XML: Extensible Markup Language

1
XML Extensible Markup Language

• FST-UMAC
• Gong Zhiguo

2
How the Web is Today

• HTML documents
• all intended for human consumption
• many generated automatically by applications

Easy to fetch any Web page, from any server, any
platform
3
Limits of the Web Today

• Application cannot consume HTML
• HTML wrapper technology is brittle
• screen scraping
• OO technology (Corba) requires controlled
  environment
• Companies merge, form partnerships need
  interoperability fast

4
Paradigm Shift on the Web

• new Web standard XML
• XML generated by applications
• XML consumed by applications
• data exchange
• across platforms enterprise interoperability
• across enterprises

Web from collection of documents to data and
documents
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XML

• a W3C standard to complement HTML
• origins structured text SGML
• motivation
• HTML describes presentation
• XML describes content
• http//www.w3.org/TR/REC-xml (2/98)

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From HTML to XML
HTML describes the presentation
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HTML
lth1gt Bibliography lt/h1gt ltpgt ltigt Foundations of
Databases lt/igt Abiteboul, Hull, Vianu
ltbrgt Addison Wesley, 1995 ltpgt ltigt Data on
the Web lt/igt Abiteoul, Buneman, Suciu
ltbrgt Morgan Kaufmann, 1999
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XML
ltbibliographygt ltbookgt lttitlegt Foundations
lt/titlegt ltauthorgt Abiteboul
lt/authorgt ltauthorgt Hull
lt/authorgt ltauthorgt Vianu
lt/authorgt ltpublishergt Addison
Wesley lt/publishergt ltyeargt 1995
lt/yeargt lt/bookgt lt/bibliographygt
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XML Terminology

• tags book, title, author,
• start tag ltbookgt, end tag lt/bookgt
• elements ltbookgtltbookgt,ltauthorgtlt/authorgt
• elements are nested
• empty element ltredgtlt/redgt abbrv. ltred/gt
• an XML document single root element

well formed XML document if it has matching tags
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More XML Attributes
ltbook price 55 currency USDgt lttitlegt
Foundations of Databases lt/titlegt ltauthorgt
Abiteboul lt/authorgt ltyeargt 1995
lt/yeargt lt/bookgt
attributes are alternative ways to represent data
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Query Languages Motivation

• granularity of the HTML Web one file
• granularity of Web data varies
• single data item get Johns salary
• entire database get all salaries
• aggregates get average salary
• need query language to define granularity

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XML-QL A Query Language for XML

• http//www.w3.org/TR/NOTE-xml-ql (8/98)
• features
• regular path expressions
• patterns, templates
• Skolem Functions
• based on OEM data model

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Pattern Matching in XML-QL
where ltbook languagefrenchgt
ltpublishergt ltnamegt
Morgan Kaufmann lt/namegt
lt/publishergt ltauthorgt a
lt/authorgt lt/bookgt in
www.a.b.c/bib.xml construct a
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Simple Constructors in XML-QL
where ltbook language lgt
ltauthorgt a lt/gt lt/gt in
www.a.b.c/bib.xml construct ltresultgt ltauthorgt
a lt/gt ltlanggt l lt/gt lt/gt
Note lt/gt abbreviates lt/bookgt or lt/resultgt or ...
ltresultgt ltauthorgtSmithlt/authorgtltlanggtEnglishlt/lang
gtlt/resultgt ltresultgt ltauthorgtSmithlt/authorgtltlanggtMa
ndarinlt/langgtlt/resultgt ltresultgt
ltauthorgtDoelt/authorgtltlanggtEnglishlt/langgtlt/resultgt
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Schemas in XML

• Document Type Definition (DTD)
• XML Schema
• RDF Schema

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Document Type Definition DTD

• part of the original XML specification
• an XML document may have a DTD
• terminology for XML
• well-formed if tags are correctly closed
• valid if it has a DTD and conforms to it
• validation is useful in data exchange

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DTDs as Grammars
lt!DOCTYPE paper lt!ELEMENT paper
(section)gt lt!ELEMENT section ((title,section)
text)gt lt!ELEMENT title (PCDATA)gt
lt!ELEMENT text (PCDATA)gt gt
ltpapergt ltsectiongt lttextgt lt/textgt lt/sectiongt
ltsectiongt lttitlegt lt/titlegt ltsectiongt
lt/sectiongt
ltsectiongt lt/sectiongt
lt/sectiongt lt/papergt
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DTDs as Schemas

• Not so well suited
• impose unwanted constraints on order
  lt!ELEMENT person (name,phone)gt
• references cannot be constrained
• can be too vague
• lt!ELEMENT person ((namephoneemail))gt

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XML Storage

• text file (XML)
• store in ternary relation
• use DTD to derive schema
• mine data to derive schema
• build special purpose repository (Lore)

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XML Storage Text File

• advantages
• simple
• less space than one thinks
• reasonable clustering
• disadvantage
• no updates
• require special purpose query processor

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Store XML in Ternary Relation
o1
paper
o2
year
title
author
author
o3
o4
o5
o6


1986
Florescu, Kossman 1999
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Use DTD to derive Schema

• DTD
• ODMG classes
• Christophides et al. 1994 , Shanmugasundaram et
  al. 1999

lt!ELEMENT employee (name, address,
project)gt lt!ELEMENT address (street, city,
state, zip)gt
class Employee public type tuple (namestring,
addressAddress, projectList(Project)) class
Address public type tuple (streetstring, )
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Mine Data to Derive Schema
Deutsch et al. 1999
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XML and Databases (1)

• Is XML a database?

