Introduction to XML

1
Introduction to XML

• Leonidas Fegaras

2
Traditional DB Applications

• Typically business oriented
• Large amount of data
• Data is well-structured, normalized, with
  predefined schema
• Large number of concurrent users (transactions)
• Simple data, simple queries, and simple updates
• Typically update intensive
• Small transactions
• High performance, high availability, scalability
• Data integrity and security are of major
  importance
• Good administrative support, nice GUIs

3
Document Applications

• Human friendly what-you-see-is-what-you-get
  paradigm
• Focus on presentation
• Information is divided into multiple small
  documents
• Mostly static
• Implicit structure section, subsection,
  paragraph, etc
• Meta-data title, author, date, indexing
  keywords, etc
• Content structure form/layout,
  inter-relationships, references
• Tagging eg, ltpgt for new paragraph
• Operations retrieving, editing, spell-checking,
  printing, etc
• Information retrieval keyword queries
• most successful in web search engines (eg, Google)

4
Internet Applications

• Internet applications
• use heterogeneous, complex, hierarchical,
  fast-evolving, unstructured/semistructured data
• access mostly read-only data
• need 100 availability
• manage millions of users world-wide
• have high-performance requirements
• are concerned with security (encryption)
• like to customize data in a personalized manner
• expect to gain users trust for
  business-to-consumer transactions.
• Internet users choose speed and availability over
  correctness

5
Electronic Commerce

• Currently, mostly business-to-business (B2B)
  rather than business-to-consumer (B2C)
  interactions
• Focus on selling and buying
• Order management
• Product catalogs
• Product configuration
• Sales and marketing
• Education and training
• Web services
• Communities

6
Other Web Applications

• Web services
• Many standards SOAP, WSDL, UDDI
• Web integration
• Heterogeneous data sources and types
• Thousands of web-accessible data sources
• Dynamic data
• Data warehouses
• Web publishing
• Access different types of content from browsers
  (PDF, HTML, XML)
• Structured, dynamic, customized/personalized
  content
• Integration with application
• Accessible via major gateways and search engines
• Application integration
• Transformation between different data formats
  (eg, XML, HTML)
• Integration of multiple applications

7
Current Internet Application Architectures

• Architecture
• Server-Tier relational databases and gateways to
  diverse data sources, such as, files, OLE/DB etc.
  Use of enterprise servers
• Middle-Tier provides data integration
  distribution, query, etc. Consists of a web
  server and an application server
• Client-Tier mostly a web browser, may use CGI
  scripts or Java
• Characteristics
• Customization is achieved at the server site
  (customer data in a database) with some data at
  the client site (cookies)
• Load balancing is typically hardware based
  (multiple servers, DNS routers)

8
HTML

• lthtmlgt
• ltheadgtlttitlegtMy Web Pagelt/titlegtlt/headgt
• ltbodygt
• lth1gtIntroductionlt/h1gt
• Look at lta hrefhttp//lambda.uta.edu/index.ht
  mlgtthis documentlt/agt
• ltimg srcimage.jpg width100 height50gt
• lt/bodygt
• lt/htmlgt
• It is very simple human readable, can be edited
  by any editor
• It reflects document presentation, not the
  semantics or structure of data
• Universal portable to any platform
• HTML pages are connected through hypertext links
• HTML pages can be located using web search engines

hypertext link
opening tag
closing tag
attribute name
attribute value
9
XML

• XML (eXtensible Markup Language) is a textual
  language for representing and exchanging data on
  the web
• It is designed to improve the functionality of
  the Web by providing more flexible and adaptable
  information identification
• Based on SGML
• It was developed around 1996
• It is called extensible because
• it is not a fixed format like HTML (a single,
  predefined markup language)
• it is actually a metalanguage (a language for
  describing other languages) which lets you design
  your own customized markup languages for
  limitless different types of documents

10
XML (cont.)

• XML can be untyped (semistructured), but there
  are standards now for schema conformance
• DTD
• XML Schema
• Without schema, an XML document is well-formed if
  it satisfies simple syntactic constraints
• proper nesting of start and end tags
• With a schema, an XML document is valid if its
  structure conforms to a DTD or an XML Schema

