XML

1
XML

• eXtensible Markup Language

2
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

ltconfiggt ltdescriptor type"FILE"
name"source"gt ltattribute name"media_type"
type"svalue"/gt ltattribute name"frame_rate"
type"svalue"/gt lt/descriptorgt lt/configgt
3
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

4
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 lt/gt and end tags get in the way of the
  data

5
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 dont even have to do that
• XSLT allows you to transform from an XML
  document into another document, possibly not even
  standard XML
• Etc.

6
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

7
Defining A Dialect

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

lt?xml version"1.0" encoding"UTF-8"?gt ltxsdschema
xmlnsxsd"http//www.w3.org/2001/XMLSchema"
targetNamespace"http//lamp.cfar.umd.ed
u/viper" xmlnsviper"http//lamp.cfar
.umd.edu/viper" elementFormDefault"qu
alified"gt ltxsdelement name"viper"/gt
ltxsdelement name"config"/gt lt/xsdschemagt
8
Schema Datatypes

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

ltxsdelement name"data" type"xsdbase64Binary"/gt
ltxsdattribute name"span" type"viperframespanT
ype"/gt ltxsdsimpleType name"framespanType"gt ltxsd
restriction base"xsdstring"gt ltxsdpattern
value"\d\\d" /gt lt/xsdrestrictiongt lt/xsdsim
pleTypegt
9
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

10
Extensiblity

• Inheritance
• Can extend complex elements by adding more
  attributes and elements to the bottom
• Can restrict the data using the ltrestriction/gt
  elements
• The ltany/gt and ltanyAttribute/gt elements
• The ultimate in extensibility, allow any valid
  XML in from a given namespace or range of
  namespaces

11
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

12
Xpath

• The common language for selecting individual
  pieces of an XML document shared between X-Link
  and XSLT
• 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_at_typeFILE
• Selects all of the file descriptor nodes that are
  of type FILE

13
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