Chapter 4 - Quality Control with Schemas Learning XML by Erik T. Ray

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Chapter 4 - Quality Control with Schemas
Learning XMLbyErik T. Ray

• Slides were developed by Jack DavisCollege of
  Information Scienceand TechnologyRadford
  University

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Schemas

• define an XML tag set- primarily elements,
  attributes, entities and structure
• is a pass or fail test for XML documents
  (validation)
• insure that a document fulfills a minimum set of
  requirements, finding flaws that could result in
  anomalous processing
• are not required
• A validating XML parser takes an XML instance as
  input and produces a validation report as output.
  This report typically lists errors found in the
  document (where it does not conform to the
  schema)
• Validation considersstructure, data typing,
  integrity (status of links between nodes and
  resources), business rules (spell checking,
  checksum)

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Schema Types

• DTD - Document Type DefinitionThe oldest and
  most widely supported schema language.DTD's
  don't support namespaces (can't mix tag sets
  within a single DTD) and have very weak data
  typing.
• The W3C built XML SchemaXML Schemas are
  themselves XML documents, so they can be checked
  for well-formedness and validity.XML Schema
  support namespaces and have a much broader
  ability to specify data types, including things
  like date types.
• Other schema definition languages are available
  (RELAX NG, Schematron, ).

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DTD's

• XML elements and attributes are defined in a DTD
• DTD's are extensible - meaning they can be
  extended to meet the needs of the current task
• A DTD can be specified within an XML document
  (internal) or in a separate file (external).
• Many free DTD's exist on the internet today and
  can be freely downloaded
• DTD's declare a set of allowed elements. A
  conforming XML document can't use any elements
  not defined in this set.
• DTD's define a content model for each element.
  This describes what elements or data can go
  inside an element, in what order, in what number,
  and whether they are required or optional.
• DTD's declare a set of allowed attributes for
  each element with data types and default values.
• DTD's provide mechanisms to manage the model,
  providing links to other components.

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Element Declarations

• Element declarationlt!ELEMENT element_name
  (content model)gtContent ModelText
• Description text or character data
• Syntax (PCDATA)
• Elements
• Description contains other elements
• Syntax (element_1, element_2, )
• Mixed Content
• Description contains both text and other
  elements
• Syntax (PCDATA element_1 element2 )
• Empty
• Description does not contain any content
• Syntax EMPTY
• Any
• Description can contain text or elements
• Syntax ANY

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Element Declaration Syntax

• Declaration syntax is flexible when it comes to
  whitespace. You can add extra space anywhere
  except in the string of characters at the
  beginning that identifies the declaration
  type.For example, these are all
  acceptablelt!ELEMENT thingie
  ALLgtlt!ELEMENT thingie
  ALLgtlt!ELEMENT thingie ( foo
  
  bar
  zap )gt

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Element Character Notations

• Question Mark
• Character ?
• Description element may occur zero or one time
• Usage email?
• Asterisk
• Character
• Description element may occur zero or more times
• Usage email
• Plus
• Character
• Description element may occur one or many times
• Usage email

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Element Character Notations (cont.)

• Parentheses
• Character ( )
• Description used to indicate a set
• Usage (name, address, zip_code)
• Vertical bar
• Character
• Description used to indicate a set of values
• Usage a b c
• Comma
• Character ,
• Description used to indicate element sequence
• Usage (a, b, c)

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Attribute Declarations

• lt!ATTLIST element_name
• attribute_name-1 datatype default_value
• attribute_name-2 datatype default_value
• attribute_name-3 datatype default_valuegt
• lt!ATTLIST student
• level CDATA
  REQUIREDgt
• lt!ATTLIST student level (fr
  soph jr sr) "fr"gt

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Attribute Data Types

• Data type CDATA
• Description character data
• Data type ID
• Description unique identifier to give an
  element a label
• Data type Enumerated List (i.e., (a, b, c) )
• Description list of all possible values that the
  attribute can contain

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Attributes Default Values

• Attribute type FIXED
• Description value of the attribute must match
  the value assigned in the DTD
• Attribute type REQUIRED
• Description element must contain the attribute
  to be valid
• Attribute type IMPLIED
• Description attribute is optional

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Example XML Document

• lt?xml version1.0 standaloneyes?gt
• ltemailsgt
• ltmessage numa1 tojoe64acmeshipping.com
• frombrenda64xyzcompany.com
  date02/09/01gt
• ltsubject titleOrder 10011/gt
• ltbodygt
• Joe, Please let me know if order number 10011
  has shipped.
• Thanks,
• Brenda
• lt/bodygt
• ltreply status"yes"/gt
• lt/messagegt
• lt/emailsgt

