XML Basics

XML Series Part 1

Introduction

Some definitions

XML - Extensible Markup Language; subset of SGML (Standard Generalized ML); Meta-language to create other markup languages.

Markup - Set of codes or tags added to content of document in order to indicate its meaning or representation. Non-standardized markup languages such as RTF have many disadvantages – (1) Proprietary and specific to particular applications (2) Difficult to read and manage (3) Poor problem-domain semantics – more based on presentation rather than the data itselfSGML was the original meta-language created based on markups. SGML gave the rules on how the markups should be used. Based on these rules, the user or the application could create custom markup language for specific problem domain. HTML was an application of SGML. However, because of SGML’s complexity, it failed to be accepted completely. XML was then created using a subset of SGML features. In effect XML could capture 80% of SGML power with mere 20% of its complexity.

XML suite of standards – XML is complemented by many other related standards to enrich with features such as linking, transformation and advanced data modeling – called the core or fundamental specifications. Some of the key words here are XML Info seet, DOM, SAX, XML 1.0, Namespace, Xpath, Xpointer, Xlink, XSLT, XSL, XML Schema, DTD.

XML Document

XML Element – Basic content unit in XML; delimited by tags; three types of elements –

• elements containing character data
• elements containing other elements
• elements containing character data and other elements called mixed-content element

If the elements are nested correctly, then it is said to be well-formed

Logical Structure

• Recommended and Optional Prolog - contains XML declaration and optional DTD
• Root Element - Also called Document element which contains all other markups and charecter data
• Not recommended and Optional Epilog - contains comments and other non element markups

Prolog – option for the first line to declare the version of XML being used and the character encoding of the document – for example if a document was written in XML 1.0 version using UTF-8 encoding, then the XML document could have an XML declaration as -

<?xml version=”1.0”?>

Physical structure

Made out of pieces of text composed of character data and markup. The following are instances of markups – start, end and empty tags, attributes, comments, PI – processing instructions, CDATA section delimiters, Entity references, Character references and DTD. Everything else in the document, which is not the above, is considered character data. For example in

;
<movie>
<title> L.A.Confidential </title>
</movie>