11
Example

• ltpeoplegt
• ltpersongt
• ltnamegt Leonidas Fegaras lt/namegt
• lttelgt [phone] lt/telgt
• ltemailgt fegaras_at_cse.uta.edu lt/emailgt
• lt/persongt
• ltpersongt
• ltnamegt Ramez Elmasri lt/namegt
• lttelgt [phone] lt/telgt
• ltemailgt elmasri_at_cse.uta.edu lt/emailgt
• lt/persongt
• lt/peoplegt

12
Why XML is so Popular?

• It looks like HTML
• simple, human-readable, easy to learn, universal
• Flexible extensible, since you can represent
  any kind of data
• unlike HTML
• HTML describes the presentation while XML
  describes the content
• Precise
• well-formed properly nested XML tags
• valid its structure may conform to a DTD or an
  XML Schema
• Supported by the W3C
• trusted and adopted by industry
• Many standards around XML schemas, query
  languages, etc

13
What XML has to do with Databases?

• XML is an important standardization for data
  representation and exchange, but still needs
• to store and query large repositories of XML
  documents
• data models and schema representations
• query languages, data indexing, query optimizers
• updates, view maintenance
• concurrency, distribution, security, etc
• Example application
• an XML data repository distributed in a
  peer-to-peer network
• answer queries, such as
• find all books whose author is Smith and whose
  title contains the word Web
• much like a web search engine, but for XML, ...
  and for more precise querying

14
XML Syntax

• XML consists of tags and text
• XML documents conform to the following grammar
• XMLdocument Pi Element Pi
• Element Stag (char Pi Element) Etag
• Stag 'lt' Name Attributes 'gt'
• Etag 'lt/' Name 'gt'
• Pi 'lt?' char '?gt'
• Attributes ( Name '' String )
• String '"' char '"'
• Tags come in pairs ltdategt8/25/2004lt/dategt and
  must be properly nested
• ltpersongt ltnamegt ... lt/namegt ... lt/persongt ---
  valid nesting
• ltpersongt ltnamegt ... lt/persongt ... lt/namegt ---
  invalid nesting
• Text is bounded by tags. PCDATA parsed character
  data. eg,
• lttitlegt The Big Sleep lt/titlegt
• ltyeargt 1935 lt/ yeargt

15
XML Elements

• An element is a segment of an XML document
  between an opening and the matching closing tags
• ltpersongt
• ltnamegt Ramez Elmasri lt/namegt
• lttelgt [phone] lt/telgt
• ltemailgt elmasri_at_cse.uta.edu lt/emailgt
• lt/persongt
• An element may contain a mixture of sub-elements
  and PCDATA
• lttitlegtAn ltemgtelementlt/emgt is a segmentlt/titlegt
• An abbreviation for an element with empty
  content, we can use
• lttagname ... /gt
• instead of
• lttagname ...gtlt/tagnamegt

16
Representing Data Using XML

• Nesting tags can be used to express various
  structures, such as a record
• ltpersongt
• ltnamegt Ramez Elmasri lt/namegt
• lttelgt [phone] lt/telgt
• ltemailgt elmasri_at_cse.uta.edu lt/emailgt
• lt/persongt
• We can represent a list by using the same tag
  repeatedly
• ltaddressesgt
• ltpersongt ... lt/persongt
• ltpersongt ... lt/persongt
• ltpersongt ... lt/persongt
• ...
• lt/addressesgt

17
XML structure

• XML
• ltpersongt
• ltnamegt Ramez Elmasri lt/namegt
• lttelgt [phone] lt/telgt
• ltemailgt elmasri_at_cse.uta.edu lt/emailgt
• lt/persongt
• is Lisp-like
• (person (name Ramez Elmasri)
• (tel [phone])
• (email elmasri_at_cse.uta.edu))
• and tree-like

person
name
tel
email
Ramez Elmasri
[phone]
elmasri_at_cse.uta.edu
18
Attributes

• An opening tag may contain attributes
• typically used to describe the content of an
  element
• ltauthor ssn"2787901"gt
• ltnamegtRamez Elmasrilt/namegt
• ltemailgt elmasri_at_cse.uta.edu lt/emailgt
• lt/authorgt
• It's not always clear when to use attributes
• ltauthorgt
• ltssngt2787901lt/ssngt
• ltnamegtRamez Elmasrilt/namegt
• ltemailgt elmasri_at_cse.uta.edu lt/emailgt
• lt/authorgt
• ID attributes are special must be unique within
  the document
• An IDref attribute must refer to an existing ID
  in the same doc