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Internal DTD

• lt!DOCTYPE emails lt!ELEMENT emails
  (message)gt
• lt!ELEMENT message (subject?, body, reply)gt
• lt!ATTLIST message
• num ID REQUIRED
• to CDATA REQUIRED
• from CDATA FIXED brenda64xyzcompany.com
• date CDATA REQUIREDgt
• lt!ELEMENT subject EMPTYgt
• lt!ATTLIST subject
• title CDATA IMPLIEDgt
• lt!ELEMENT body ANYgt
• lt!ELEMENT reply EMPTYgt
• lt!ATTLIST reply
• status (yes no) "no"gt
• gt
• In a standalone XML document this is prepended
  to the XML document. If it's an external DTD the
  XML document must contain a declaration like the
  following.lt!DOCTYPE emails SYSTEM "emails.dtd"gt
  orlt!DOCTYPE emails SYSTEM "http//"gt

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DTD Census Example

• Here's an example XML document. The information
  in this example is a census document. The
  following example is a typical Census example XML
  document. It's created after an interview with
  one family. Consider that all such documents
  could be compiled and overall statistics
  generated. example 4-1
• Here's the DTD that generates the rules by which
  the Census XML documents are created.example
  4.2

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DTD Design

• DTD design and construction is part science and
  part art form. The basic concepts are simple,
  but maintaining hundreds of element and attribute
  declarations while keeping them readable and
  bug-free can be a challenge.
• Keep it organizedGood comments can save hours of
  scrutinizing later, do not wait until the end to
  document. Keep declarations separated into
  sections by their purpose.Pad declarations with
  lots of whitespace. Content models and attribute
  lists suffer from dense syntax, so spacing out
  the parts, even placing them on separate lines,
  helps. Indent lines inside declarations to make
  the delimiters clearer. Use extra space between
  logical divisions.DTD's will require updating
  as requirements change. Number versions to avoid
  lots of confusion later.

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DTD Design (cont.)

• Parameter entitiesParameter entities can hold
  recurring parts of declarations and allow you to
  edit them in one place. In the external subset,
  they can be used in element-type declarations to
  hold element groups and content models, or in
  attribute list declarations to hold attribute
  definitions. For example, assume you want every
  element to have an optional ID attribute for
  linking and an optional class attribute to assign
  specific role information. Parameter entities,
  which apply only in DTDs, look much like ordinary
  general entities, but have an extra in the
  declaration. You can declare a parameter entity
  as in the followinglt!ENTITY common.atts "
  id ID implied class
  CDATA implied" gtthe entity can be used in
  attribute list declarationslt!ATTLIST foo
  common.attsgtlt!ATTLIST bar common.atts
  extra CDATA FIXED "blah"gt

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Attributes vs. Elements

• Making a DTD from scratch is not easy. You have
  to break information down into its conceptual
  atoms and package it as a hierarchical structure,
  but it's not always clear how to divide the
  information. Choose names that make sense.
  Element names like thing, object, and chunk are
  nearly impossible to figure out.Hierarchy adds
  information. A newspaper has articles that
  contain paragraphs and heads. Containers create
  boundaries to make it easier to write stylesheets
  and processing applications. Strive for a tree
  structure that resembles a wide, bushy shrub. If
  you go too deep, the markup begins to overwhelm
  the content and it becomes harder to edit a
  document too shallow and the information
  content is diluted.

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Attributes vs. Elements (cont.)

• Know when to use elements over attributes. An
  element holds content that is part of your
  document. An attribute modifies the behavior of
  an element. The trick is to find a balance
  between using general elements with attributes to
  specify purpose and creating an element for every
  single contingency.There are advantages to
  splitting a monolithic DTD into smaller
  components, or modules. The first is that a
  modularized DTD can be easier to maintain. XML
  provides two ways to modularize your DTD. The
  first is to store parts in separate files, then
  import them with external parameter entities.
  The second is to use a syntactic device called a
  conditional section.

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Importing Modules

• To import whole DTD's or parts of DTDs, use an
  external parameter entity.lt!ELEMENT catalog
  (title, metadata, front,
  entries)gtlt!ENTITY basic.stuff
  SYSTEM
  "basics.mod"gtbasic.stufflt!ENTITY
  frnt.matter SYSTEM "front.mod"gtfrnt.matterlt!EN
  TITY metadata PUBLIC "-//Standards
  Stuff//DTD Metadata v3.2//EN"
  "http//www.standards- ."gtmetadataThis DTD
  has two local components, which are specified by
  system identifiers. Each component has a .mod
  filename extension, which is a traditional way to
  show that a file contains declarations but should
  not be used as a DTD on its own.

