XML

1

• XML

2
Outline (ambitious)

• Background documents (SGML/HTML) and databases
  (structured and semistructured data)
• XML Basics and Document Type Descriptors
• XML query languages XML-QL and XSL.
• XML additions Xlink, Xpointer, RDF, SOX,
  XML-Data
• Document Object Model (XML API's)

3
Some Useful Articles

• XML, Java, and the future of the web
• http//webreview.com/wr/pub/97/12/19/xml/index.htm
  l
• XML and the Second-Generation Web
• http//www.sciam.com/1999/0599issue/0599bosak.html
  
• Articles/standards for XML, XSL, XML-QL
  http//www.w3c.org/
• http//www.w3.org/TR/REC-xml

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Part I Background

• Whats the difference between the world of
  documents and information retrieval and databases
  and query interfaces?

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Documents vs Databases

• Document world
• plenty of small documents
• usually static
• implicit structure
• section, paragraph, toc,
• tagging
• human friendly
• content
• form/layout, annotation
• Paradigms
• Save as, wysiwyg
• meta-data
• author name, date, subject

• Database world
• a few large databases
• usually dynamic
• explicit structure (schema)
• records
• machine friendly
• content
• schema, data, methods
• Paradigms
• Atomicity, Concurrency, Isolation, Durability
• meta-data
• schema description

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What to do with them

• Documents
• editing
• printing
• spell-checking
• counting words
• retrieving (IR)
• searching

• Database
• updating
• cleaning
• querying
• composing/transforming

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HTML

• Publishing hypertext on the World Wide Web
• Designed to describe how a Web browser should
  arrange text, images and push-buttons on a page.
• Easy to learn, but does not convey structure.
• Fixed tag set.

Text (PCDATA)
Opening tag
Welcome to the XML
course Introduction
Closing tag
Bachelor tag
Attribute name
Attribute value
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The Structure of XML

• XML consists of tags and text
• Tags come in pairs ...
• They must be properly nested
• ... ... --- good
• ... ... --- bad
• (You cant do ... ... ... in
  HTML)

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XML text

• XML has only one basic type -- text.
• It is bounded by tags e.g.
• The Big Sleep
• 1935 --- 1935 is still text
• XML text is called PCDATA (for parsed
• character data). It uses a 16-bit encoding,
• e.g. \\x0152 for the Hebrew letter Mem
• Later we shall see how new types are specified by
  XML-data

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XML structure

• Nesting tags can be used to express various
  structures. E.g. A tuple (record)

Malcolm Atchison
[phone]
mp_at_dcs.gla.ac.sc
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XML structure (cont.)

• We can represent a list by using the same
• tag repeatedly

...
... ...
...
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Terminology

• The segment of an XML document between an opening
  and a corresponding closing tag is called an
  element.

Malcolm Atchison
[phone]
[phone] mp_at_dcs.gla.ac.sc

element
element, a sub-element of
not an element
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XML is tree-like
Malcolm Atchison
[phone]
[phone]
mp_at_dcs.gla.ac.sc
Semistructured data models typically put the
labels on the edges
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Mixed Content

• An element may contain a mixture of sub-elements
  and PCDATA
• British Airways
• Worlds
  favorite airline
• Data of this form is not typically generated from
  databases. It is needed for consistency with HTML

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A Complete XML Document

• Malcolm Atchison
• [phone]
• mp_at_dcs.gla.ac.sc

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Two ways of representing a DB

projects
title budget managedBy
employees
name ssn age
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Project and Employee relations in XML
Projects and employees are intermixed

• Pattern recognition
• 10000
• Joe
• Joe
• 344556
• 34

Sandra
2234 35
Auto
guided vehicle 70000
Sandra

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Project and Employee relations in XML (contd)
Employees follow projects
Joe
344556
34
Sandra 2234
35


Pattern recognition
10000
Joe
Auto guided vehicles
70000
Sandra

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Project and Employee relations in XML (contd)
Or without separator tags
Pattern
recognition 10000
Joe
Auto guided vehicles
70000 Sandra

Joe
344556 34
Sandra 2234
35
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Attributes

• An (opening) tag may contain attributes. These
  are typically used to describe the content of
  an element
• cheese
• fromage
• branza
• A food made
• Order of attributes in an element does not matter
• XML elements are ordered

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Attributes (contd)

• Another common use for attributes is to express
  dimension or type
• 2400
• 96
• M05-.C_at_02!G96YE
• A document that obeys the nested tags rule and
  does not repeat an attribute within a tag is said
  to be well-formed .