19
Referencing Elements Using IDs/IDrefs

• ltfamilygt
• ltperson id"jane" mother"mary"
  father"john"gt
• ltnamegt Jane Doe lt/namegt
• lt/persongt
• ltperson id"john" children"jane jack"gt
• ltnamegt John Doe lt/namegt ltmother/gt
• lt/persongt
• ltperson id"mary" children"jane jack"gt
• ltnamegt Mary Doe lt/namegt
• lt/persongt
• ltperson id"jack" mothermary"
  father"john"gt
• ltnamegt Jack Doe lt/namegt
• lt/persongt
• lt/familygt

20
A Complete Example

• lt?xml version"1.0"?gt
• lt!DOCTYPE bib SYSTEM "bib.dtd"gt
• ltbibgt
• ltvendor id"id0_1"gt
• ltnamegtAmazonlt/namegt
• ltemailgtwebmaster_at_amazon.comlt/emailgt
• ltphonegt1-800-555-9999lt/phonegt
• ltbookgt
• lttitlegtUnix Network Programminglt/titlegt
• ltpublishergtAddison Wesleylt/publishergt
• ltyeargt1995lt/yeargt
• ltauthorgt
• ltfirstnamegtRichardlt/firstnamegt
• ltlastnamegtStevenslt/lastnamegt
• lt/authorgt
• ltpricegt38.68lt/pricegt
• lt/bookgt
• ltbookgt
• lttitlegtAn Introduction to
  Object-Oriented Designlt/titlegt

21
OODB Schema

• class Movie
• ( extent Movies, key title )
• attribute string title
• attribute string director
• relationship setltActorgt casts
• inverse Actoracted_In
• attribute int budget

class Actor ( extent Actors, key name
) attribute string name relationship
setltMoviegt acted_In inverse
Moviecasts attribute int age
attribute setltstringgt directed
22
In XML
ltdbgt ltmovie idm1gt lttitlegtWaking Ned
Divinelt/titlegt ltdirectorgtKirk Jones
IIIlt/directorgt ltcast idrefsa1
a3gtlt/castgt ltbudgetgt100,000lt/budgetgt
lt/moviegt ltmovie idm2gt
lttitlegtDragonheartlt/titlegt ltdirectorgtRob
Cohenlt/directorgt ltcast idrefsa2 a9
a21gtlt/castgt ltbudgetgt110,000lt/budgetgt
lt/moviegt ltmovie idm3gt
lttitlegtMoondancelt/titlegt ltdirectorgtDagmar
Hirtzlt/directorgt ltcast idrefsa1
a8gtlt/castgt ltbudgetgt90,000lt/budgetgt
lt/moviegt
ltactor ida1gt ltnamegtDavid
Kellylt/namegt ltacted_In idrefsm1 m3 m78
gt lt/acted_Ingt lt/actorgt ltactor
ida2gt ltnamegtSean Connerylt/namegt
ltacted_In idrefsm2 m9 m11gt lt/acted_Ingt
ltagegt68lt/agegt lt/actorgt ltactor
ida3gt ltnamegtIan Bannenlt/namegt
ltacted_In idrefsm1 m35gt lt/acted_Ingt
lt/actorgt lt/dbgt
23
DTD Document Type Descriptor

• A DTD imposes a structure on an XML document
• Not quite a typing system
• it is purely syntactic
• now replaced by XML Schema
• Uses regular expressions to specify structure
• firstname an element with tag name firstname
• book zero or more books
• year? an optional year
• firstname,lastname a firstname followed by
  lastname
• book journal either a book or a journal

