XML

eXtensible Markup Language

XML

• A method of defining a format for exchanging documents and data.– Allows one to define a dialect of XML– A library of tags, with associated

structure<config>

<descriptor type="FILE" name="source">

<attribute name="media_type" type="svalue"/>

<attribute name="frame_rate" type="svalue"/>

</descriptor>

</config>

The Social Benefits

• Can specify an interchange format concisely and accurately enough to set up a validation service easily

• There is plenty of available software for dealing with XML files and translating from one format into another

Downsides

• Sometimes defining a representation can be a pain– Deciding what to leave as content and what to

move to attributes.– XML Schemas are confusing, while DTDs do not

offer enough control

• Verbose– ViPER increased about 2x uncompressed, 4/3x

gzip compressed

• Difficult to read– Lots of </…> and end tags get in the way of the

data

The Real Benefits to The Programmer

• XML Schema (or DTDs) allow you to validate a document without having to examine it

• Xpath allows you to specify a node, or set of nodes, in a document quickly and easily

• SAX makes it easy to write a quick parser• DOM makes it so you don’t even have to do

that• XSL:T allows you to transform from an XML

document into another document, possibly not even standard XML

• Etc.

XML As A File Format

• Makes parsing simpler, but currently no methods for making saving easier

• Saves you from dealing with things like character encoding and date formatting

• No more difficult than making up your own

• An unfamiliar or forgotten file grants more affordances than an XML or binary file

Defining A Dialect

• XML Schema – Structure and Data– Define elements and attributes– Associate them with data types

<?xml version="1.0" encoding="UTF-8"?>

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"

targetNamespace="http://lamp.cfar.umd.edu/viper"

xmlns:viper="http://lamp.cfar.umd.edu/viper"

elementFormDefault="qualified">

<xsd:element name="viper"/>

<xsd:element name="config"/>

</xsd:schema>

Schema Datatypes

• Can create and assign datatypes to attributes and elements. For example:

<xsd:element name="data" type="xsd:base64Binary"/>

<xsd:attribute name="span" type="viper:framespanType"/>

<xsd:simpleType name="framespanType">

<xsd:restriction base="xsd:string">

<xsd:pattern value="\d+\:\d+" />

</xsd:restriction>

</xsd:simpleType>

Schema Structures

• Can specify order and contents of elements– Sequence, choice, mixed, etc. allow

specifying how and where elements appear

– Substitution groups allow one tag to take the place of another

• Can group elements without placing the into types

Extensiblity

• Inheritance– Can extend complex elements by adding

more attributes and elements to the bottom– Can restrict the data using the

<restriction/> elements

• The <any/> and <anyAttribute/> elements– The ultimate in extensibility, allow any valid

XML in from a given namespace or range of namespaces

Parsing

• Using the DOM:– The DOM provides a tree structure

that represents the document– Memory heavy

• Using SAX:– Event driven– Lightweight– Better for large documents

Xpath

• The common language for selecting individual pieces of an XML document shared between X-Link and XSL:T– Also used for defining uniqueness constraints

in Schemas– DOM Level 3 will support selecting by Xpath

• Looks sort of like a JavaScript DOM call:– /viper/config/descriptor[@type=“FILE”]

• Selects all of the file descriptor nodes that are of type “FILE”

Resources

• www.xml.com– O'Reilly's XML resource

• www.w3.org – The standards themselves, and lots of

good links to implementations.

• xml.apache.org– DOM, SAX, and XSLT for C and Java