Extensible Markup Language XML

1
Extensible Markup LanguageXML

• MSI 602 Spring 2003

2
DatabasesTypes

• Data is facts and figures
• Database is a related set of data
• Kinds of databases
• Unstructured
• Meaning of data interpreted by user
• Semi-Structured
• Structure of data wrapped around data
• Structured
• Fixed structure of data
• Data added to the fixed structure

3
XMLDefinition and Example

• XML is a text based markup language that is fast
  becoming a standard of data interchange
• An open standard from W3C
• A direct descendant from SGML
• Example Product Inventory Data
• ltProductgt
• ltNamegtRefrigeratorlt/Namegt
• ltModel NumbergtR3456d2hlt/Model Numbergt
• ltManufacturergtGeneral Electriclt/Manufacturergt
• ltPricegt1290.00lt/Pricegt
• ltQuantitygt1200lt/Quantitygt
• lt/Productgt

4
XMLData Interchange

• XMLs key role is data interchange
• Two business partners want to exchange customer
  data
• Agree on a set of tags
• Exchange data without having to change internal
  databases
• Other business partners can participate by using
  the same tagset
• New tags can be added to extend the functionality

Key to successful data interchange is building
consensus and standardizing of tag sets
5
XML Universal Data

• TCP/IP ? Universal Networking
• HTML ? Universal Rendering
• Java ? Universal Code
• XML ? Universal Data
• Numerous standard bodies are set up for
  standardization of tags in different domains
• ebXML
• XBRL
• MML
• CML

6
HTML vs. XML Comparison

• Both are markup languages
• HTML has fixed set of tags
• XML allows user to specify the tags based on
  requirements
• Usage
• HTML tags specify how to display data
• XML tags specify semantics of the data
• Tag Interpretation
• HTML specifies what each tag and attribute means
• XML tags delimit data leave interpretation to
  the parsing application
• Well formedness
• HTML very tolerant of rule violations (nesting,
  matching tags)
• XML very strictly follows rules of well
  formedness

7
XML Structure

• Prolog
• Instructs the parser as to what it it parsing
• Contains processing instructions for processor
• Body
• Tags - Entities
• Attributes - Properties of Entities
• Comments - Statements for clarification in the
  document
• Example
• lt?xml version1.0 encodingUTF-8
  standaloneyes ?gt ? Prolog
• ltcontactgt
• ltnamegt
• ltfirst namegtSanjaylt/first namegt
• ltlast namegtGoellt/last namegt
• lt/namegt
• ltaddressgt ? Body
• ltstreetgt56 Della Streetlt/streetgt
• ltcitygtPhoenixlt/citygt
• ltstategtAZlt/stategt
• ltzipgt15784lt/zipgt

8
XML Prolog

• Syntax lt?xml version1.0 encodingUTF-8
• standaloneyes ?gt
• Contains eclaration that identifies a document as
  xml
• Version
• Version of XML markup language used in the data
• Not optional
• Encoding
• Identifies the character set used to encode the
  data
• Default compressed Unicode UTF-8
• Standalone
• Tells whether or not this document references
  external entity
• May contain entity definitions and tag
  specifications

9
XML Syntax Elements Attributes

• Uses less-than and greater-than characters (ltgt)
  as delimiters
• Every opening tag must having an accompanying
  closing tag
• ltFirst NamegtSanjaylt/First Namegt
• Empty tags do not require an accompanying closing
  tag.
• Empty tags have a forward slash before the
  greater-than sign e.g. ltName/gt
• Tags can have attributes which must be enclosed
  in double quotes
• ltname firstSanjay lastGoel)
• Elements should be properly nested
• The nesting can not be interleaved
• Each document must have one single root element
• Elements and attribute names are case sensitive

10
Tree Structure Elements

• XML documents have a tree structure containing
  multiple levels of nested tags.
• Root element is a single XML element which
  encloses all of the other XML elements and data
  in the document
• All other elements are children of the root
  element
• lt?xml version1.0 encodingUTF-8
  standaloneyes ?gt
• ltcontactgt ? Root Element
• ltnamegt
• ltfirst namegtSanjaylt/first namegt
• ltlast namegtGoellt/last namegt
• lt/namegt
• ltaddressgt
• ltstreetgt56 Della Streetlt/streetgt ? Child
  Elements
• ltcitygtPhoenixlt/citygt
• ltstategtAZlt/stategt
• ltzipgt15784lt/zipgt
• lt/addressgt
• lt/contactgt