24
Example of XML Data

• ltbibgt
• ltvendor id"id0_1"gt
• ltnamegtAmazonlt/namegt
• ltemailgtwebmaster_at_amazon.comlt/emailgt
• ltphonegt1-800-555-9999lt/phonegt
• ltbookgt
• lttitlegtUnix Network Programminglt/titlegt
• ltpublishergtAddison Wesleylt/publishergt
• ltyeargt1995lt/yeargt
• ltauthorgt
• ltfirstnamegtRichardlt/firstnamegt
• ltlastnamegtStevenslt/lastnamegt
• lt/authorgt
• ltpricegt38.68lt/pricegt
• lt/bookgt
• ...
• lt/vendorgt
• lt/bibgt

25
DTD Example

• lt?xml encoding"ISO-8859-1"?gt
• lt!ELEMENT bib (vendor)gt
• lt!ELEMENT vendor (name, email, book)gt
• lt!ATTLIST vendor id ID REQUIREDgt
• lt!ELEMENT book (title, publisher?, year?,
  author, price)gt
• lt!ELEMENT author (firstname?, lastname)gt
• lt!ELEMENT name (PCDATA)gt
• lt!ELEMENT email (PCDATA)gt
• lt!ELEMENT title (PCDATA)gt
• lt!ELEMENT publisher (PCDATA)gt
• lt!ELEMENT year (PCDATA)gt
• lt!ELEMENT firstname (PCDATA)gt
• lt!ELEMENT lastname (PCDATA)gt
• lt!ELEMENT price (PCDATA)gt

26
Summary of the DTD Syntax

• A tagged element in a DTD is defined by
• lt!ELEMENT name egt
• where e is a DTD expression
• If e, e1, e2 are DTD expressions, then so are
• EMPTY empty content
• PCDATA any text
• A an element with tag name A
• e1,e2 e1 followed by e2
• e1 e2 either e1 or e2
• e zero or more occurrences of e
• e one or more occurrences of e
• e? optional e (zero or one occurrences)
• (e)
• Note tagged elements are global
• must be defined once in a DTD

27
DTD Syntax (cont.)

• Attribute specification
• lt!ATTLIST name (attribute-name type accuracy?)gt
• type is
• ID must be unique within the document
• IDREF a reference to an existing ID
• IDREFS multiple IDREFs
• CDATA any string
• accuracy is REQUIRED, IMPLIED, FIXED 'value',
  value 'v1 ... vn'
• ID, IDref, and IDrefs attributes are not typed!
• Example
• lt!ELEMENT person (PCDATA)gt
• lt!ATTLIST person
• id ID REQUIRED
• children IDrefs IMPLIED gt
• the id attribute is required while the children
  attribute is optional

28
Connecting an XML document to a DTD

• In-line the DTD into the XML file
• lt?xml version1.0?gt
• lt!DOCTYPE db
• lt!ELEMENT person ...gt
• ...
• gt
• ltdbgt
• ltpersongt ... lt/persongt
• ...
• lt/dbgt
• Better put the DTD in a separate file and
  reference it by URL
• lt!DOCTYPE db SYSTEM http//lambda.uta.edu/person.
  dtdgt
• Documents are validated against their DTD before
  they are used

DTD
XML data
29
Recursive DTDs

• We want to capture a person with a mother and a
  father
• First attempt
• lt!ELEMENT person (name, address, person, person)gt
• where the first person is the mother while the
  second is the father
• Second attempt
• lt!ELEMENT person (name, address, person?,
  person?)gt
• Third attempt
• lt!ELEMENT person (name, address)gt
• lt!ATTLIST person
• id ID REQUIRED
• mother IDREF IMPLIED
• father IDREF IMPLIEDgt

30
Back to the OODB Schema

• class Movie
• ( extent Movies, key title )
• attribute string title
• attribute string director
• relationship setltActorgt casts
• inverse Actoracted_In
• attribute int budget

class Actor ( extent Actors, key name
) attribute string name relationship
setltMoviegt acted_In inverse
Moviecasts attribute int age
attribute setltstringgt directed
31
DTD