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When to use attributes

• Its not always clear when to use attributes

F. MacNiel
fmacn_at_dcs.barra.ac.sc
...
123 45 6789
F. MacNiel
fmacn_at_dcs.barra.ac.sc
...
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XML Misc.

• Apart from elements and attributes, XML allows
  processing instructions and comments. A
  processing instruction is a statement of the
  form
• A comment takes the following form enclose
  comments between

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Part III Document Type Descriptors

• Imposing structure on XML documents

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Document Type Descriptors

• Document Type Descriptors (DTDs) impose structure
  on an XML document.
• There is some relationship between a DTD and a
  schema, but it is not close -- hence the need for
  additional typing systems.
• The DTD is a syntactic specification.

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Example The Address Book

• MacNiel, John
• Dr. John MacNiel
• 1234 Huron Street
• Rome, OH 98765
• [phone]
• [phone]
• [phone]
• jm_at_abc.com

Exactly one name
At most one greeting
As many address lines as needed (in order)
Mixed telephones and faxes
As many as needed
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Specifying the structure

• name to specify a name element
• greet? to specify an optional (0 or 1)
  greet elements
• name,greet? to specify a name followed by an
  optional greet

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Specifying the structure (cont)

• addr to specify 0 or more address lines
• tel fax a tel or a fax element
• (tel fax) 0 or more repeats of tel or fax
• email 0 or more email elements

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Specifying the structure (cont)

• So the whole structure of a person entry is
  specified by
• name, greet?, addr, (tel fax), email
• This is known as a regular expression. Why is it
  important?

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Regular Expressions

• Each regular expression determines a
  corresponding finite state automaton. Lets start
  with a simpler example
• name, addr, email
• This suggests a simple parsing program

addr
name
email
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Another example

• name,address,(tel fax),email

address
email
tel
tel
name
email
fax
fax
email
Adding in the optional greet further complicates
things
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A DTD for the address book

• (name, greet?, address, (fax tel),
  email)

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Our relational DB revisited

projects
title budget managedBy
employees
name ssn age
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Two DTDs for the relational DB
(projects,employees) (project)
managedBy) age) ...

employee) budget, managedBy) ssn, age) ...
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Some things are hard to specify

• Each employee element is to contain name, age and
  ssn elements in some order.
• ( (name, age, ssn) (age, ssn, name)
• (ssn, name, age) ...
• )
• Suppose there were many more fields !

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Summary of XML regular expressions

• A The tag A occurs
• e1,e2 The expression e1 followed by e2
• e 0 or more occurrences of e
• e? Optional -- 0 or 1 occurrences
• e 1 or more occurrences
• e1 e2 either e1 or e2
• (e) grouping

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Specifying attributes in the DTD

• dimension CDATA REQUIRED
• accuracy CDATA IMPLIED
• The dimension attribute is required the accuracy
  attribute is optional.
• CDATA is the type of the attribute -- it means
  string.

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The DTD Language

• Default modifiers in DTD attributes

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The DTD Language

• Datatypes in DTD attributes

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Consistency of ID and IDREF attribute values

• If an attribute is declared as ID
• the associated values must all be distinct (no
  confusion)
• Id is a poor cousin of a key in relational
  databases.
• If an attribute is declared as IDREF
• the associated value must exist as the value of
  some ID attribute (no dangling pointers)
• IDREF is a poor cousin of foreign key in
  relational databases.
• Similarly for all the values of an IDREFS
  attribute
• An attribute of type IDREFS represent a
  space-separated list of strings of references to
  valid IDs.
• ID and IDREF attributes are not typed

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Specifying ID and IDREF attributes

• id ID REQUIRED
• mother IDREF IMPLIED
• father IDREF IMPLIED
• children IDREFS IMPLIED

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Some conforming data

• father"john"
• Jane Doe
• John Doe
• Mary Doe
• father"john"
• Jack Doe

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An alternative specification

• children?)