11
Attributes Definition and Example

• Attributes are properties associated with an
  element
• Each attribute is a name value pair
• No element may contain two attributes with same
  name
• Name and value are strings
• Example
• lt?xml version1.0 encodingUTF-8
  standaloneyes ?gt
• ltcontactgt
• ltname firstSanjay lastGoelgtlt/namegt ?
  Attributes
• ltaddressgt
• ltstreetgt56 Della Streetlt/streetgt ? Nested
  Elements
• ltcitygtPhoenixlt/citygt
• ltstategtAZlt/stategt
• ltzipgt15784lt/zipgt
• lt/addressgt
• lt/contactgt

12
Elements vs. Attributes Comparison

• Data should be stored in Elements
• Information about data (meta-data) should be
  stored in attributes
• When in doubt use elements
• Rules of thumb
• Elements should have information which some one
  may want to read.
• Attributes are appropriate for information about
  document that has nothing to do with content of
  document
• e.g. URLs, units, references, ids belong to
  attributes
• What is your meta-data may be some ones data

13
CommentsBasics

• XML comments begin with lt!--and end with --gt
• All data between these delimiters is discarded
• lt!-- This is a list of names of people --gt
• Comments should not come before XML declaration
• Comments can not be placed inside a tag
• Comments may be used to hide and surround tags
• ltNamegt
• ltfirstgtSanjaylt/firstgt
• lt!-- ltlastgtGoellt/lastgt --gt ? Last tag is
  ignored
• lt/Namegt
• -- string may not occur inside a comment except
  as part of its opening and closing tag
• lt!-- the Red door -- that is the second --gt ?
  Illegal

14
Namespaces Basics

• XML documents come from different sources
• Combining elements from different sources can
  result in name conflict
• Namespaces allow the interpreter to resolve the
  elements
• Namespaces
• Declared within element start-tag using attribute
  xmlns
• Represented as an actual URI (since namespaces
  are globally unique)
• e.g. ltCollection xmlnsbook"http//www.mjyOnline.
  com/books"
• xmlnscdhttp//www.mjyOnline.com/booksgt
• Here book and cd are short hands for the full
  namespace name
• Default namespace is used if no other namespace
  is defined
• It does not have any prefix associated with it

15
Namespaces Example

• lt?xml version"1.0"?gt
• lt!-- File Name Collection.xml --gt
• ltCOLLECTION
• xmlnsbook"http//www.mjyOnline.com/books"
• xmlnscd"http//www.mjyOnline.com/cds"gt
• ltITEM Status"in"gt
• ltTITLEgtThe Adventures of Huckleberry
  Finnlt/bookTITLEgt
• ltAUTHORgtMark Twainlt/bookAUTHORgt
• ltPRICEgt5.49lt/bookPRICEgt
• lt/ITEMgt
• ltITEM Status"in"gt
• ltTITLEgtThe Marble Faunlt/TITLEgt
• ltAUTHORgtNathaniel Hawthornelt/AUTHORgt
• ltPRICEgt10.95lt/PRICEgt
• lt/ITEMgt
• ltITEMgt
• ltITEM Status"out"gt
• ltTITLEgtLeaves of Grasslt/TITLEgt
• ltAUTHORgtWalt Whitmanlt/AUTHORgt

lt?xml version"1.0"?gt lt!-- File Name
Collection.xml --gt ltCOLLECTION ltITEMgt
ltTITLEgtViolin Concertos Numbers 1, 2, and
3lt/TITLEgt ltCOMPOSERgtMozartlt/COMPOSERgt
ltPRICEgt16.49lt/PRICEgt lt/ITEMgt ltTITLEgtViolin
Concerto in Dlt/TITLEgt ltCOMPOSERgtBeethovenlt/C
OMPOSERgt ltPRICEgt14.95lt/PRICEgt
lt/ITEMgt lt/COLLECTIONgt
Books and CDs are tracked in different files if
combined will lead to conflicts
16
Namespaces Example