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Examples

• standalone.xml
• itfac.xmlReview the itfac.xml document, then
  students should develop the dtd.
• faculty.dtd
• faculty.css

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XML Schema Overview

• XML Schema specification released by the W3C in
  May 2001, and contains two parts
• Part I - structure
• Part II - data types
• Developed as an alternative to DTDs and is much
  more powerful
• Features
• Pattern matching
• Rich set of data types
• Attribute grouping
• Supports XML namespaces
• Follows XML syntax

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XML Schemas

• The XML Schema specification was released by the
  W3C in May of 2001
• XML Schemas, like DTDs, are used to describe the
  structure of an XML document
• The XML Schema specification consists of two
  parts
• XML Schema Structures. This specification
  consists of a definition language for describing
  and constraining the content of XML documents
• XML Schema Datatypes. This specification defines
  the datatypes to be used in XML schemas.
• The namespace for XML Schema is
• http//www.w3.org/2001/XMLSchema

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XML Schema - advantages

• XML Schema allows you to import vocabularies (tag
  sets).
• XML Schemas are XML documents, so they can be
  validated
• The XML Schema specification contains a number of
  built-in datatypes, and also allows developers to
  create their own datatypes
• Some of the datatypes arexsstring
  textxstoken contains textual tokens
  xsQName namespace-qualified
  namexsdecimal pos neg floats and
  int'sxsinteger integersxsfloat
  floating pt. numberxsID,IDREF
  identification tokenxsboolean true or
  falsexstime HHMMSSxsdate
  CCYY-MM-DDxsdateTime CCYY-MM-DDTHHMMSS-Z
  one

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Complex Elements

• Most elements are not simple. They can contain
  elements, attributes, and character data with
  specialized formats. So, complex elements can be
  defined.Here's an example complex type
  definition.ltxselement name"date"gt
  ltxscomplexTypegt ltxsallgt ltxselement
  ref"year"/gt ltxselement ref"mo"/gt
  ltxselement ref"day"/gt lt/xsallgt
  lt/xscomplexTypegtlt/xselementgtltxselement
  name"year" type"xsinteger"/gtltxselement
  name"mo" type"xsinteger/gtltxselement
  name"day" type"xsinteger/gt

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Restriction Elements

• In the previous example the month number was just
  given as type integer. However, this would allow
  the user to insert any integer into the document
  for the month number, obviously we'd like to
  restrict the month number to 1-12.ltxssimpleType
  name"monthNum"gt ltxsrestriction
  base"xsinteger"gt ltxsminInclusive value"1"
  /gt ltxsmaxInclusive value"12" /gt
  lt/xsrestrictiongtlt/xssimpleTypegtltxselement
  name"mo" type"monthNum"/gt

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Restriction Elements (cont.)

• Restrictions can create fixed values, constrain
  the length of strings, and match patterns with
  regular expressions. Here's an example that
  restricts a postal code (three digits followed by
  three capital letters).ltxselement
  name"postalcode"
  type"pcode"/gtltxssimpleType name"pcode"gt
  ltxsrestriction base"xstoken"gt ltxspattern
  value"0-93A-Z3"/gt lt/xsrestrictiongtlt/x
  ssimpleTypegt
• Can also implement enumeration typesltxs
  simpleType name"gender"gt ltxsrestriction
  base"xstoken"gt ltxsenumeration
  value"female"/gt ltxsenumeration
  value"male"/gt lt/xsrestrictiongtlt/xssimpleType
  gt

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XML Schema Occurrence Constraints

• Occurrence constraints define the number of times
  a particular element can or must occur
• Attributes
• minOccursDefines the minimum number of times an
  element can occur. Default value is 1
• maxOccurs Defines the maximum number of times
  an element can occur. Default value is 1
• Can set the value of the maxOccurs attribute to
  unbounded to indicate that there is no maximum
  number of times the element can occur

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XML Schema Simple Type Example

• XML schemas are put together like DTD's with
  element and attribute declarations along with
  type declarations. A simple example shows the
  structure.
• XML filelt?xml version1.0?gt ltemail
• xmlnsxsi"http//www.w3.org/2001/XMLSchema-insta
  nce"
• xsinoNamespaceSchemaLocation
  "email_schema.xsd"gt
• This is my e-mail message
• lt/emailgt
• Schema file
• lt?xml version1.0?gtltxsdschema xmlnsxsd
  http//www.w3.org/2001/XMLSchemagt
  ltxsdelement nameemail
  typexsdstring/gtlt/xsdschemagt

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XML Schemas

• XML Schemas utilizetype extensiontype
  restrictionlistsunionsnamespace
  featuresand much, much more.This brief
  presentation only scratches the surface of XML
  schemas.

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XML Schema Example

• Here's a schema for the Census example that a DTD
  was defined for. Note the differences.example
  4-6