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The revised data

• Jane Doe
• John Doe
• ...

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Types of Attributes

• Enumerated - List of values (You must use one of
  the items)
• ALIGN (LEFT CENTER RIGHT) "LEFT"
• Programming XML in Java ALIGN"CENTER" Programming XML in Java

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Types of Attributes

• NMTOKEN - The characters of an NMTOKEN value must
  be a letter, digit, '.', '-', '_', or ''.
• It may not include whitespace.
• student_name
• student_no NMTOKEN REQUIRED
• Jo
  Smith

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Types of Attributes

• Entities
• XML's way of referring to a data item.
• Text or Binary data.
• General Entity
• Use in the content of XML document
• References start with '' and end with ''
• Parameter Entity
• Use in a DTD
• References start with '' and end with ''
• Internal Entity - Defined in XML Document
• External Entity - Defined in a external source
  file, URI.

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Types of Attributes

• Internal General Entities
• Example 1DATE (PCDATA)
• 
  TODAY
• Example 2

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Types of Attributes

• External General Entities
• FPI URIExample
• 
  TODAY

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Types of Attributes

• Predefined General Entity References
• amp  - The character
• apos - The ' character
• gt   - The character
• lt   - The
• quot - The " character
• lkhanat_newutdal
  las.edu

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Types of Attributes

• Parameter Entities
• Internal
• Externalname PUBLIC FPI URI
• Example
• 
  

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A useful abbreviation

• When an element has empty content we can use
• for blahblahbla
• For example
• Jane Doe
• ...

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Schema.dtd

• )
• REQUIRED

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Schema.dtd (contd)

• directed)
• REQUIRED

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The DTD Language

• Example Sales Order Document
• An order document is comprised of several sales
  orders. Each individual order has a number and it
  contains the customer information, the date when
  the order was received, and the items ordered.
  Each customer has a number, a name, street, city,
  state, and ZIP code. Each item has an item
  number, parts information and a quantity. The
  parts information contains a number, a
  description of the product and its unit price.
• The numbers should be treated as attributes.

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The DTD Language

• Example Sales Order Document DTD

)










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The DTD Language

• Example Sales Order XML Document
• 
  ABC
  Industries 123 Main
  St. Chicago
  IL 60609
  10222000
  
  Turkey wrench
  9.95
  10
  

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Connecting the document with its DTD

• In line
• ...
• Another file
• A URL
• "http//www.schemaauthority.com/
  schema.dtd"

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Well-formed and Valid Documents

• Well-formed applies to any document (with or
  without a DTD) proper nesting of tags and unique
  attributes
• Valid specifies that the document conforms to the
  DTD conforms to regular expression grammar,
  types of attributes correct, and constraints on
  references satisfied

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DTDs v.s Schemas (or Types)

• By database (or programming language) standards
  DTDs are rather weak specifications.
• Only one base type -- PCDATA
• No useful abstractions e.g., sets
• IDREFs are untyped. You point to something, but
  you dont know what!
• No constraints e.g., child is inverse of parent
• No methods
• Tag definitions are global
• Some of the XML extensions impose something like
  a schema or type on an XML document. Well see
  these later

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Lots of possibilities for schemas

• XML Schema (under W3Cs spotlight)
• XDR (Microsofts BizTalk)
• SOX (Schema for Object-Oriented XML)
• Schematron
• DSD (ATT Labs and BRICS)
• and more.

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Some tools

• XML Authority http//www.extensibility.com/tibco/s
  olutions/xml_authority/index.htm
• XML Spy http//www.xmlspy.com
  /download.html

63
Summary

• XML is a new data format. Its main virtues are
  widespread acceptance and the (important) ability
  to handle semistructured data (data without
  schema)
• DTDs provide some useful syntactic constraints on
  documents. As schemas they are weak
• How to store large XML documents?
• How to query them?
• How to map between XML and other representations?