• lt?xml version"1.0"?gt
• lt!-- File Name Collection.xml --gt
• ltCOLLECTION
• xmlnsbook"http//www.mjyOnline.com/books"
• xmlnscd"http//www.mjyOnline.com/cds"gt
• ltbookITEM Status"in"gt
• ltbookTITLEgtThe Adventures of Huckleberry
  Finnlt/bookTITLEgt
• ltbookAUTHORgtMark Twainlt/bookAUTHORgt
• ltbookPRICEgt5.49lt/bookPRICEgt
• lt/bookITEMgt
• ltcdITEMgt
• ltcdTITLEgtViolin Concerto in Dlt/cdTITLEgt
• ltcdCOMPOSERgtBeethovenlt/cdCOMPOSERgt
• ltcdPRICEgt14.95lt/cdPRICEgt
• lt/cdITEMgt
• ltbookITEM Status"out"gt
• ltbookTITLEgtLeaves of Grasslt/bookTITLEgt
• ltbookAUTHORgtWalt Whitmanlt/bookAUTHORgt
• ltbookPRICEgt7.75lt/bookPRICEgt

ltcdITEMgt ltcdTITLEgtViolin Concertos
Numbers 1, 2, and 3lt/cdTITLEgt
ltcdCOMPOSERgtMozartlt/cdCOMPOSERgt
ltcdPRICEgt16.49lt/cdPRICEgt lt/cdITEMgt
ltbookITEM Status"out"gt ltbookTITLEgtThe
Legend of Sleepy Hollowlt/bookTITLEgt
ltbookAUTHORgtWashington Irvinglt/bookAUTHORgt
ltbookPRICEgt2.95lt/bookPRICEgt lt/bookITEMgt
ltbookITEM Status"in"gt ltbookTITLEgtThe
Marble Faunlt/bookTITLEgt ltbookAUTHORgtNathan
iel Hawthornelt/bookAUTHORgt
ltbookPRICEgt10.95lt/bookPRICEgt
lt/bookITEMgt lt/COLLECTIONgt
17
Display XML Style Sheets

• A style sheet is a file that contains
  instructions for rendering individual elements in
  an XML document
• Two kinds of style sheets exist
• Cascading Style Sheets (CSS)
• Extensible Stylesheet language (XSLT)
• Please refer to the following web site for
  comprehensive information on style sheets
• http//www.w3schools.com/css/default.asp

18
Cascading Style SheetsExample

• lt?xml version"1.0"?gt
• lt!-- File Name Inventory01.xml --gt
• lt?xml-stylesheet type"text/css"
  href"Inventory01.css"?gt
• ltINVENTORYgt
• ltBOOKgt
• ltTITLEgtThe Adventures of Huckleberry
  Finnlt/TITLEgt
• ltAUTHORgtMark Twainlt/AUTHORgt
• ltBINDINGgtmass market paperbacklt/BINDINGgt
• ltPAGESgt298lt/PAGESgt
• ltPRICEgt5.49lt/PRICEgt
• lt/BOOKgt
• ltBOOKgt
• ltTITLEgtLeaves of Grasslt/TITLEgt
• ltAUTHORgtWalt Whitmanlt/AUTHORgt
• ltBINDINGgthardcoverlt/BINDINGgt
• ltPAGESgt462lt/PAGESgt
• ltPRICEgt7.75lt/PRICEgt
• lt/BOOKgt

ltBOOKgt ltTITLEgtThe Legend of Sleepy
Hollowlt/TITLEgt ltAUTHORgtWashington
Irvinglt/AUTHORgt ltBINDINGgtmass market
paperbacklt/BINDINGgt ltPAGESgt98lt/PAGESgt
ltPRICEgt2.95lt/PRICEgt lt/BOOKgt ltBOOKgt
ltTITLEgtThe Marble Faunlt/TITLEgt
ltAUTHORgtNathaniel Hawthornelt/AUTHORgt
ltBINDINGgttrade paperbacklt/BINDINGgt
ltPAGESgt473lt/PAGESgt ltPRICEgt10.95lt/PRICEgt
lt/BOOKgt ltBOOKgt ltTITLEgtMoby-Dicklt/TITLEgt
ltAUTHORgtHerman Melvillelt/AUTHORgt
ltBINDINGgthardcoverlt/BINDINGgt
ltPAGESgt724lt/PAGESgt ltPRICEgt9.95lt/PRICEgt
lt/BOOKgt lt/INVENTORYgt
19
Cascading Style Sheets Example