• lt!ELEMENT db (movie, actor)gt
• lt!ELEMENT movie (title, director, cast, budget)gt
• lt!ATTLIST movie id ID REQUIREDgt
• lt!ELEMENT title (PCDATA)gt
• lt!ELEMENT director (PCDATA)gt
• lt!ELEMENT cast EMPTYgt
• lt!ATTLIST cast IDREFS REQUIREDgt
• lt!ELEMENT budget (PCDATA)gt
• lt!ELEMENT actor (name, acted_In, age, directed)gt
• lt!ELEMENT name (PCDATA)gt
• lt!ELEMENT acted_In EMPTYgt
• lt!ATTLIST acted_In IDREFS REQUIREDgt
• lt!ELEMENT age (PCDATA)gt
• lt!ELEMENT directed (PCDATA)gt

32
XML Namespaces

• When merging multiple docs together, name
  collisions may occur
• A namespace is a mechanism for uniquely naming
  tagnames and attribute names to avoid name
  conflicts
• Tag/attribute names are now qualified names
  (QNames)
• (namespace '')? localname
• example bibauthor
• A document may use multiple namespaces
• A DTD has its own namespace in which all names
  are unique
• A namespace in an XML doc is defined as an
  attribute
• xmlnsbibhttp//lambda.uta.edu/biblio.dtd
• where bib is the namespace name and the URL is
  the location of the DTD
• The default namespace is defined as
• xmlnsURL
• If not defined, it is the global namespace

33
Example

• ltitem xmlnshttp//www.acme.com/jpsupplies
• xmlnstoy http//www.acme.com/jptoysgt
• ltnamegtbackpacklt/namegt
• ltfeaturegt
• lttoyitemgt
• lttoynamegtcyberpetlt/toynamegt
• lt/toyitemgt
• lt/featuregt
• lt/itemgt

34
Query Languages for XML

• Need a language for XML data for
• extracting fragments (querying)
• restructuring (data transformation)
• integrating (eg, combining multiple XML
  documents)
• browsing
• presentation (eg, from XML to HTML)
• We will first learn XPath
• used in extracting fragments from a single
  document
• many XML query languages are based on XPath
• We will briefly discuss XSLT
• for extracting, restructuring, and presentation
  over a single document
• We will focus later on XQuery
• a full-fledged query language
• much like OQL

35
XPath

• Describes a single navigation path in an XML
  document
• Selects a sequence of nodes reachable by the path
• the order of nodes is the document order
• Main construct axis navigation
• Consists of one or more navigation steps
  separated by /
• A navigation step is a triplet
• axis node-test list-of-predicates
• Each navigation path is evaluated relative to a
  context node
• Examples
• /childbib /descendantauthor
• /descendantbook /childauthor Smith
  /childtitle
• Most people use shorthands
• /bib//author
• //bookauthorSmith/title

36
Axis Navigation

• In the beginning, the context node is the
  document root
• Dot (.) identifies the context node
• Some navigation steps
• / the root node
• // the root node and its descendants
• ./author all the children of the context node
  with tagname author the context node of the
  next step is each of these children
• .// the context node and all its descendants the
  context node of the next step is each of the
  nodes
• _at_mother the attribute value of the attribute name
  mother of the context node
• ./ all the children of the context node
• .. parent of context node
• text() all the text children of the context node
• Shortcut you can remove ./

37
Example

• ltagt
• ltbgt
• ltcgtlt/cgt
• ltbgtlt/bgt
• lt/bgt
• ltdgt
• ltcgtlt/cgt
• lt/dgt
• ltbgt
• ltdgtlt/dgt
• lt/bgt
• lt/agt

a
1
b
d
b
3
2
4
c
d
c
b
7
5
6
8
/./a or /a --gt 1 /./a./b or /a/b --gt
2,4 /a/c --gt /a//c --gt 5,7 //b --gt
2,6,4 //b/c --gt 5 /a//c --gt 5,7
38
Predicates

• Many variations
• 10 the tenth child node of the context node
• last() the last child node of the context node
• author true, if the context node has at least
  one child tagged author
• author/name true, if the XPath ./author/name is
  nonempty
• authornameSmith true if the author name is
  Smith
• Examples
• /bib/book_at_price lt 100/title
• /bib/bookauthor/text()
• authorname/firstnameJohn and
  name/lastnameSmith/title
• /bib/book/authorname/firstnameaddress//zipci
  ty/name/lastname