Extensible Markup Language

Introduction

Extensible Markup Language (XML) is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. The World Wide Web Consortium's (W3C) design goals for XML emphasize simplicity, generality, and usability across the Internet. It is a textual data format, with strong support via Unicode for the languages of the world. Although XML's design focuses on documents, it is widely used for the representation of arbitrary data structures, for example in web services.

Design

XML is designed to store and transport data. XML data is known as self-descriptive or self-defining, meaning that the structure of the data is embedded with the data, thus when the data arrives there is no need to predefine the data structure. XML is a W3C Recommendation, and it is a fee-free open standard. The W3C recommendation specifies both the lexical grammar and the requirements for parsing.

Structure

An XML document consists of elements, each element has a start tag, content and an end tag. An element can contain other elements, plain text or a mixture of both. Elements can have attributes. XML tags are not predefined like HTML. You must define your own tags.

Syntax

XML syntax refers to the rules that determine how an XML application can be written. The XML syntax is straightforward, but rigid. Tags are case sensitive and must be closed. XML tags are not predefined. You must define your own tags. XML documents must have a root element. XML documents must be well-formed.

Applications

XML is used extensively to underpin various publishing formats, and is widely used in the field of scientific research, in particular in bioinformatics. In many application areas, XML is used to store and process data on the web. Some of these areas include:

Sharing data: XML is used to encode documents and serialize data; that is, it is used to turn data into a code and to convert data to a string of characters. This allows data to be shared between different applications, and even between different organizations.

Displaying data: XML is used to create documents that can be displayed on the web. This is done by embedding XML documents in HTML documents.

Storing data: XML is used to store data in a structured way. This allows data to be retrieved and manipulated easily.

Transmitting data: XML is used to transmit data between different systems. This is done by encoding the data in XML and then sending the XML document to the receiving system.

A screenshot of XML code in a text editor.

Advantages and Disadvantages

Like any technology, XML has its pros and cons. It is important to understand these when deciding whether to use XML in a particular application.

Advantages of XML include:

Self-descriptive: XML is a self-descriptive language. This means that the tags in an XML document describe the data that is contained in the document.

Platform independent: XML is platform independent. This means that it can be used on any system or platform.

Language independent: XML is language independent. This means that it can be used with any programming language.

Supports Unicode: XML supports Unicode. This means that it can represent characters from any human language.

Disadvantages of XML include:

Verbosity: XML is verbose. This means that it uses a lot of words and phrases to describe data. This can make XML documents large and cumbersome.

Complexity: XML is complex. This means that it can be difficult to understand and use.

Lack of indexing: XML does not support indexing. This means that it can be slow to search through large XML documents.