• / File Name Inventory02.css /
• BOOK
• displayblock
• margin-top12pt
• font-size10pt
• TITLE
• displayblock
• font-size12pt
• font-weightbold
• font-styleitalic
• AUTHOR
• displayblock
• margin-left15pt
• font-weightbold

BINDING displayblock
margin-left15pt PAGES
displaynone PRICE displayblock
margin-left15pt
20
Cascading Style Sheets Display
21
Formal Languages/Grammars Basics

• A formal language is a set of strings
• It is characterized by a set of rules which
  determine which strings are a part of the
  language and which are not
• In case of programming languages, programs which
  compile are grammatical corret (others are not)
• In a natural language, like English, correct
  sentences follows rules of the English language
  grammar
• More precisely grammar a defines four things
• A vocabulary out of which the strings are
  constructed (terminal symbols)
• Vocabulary that is used to formulate grammar
  rules (non terminal symbols)
• Grammar rules (productions), each of which has a
  lhs and a rhs
• A designated start symbol

22
Validated XML Document Basics

• An XML document is valid if it conforms to the
  grammar of the language
• Validity is different from well-formedness
• Two ways to specify the grammar of the language
• Document Type Definition (DTD)
• XML Schema
• Why bother with the language grammar
• It provides the blueprint of the language
• Ensures that the data is interchangable
• Eliminates processing errors in custom software
  which expects a particular document content and
  structure
• Validity of the document is checked by using a
  validator

23
Document Type Declaration Basics

• Document type declaration is a block of XML
  markup added to the prolog of the document
• It has to follow the XML declaration
• It has to be outside of other markup language
• It defines the content and structure of the
  language
• Without a document type declaration or schema a
  document is merely checked for well-formedness
  and not validity
• Why bother with the language grammar
• It provides the blueprint of the language
• Ensures that the data is interchangable
• Eliminates processing errors in custom software
  which expects a particular document content and
  structure
• The form of a document type declaration is
• lt!DOCTYPE Name DTDgt
• DTD is document type definition
• Name specifies the name of the document element

24
Document Type Definitions Basics

• Document type definition (DTD) consists of a
  series of markup declarations enclosed in square
  brackets
• lt?xml version1.0 standaloneyes?gt
• lt!DOCTYPE GREETING
• lt!ELEMENT GREETING (PCDATA)gt
• gt
• ltGREETINGgt
• Hello XML!
• lt/GREETINGgt
• A DTD can also be stored separately from the XML
  document and referenced in it.

25
Document Type Definitions Syntax

• Element Type Declaration
• Syntax lt!Element Name contentspecgt
• Name is the name of the element
• contentspec is the content specification
• Example
• lt!Element Title (PCDATA)gt
• Content specification can have four types of
  values
• EMPTY content Element must not have content
• lt!Element Image EMPTYgt
• ANY Content Can contain any thing
• lt!Element misc ANYgt
• Element Content Child elements but no character
  data
• lt!DOCTYPE BOOK
• lt!ELEMENT BOOK (TITLE, AUTHOR)gt
• lt!ELEMENT TITLE (PCDATA)gt
• lt!ELEMENT AUTHOR (PCDATA)gt
• Mixed Content character data and child elements
  interspersed

26
Element Content Specification Types

• Content Specification indicates allowed child
  elements and their order
• If element has element content it can not contain
  any character data
• Types of content specifications
• Sequence Indicates that each element must have a
  specific sequence of child elements
• Example
• lt!Doctype Mountain
• lt!ELEMENT MOUNTAIN (NAME, HEIGHT, STATE)gt
• lt!ELEMENT NAME (PCDATA)
• lt!ELEMENT HEIGHT (PCDATA)
• lt!ELEMENT STATE (PCDATA)
• gt
• Valid XML
• ltMOUNTAINgt
• ltNAMEgtWheelerlt/NAMEgt
• ltHEIGHTgt13161lt/HEIGHTgt
• ltSTATEgtNew Mexicolt/STATEgt
• lt/MOUNTAINgt

27
Element Content Specification Types

• Types of content specifications
• Choice Indicates that element can have one of a
  series of child elements
• Each element is separated by a sign
• Example
• lt!Doctype FILM
• lt!ELEMENT FILM (STAR NARRATOR INSTRUCTOR)gt
• lt!ELEMENT STAR (PCDATA)gt
• lt!ELEMENT NARRATOR (PCDATA)gt
• lt!ELEMENT INSTRUCTOR (PCDATA)gt
• gt
• Valid XML
• ltFILMgt
• ltSTARgtROBERT REDFORDlt/STARgt
• lt/FILMgt
• Invalid XML
• ltFILMgt
• ltNARRATORgtSir Gregory Parsloelt/NARRATORgt
• ltINSTRUCTORgtGalahad Threepwoodlt/INSTRUCTORgt
• lt/FILMgt

