the internet and create data that can be read by many different types of applications 

Ex: XML can be used in B2B (Business to Business) systems

Data Sharing with XML

Since XML is independent of hardware, software and application, you can make your data available to other than only standard HTML browsers 

Applications can access your XML files as data sources, like they are accessing databases 

XML Tags

In XML, it is illegal to omit closing tags

This is contrary to HTML

In XML, all elements must have a closing tag

The XML Declaration is not an element. So, it won’t have a closing tag.

XML tags are case-sensitive

Obviously, the tags must be properly nested

Other facets of XML

Attribute values must be quoted (“ ” or ‘ ‘)

In XML, whitespaces are preserved; Not truncated as in HTML

XML Elements

XML Elements are extensible

An application that is created to extract the elements in a XML document and produce an output should be able to produce the same output, even if an extra element is added to the document.

XML elements have relationships – Parent-Child relationships

XML Elements

An XML element is everything from (including) the element's start tag to (including) the element's end tag

An element can have element content, mixed content, simple content, or empty content

An element can also have attributes 

Naming Convention for Elements

Names can contain letters, numbers, and other characters 

Names must not start with a number or punctuation character 

Names must not start with the letters xml (or XML or Xml ..) 

Names cannot contain spaces

Any name can be used, no words are reserved, but the idea is to make names descriptive 

XML Documents

Two kinds of XML Documents

Valid XML

Well-formed XML

XML validated against a DTD is Valid XML

XML with correct syntax, but without a DTD, is Well-formed XML

XML DTD

Document Type Definition

A DTD defines the legal elements of an XML document

It defines the document structure with a list of legal elements

A DTD can be declared inline in your XML document, or as an external reference 

Why is a DTD needed?

With DTD, each of your XML files can carry a description of its own format with it

With a DTD, independent groups of people can agree to use a common DTD for interchanging data

Your application can use a standard DTD to verify that the data is valid 

XML Building Blocks – DTD Viewpoint

Elements -  “to”, “from”, “heading”, etc.

Tags  - used to markup elements

Attributes - extra information about elements

Entities

PCDATA 

CDATA 

Entities

Entities are variables used to define common text

Entity references are references to entities 

Entities are expanded when a document is parsed by an XML parser

PCDATA

PCDATA means parsed character data

Text found between the start tag and the end tag of an XML element

Text that will be parsed

Tags inside the text will be treated as markup and entities will be expanded

CDATA

CDATA means character data

Text that will NOT be parsed

Tags inside the text will NOT be treated as markup and entities will not be expanded

XML Schema

W3C supports an alternative to DTD called XML Schema

An XML schema describes the structure of an XML document

XML Schema Language is used for schema definition

XML Schema

defines elements that can appear in a document 

defines attributes that can appear in a document 

defines which elements are child elements 

defines the order of child elements 

defines the number of child elements 

defines whether an element is empty or can include text 

defines data types for elements and attributes 

defines default and fixed values for elements and attributes 

XML Schema – Successor of DTD

XML Schemas are extensible to future additions 

XML Schemas are richer and more useful than DTDs 

XML Schemas are written in XML 

XML Schemas support data types 

XML Schemas support namespaces (more about this in the forthcoming session)