• In a strict sense, no.
• In a more liberal sense, yes, but
• XML has
• Storage (the XML document)
• A schema (DTD)
• Query languages (XQL, XML-QL, )
• Programming interfaces (SAX, DOM)
• XML lacks
• Efficient storage, indexes, security,
  transactions, multi-user access, triggers,
  queries across multiple documents

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XML and Databases (2)

• Data versus Documents
• There are two ways to use XML in a database
  environment
• Use XML as a data transport, i.e., to get data in
  and out of the database
• Data is stored in a relational or object-oriented
  database
• Middleware converts between the database and XML
• Use a native XML database, i.e., store data in
  document form
• Use a content management system

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XML and Databases (3)

• Data-centric documents
• Fairly regular structure
• Fine-grained data
• Little or no mixed content
• Order of sibling elements often not significant
• Document-centric documents
• Irregular structure
• Larger-grained data
• Lots of mixed content
• Order of sibling elements is significant

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XML and Databases (4)

• Data-centric storage and retrieval systems
• Use a database
• Add middleware to convert to/from XML
• Use an XML server (specialized product for
  e-commerce)
• Use an XML-enabled web server with a database
  backend
• Document-centric storage and retrieval systems
• Content management system
• Persistent DOM implementation

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XML and Databases (5)

• Mapping document structure to database structure
• Template-driven
• No predefined mapping
• Embedded commands process (retrieve) data
• Currently only available from RDBMS to XML
• lt?xml version1.0gtltFlightInfogt ltIntrogtThe
  following flights have available
  seatslt/Introgt ltSelectStmtgtSELECT Airline,
  FltNumber, Depart, Arrive FROM
  Flightslt/SelectStmtgt ltConcludegtWe hope one of
  these meets your needslt/Concludegtlt/FlightInfogt

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XML and Databases (6)

• Template-driven - Example result
• lt?xml version1.0gtltFlightInfogt ltIntrogtThe
  following flights have available
  seatslt/Introgt ltFlightsgt ltRowgt
  ltAirlinegtACMElt/Airlinegt ltFltNumbergt123lt/FltN
  umbergt ltDepartgtDec 12, 2000,
  1343lt/Departgt ltArrivegtDec 13, 2000,
  0121lt/Arrivegt lt/Rowgt lt/Flightsgt
  ltConcludegtWe hope one of these meets your
  needslt/Concludegtlt/FlightInfogt

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XML and Databases (7)

• Mapping document structure to database structure
• Model-driven
• A data model is imposed on the structure of the
  XML document
• This model is mapped to the structures in the
  database
• There are two common models
• Model the XML document as a single table or a set
  of tables
• Model the XML document as a tree of data-specific
  objects (good for OODBMS mapping)

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XML and Databases (8)

• Single table or set of tables
• lt?xml version1.0gtltdatabasegt lttablegt
  ltrowgt ltcolumn1gt...lt/column1gt
  ltcolumn2gt...lt/column2gt ... lt/rowgt
  lt/tablegtlt/databasegt

• Tree organization
• Orders SalesOrder
  / \Customer Item Item
  
• Part Part

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XML and Databases (9)

• Generating DTDs from a database schema and vice
  versa
• Many times the DTD does not change often for an
  application and does not need to be automatically
  generated.
• Some simple conversions are possible
• Example DTD from relational schema
• For each table, create an ELEMENT.
• For each column in a table, create an attribute
  or a PCDATA-only child ELEMENT.
• For each primary key/foreign key relationship
  in which a column of the table contributes the
  primary key, create a child ELEMENT.

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XML and Databases (10)

• Document-centric storage and retrieval systems
• Content management system
• Allows the storage of discrete content fragments,
  such as examples, procedures, chapters, as well
  as metadata such as author names, revision dates,
  etc.
• Many content management systems are built on top
  of relational or object-oriented database
  systems.
• Examples
• BladeRunner (Interleaf), SigmaLink (STEP),
  Parlance Content Manager (XyEnterprise),Target
  2000 (Progressive Information Technology)
• Persistent DOM implementation

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Further Readings
www. w3.org/XML www-db.stanford.edu/widom www-roc
q.inria.fr/abiteboul db.cis.upenn.edu www.researc
h.att.com/suciu Abiteboul, Buneman, Suciu Data
on the Web From Relational to Semistructured to
XML Morgan Kaufmann, 1999 (appears in October)