28
Element Content Specification Number of Elements

• Specifying the number of elements allowed
• ? zero or one
• one or more
• zero or more
• Example
• lt!Doctype Mountain
• lt!ELEMENT MOUNTAIN (NAME, HEIGHT?, STATE)gt
• lt!ELEMENT NAME (PCDATA)
• lt!ELEMENT HEIGHT (PCDATA)
• lt!ELEMENT STATE (PCDATA)
• gt
• Valid XML
• ltMOUNTAINgt
• ltNAMEgtPeublo Peaklt/NAMEgt
• ltNAMEgtTaos Mountainlt/NAMEgt
• ltSTATEgtNew Mexicolt/STATEgt
• lt/MOUNTAINgt

29
Element Content Specification Modification

• Modifying a group of elements
• Example
• lt!Doctype FILM
• lt!ELEMENT FILM (STAR NARRATOR
  INSTRUCTOR)gt
• lt!ELEMENT STAR (PCDATA)gt
• lt!ELEMENT NARRATOR (PCDATA)gt
• lt!ELEMENT INSTRUCTOR (PCDATA)gt
• gt
• Valid XML
• ltFILMgt
• ltNARRATORgtSir Gregory Parsloelt/NARRATORgt
• ltSTARgtROBERT REDFORDlt/STARgt
• ltNARRATORgtPLUG BASHMANlt/NARRATORgt
• lt/FILMgt

30
Element Content Specification Nesting

• Nesting in specification
• Example
• lt!Doctype FILM
• lt!ELEMENT FILM TITLE, CLASS,(STAR
  NARRATOR INSTRUCTOR)gt
• lt!ELEMENT TITLE (PCDATA)gt
• lt!ELEMENT CLASS (PCDATA)gt
• lt!ELEMENT STAR (PCDATA)gt
• lt!ELEMENT NARRATOR (PCDATA)gt
• lt!ELEMENT INSTRUCTOR (PCDATA)gt
• gt
• Valid XML
• ltFILMgt
• ltTITLEgtThe Netlt/TITLEgt
• ltCLASSgtActionlt/CLASSgt
• ltSTARgtSandra Bullocklt/STARgt
• lt/FILMgt

31
Element Content Specification Mixed Content Model

• Mixed Content Model Allows element to contain
• Character Data
• Child elements in any position and any frequency
  (zero or more repetitions)
• Child elements can be interspersed with data
• Character data only
• Example
• lt!ELEMENT TITLE (PCDATA)gt
• Character data and elements
• Example
• lt!ELEMENT TITLE (PCDATA SUBTITLE)gt
• lt!ELEMENT SUBTITLE (PCDATA)gt
• Valid XML
• ltTITLEgtMoby Dick ltSUBTITLEgtOr, The
  Whalelt/SUBTITLEgtlt/TITLEgt
• ltTITLEgtltSUBTITLEgtOr, The Whalelt/SUBTITLEgtMoby
  Dicklt/TITLEgt

32
Attribute Specification Basics

• All attributes in the document need to be
  specified using an attribute declaration list. It
  defines
• Defines the name of the attribute
• Defines the data type of each attribute
• Specifies whether an attribute is required or noe
• Syntax lt!ATTLIST Name Attdefsgt
• Name is the name of the element
• Attdefs is a series of one or more attribute
  definitions
• Attribute definition Syntax Name AttType
  DefaultDecl
• Name is the attribute name
• AttType is the type of the attribute (CDATA,
  Token Type, Enumerated)
• DefaultDecl specifies if attribute is required
  default values
• Example
• lt!ELEMENT FILM (TITLE, (STAR NARRATOR
  INSTRUCTOR))gt
• lt!ATTLIST FILM Class CDATA fictional Year CDATA
  REQUIREDgt

33
Entity Specification Types

• There are two kinds of entities in XML documents1
• Character entities (referred by character unicode
  number)
• Named entities, referred to by name

34
XML Parsing Definition and Types

• An XML parser is a program that reads an XML
  document and makes its contents available for
  processing
• There are two standard types of parsers for XML
• Document Object Model (DOM) which makes the
  document available as a tree
• Simple XML Parser (SAX) which associates an event
  with each tag and each block of text
• XML parsers are available from many vendors
• Each vendor conforms to the standardized XML
  interfaces
• One of the best parsers is the xerces parser
• Suns API for XML parsing is JAXP (supports basic
  classes and interfaces that a Java XML parser
  should support)
• Often SAX parsers are used for writing DOM parsers

35
SAX Parser Basics

• As the parser scans the document it sends
  notifications of events, for instance
• Element start
• Element end
• Character sequence between two elements is found
• SAX provides standard names for these callback
  functions that are triggerd by these events
• void characters (char ch, int start, int
  length) notification of character data
• void startDocument() notification of start of
  document
• void endDocument() notification of end of
  document
• void startElement(String name, AttributeList
  atts) notification of start of element
• void endElement(String name) notification of end
  of element
• void processingInstruction(String target, String
  data) notification of processing instruction

36
SAX Parser Example

• From professional JSP page 658

37
XSLT Parser Definition and Uses

• XSLT is an XML structure transforming language
• Any treee transforming language needs an ability
  to refer to tree paths
• Xpath is the sub-language underneath XSLT for
  tree path description
• There are two scenarios for use of XSLT
• Browser contains an XSLT and uses it to render
  XML documents
• XSLT is used for changing the structure of an
  existing XML document
• To run XSLT the following components are
  required
• Java 1.4 standard development kit
• James Clarks xt (xt.jar)

38
XSLT Parser Basics

• XSLT style sheet is an XML document
• Consists of two parts
• Standard XML declaration including namespace
  declaractions
• Top level elements that set up the general
  framework for the output, e.g., variables or
  import parameters from the command line
• Processing involves the following
• A current list of nodes from the source document
  is created by matching a pattern
• Output to the current node is generated by
  instantiating a template corresponding the
  current pattern
• In process of transformation new nodes can be
  added to the list
• The processing begins by processing a list
  containing the entire document
• Transformation ends when the node list is empty

39
XSLT Parser Example

• XSLT

40
Web Services
41
Web Services Definition

• Web Services are software programs that use XML
  to exchange information with other software
  programs via common Internet protocols.
• Web services communicate over the network to
  provide specific methods that other applications
  can invoke.
• Thus applications residing on different computer
  can work synergistically by invoking methods on
  each other
• Http is the key protocol used for Web Services.
• Characteristics
• Programmable
• Encapsulate a task
• XML based data exchange allows programs on
  heterogenous platforms to communicate (SOAP)
• Self-describing (WSDL)
• Discoverable (UDDI)

42
Web Services SOAP

• SOAP Simple Object Access Protocol
• Enables data transfer between systems distributed
  over a network
• A SOAP method send to the a Web Service invokes a
  method provided by the service
• Web Service may return the result via another
  SOAP message
• SOAP consists of standardized XML schemas
• Defines a format for transmitting XML messages
  over network
• Includes data types and message structure
• Layered over an Internet protocol, such as HTTP
  and can be used to transfer data across the Web
  and other networks
• Http allows message transfer across firewall
  since Http messages are usually accepted by
  firewalls

43
Web Services SOAP

• SOAP message consists of three parts
• Envelope
• Header
• Body
• Envelope wraps the entire message and contains
  header and body
• Header (optional) provides information on
  security and routing
• Body contains application specific data that is
  being transferred
• Other alternative to SOAP are XML-RPC
• SOAP de facto standard due to simplicity,
  extensibility and interoperability

44
Web Services WSDL

• WSDL Web Services Description Language
• Provides means to provide information about a web
  service
• Instructions of its use
• Capability of the service
• Provides information on connection to the service
  and communicate
• Syntax is fairly complex
• Normally created using automated tools
• Not important to understand the precise syntax of
  WSDL while developing web services

45
Web Services UDDI

• UDDI Universal Description, Discovery and
  Integration
• Allows developers and businesses to publish and
  locate web services on a network via use of
  registries
• The registries can be made private or public
• Structure similar to a phone book
• White pages contain contact information and
  textual description
• Yellow pages provides classification information
  about companies and details of companys
  electronic capability
• Green pages list technical data relating to
  services and business processes