Introduction to XML
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Introduction to XML
Leonidas Fegaras
CSE 6331
© Leonidas Fegaras
XML
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Traditional DB Applications
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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
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© Leonidas Fegaras
XML
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Document Applications
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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, <p> for new paragraph
Operations: retrieving, editing, spell-checking, printing, etc
Information retrieval: keyword queries
– most successful in web search engines (eg, Google)
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© Leonidas Fegaras
XML
3
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 user’s trust for business-to-consumer
transactions.
Internet users choose speed and availability over correctness
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© Leonidas Fegaras
XML
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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
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Sales and marketing
Education and training
Web services
Communities
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© Leonidas Fegaras
XML
5
Other Web Applications
• Web services
– Many standards: SOAP, WSDL, UDDI
• Web integration
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Heterogeneous data sources and types
Thousands of web-accessible data sources
Dynamic data
Data warehouses
• Web publishing
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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
CSE 6331
© Leonidas Fegaras
XML
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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)
CSE 6331
© Leonidas Fegaras
XML
7
HTML
<html>
<head><title>My Web Page</title></head>
<body>
hypertext link
opening tag
<h1>Introduction</h1>
Look at <a href=”http://lambda.uta.edu/index.html”>this document</a>
closing tag
<img src=”image.jpg” width=100 height=50>
</body>
</html>
attribute name
attribute value
• 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
CSE 6331
© Leonidas Fegaras
XML
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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
CSE 6331
© Leonidas Fegaras
XML
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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
CSE 6331
© Leonidas Fegaras
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Example
<people>
<person>
<name> Leonidas Fegaras </name>
<tel> (817) 272-3629 </tel>
<email> fegaras@cse.uta.edu </email>
</person>
<person>
<name> Ramez Elmasri </name>
<tel> (817) 272-2348 </tel>
<email> elmasri@cse.uta.edu </email>
</person>
</people>
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© Leonidas Fegaras
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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
CSE 6331
© Leonidas Fegaras
XML
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What XML has to do with Databases?
• XML is an important standardization for data representation and
exchange, but still needs
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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
CSE 6331
© Leonidas Fegaras
XML
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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 ::= '<' Name Attributes '>'
Etag ::= '</' Name '>'
Pi ::= '<?' char* '?>'
Attributes ::= ( Name '=' String )*
String ::= '"' char* '"'
• Tags come in pairs <date>8/25/2004</date> and must be properly nested:
<person> <name> ... </name> ... </person> --- valid nesting
<person> <name> ... </person> ... </name> --- invalid nesting
• Text is bounded by tags. PCDATA: parsed character data. eg,
<title> The Big Sleep </title>
<year> 1935 </ year>
CSE 6331
© Leonidas Fegaras
XML
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XML Elements
• An element is a segment of an XML document between an opening
and the matching closing tags
<person>
<name> Ramez Elmasri </name>
<tel> (817) 272-2348 </tel>
<email> elmasri@cse.uta.edu </email>
</person>
• An element may contain a mixture of sub-elements and PCDATA
<title>An <em>element</em> is a segment</title>
• An abbreviation: for an element with empty content, we can use:
<tagname ... />
instead of:
<tagname ...></tagname>
CSE 6331
© Leonidas Fegaras
XML
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Representing Data Using XML
●
Nesting tags can be used to express various structures, such as a
record:
<person>
<name> Ramez Elmasri </name>
<tel> (817) 272-2348 </tel>
<email> elmasri@cse.uta.edu </email>
</person>
• We can represent a list by using the same tag repeatedly:
<addresses>
<person> ... </person>
<person> ... </person>
<person> ... </person>
...
</addresses>
CSE 6331
© Leonidas Fegaras
XML
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XML structure
XML:
<person>
<name> Ramez Elmasri </name>
<tel> (817) 272-2348 </tel>
<email> elmasri@cse.uta.edu </email>
</person>
is Lisp-like:
(person (name “Ramez Elmasri”)
(tel “(817) 272-2348”)
(email “elmasri@cse.uta.edu”))
and tree-like:
person
name
Ramez Elmasri
CSE 6331
© Leonidas Fegaras
XML
tel
(817) 272-2348
email
elmasri@cse.uta.edu
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Attributes
• An opening tag may contain attributes
– typically used to describe the content of an element
<author ssn="2787901">
<name>Ramez Elmasri</name>
<email> elmasri@cse.uta.edu </email>
</author>
• It's not always clear when to use attributes
<author>
<ssn>2787901</ssn>
<name>Ramez Elmasri</name>
<email> elmasri@cse.uta.edu </email>
</author>
• ID attributes are special: must be unique within the document
• An IDref attribute must refer to an existing ID in the same doc
CSE 6331
© Leonidas Fegaras
XML
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Referencing Elements Using IDs/IDrefs
<family>
<person id="jane" mother="mary" father="john">
<name> Jane Doe </name>
</person>
<person id="john" children="jane jack">
<name> John Doe </name> <mother/>
</person>
<person id="mary" children="jane jack">
<name> Mary Doe </name>
</person>
<person id="jack" mother=”mary" father="john">
<name> Jack Doe </name>
</person>
</family>
CSE 6331
© Leonidas Fegaras
XML
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A Complete Example
<?xml version="1.0"?>
<!DOCTYPE bib SYSTEM "bib.dtd">
<bib>
<vendor id="id0_1">
<name>Amazon</name>
<email>webmaster@amazon.com</email>
<phone>1-800-555-9999</phone>
<book>
<title>Unix Network Programming</title>
<publisher>Addison Wesley</publisher>
<year>1995</year>
<author>
<firstname>Richard</firstname>
<lastname>Stevens</lastname>
</author>
<price>38.68</price>
</book>
<book>
<title>An Introduction to Object-Oriented Design</title>
<publisher>Addison Wesley</publisher>
<year>1996</year>
<author>
<firstname>Jo</firstname>
<lastname>Levin</lastname>
</author>
<author>
<firstname>Harold</firstname>
<lastname>Perry</lastname>
</author>
<price>11.55</price>
</book>
</vendor>
</bib>
CSE 6331
© Leonidas Fegaras
XML
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OODB Schema
class Movie
( extent Movies, key title )
class Actor
( extent Actors, key name )
{
{
attribute string title;
attribute string director;
relationship set<Actor> casts
inverse Actor::acted_In;
attribute int budget;
};
CSE 6331
attribute string name;
relationship set<Movie> acted_In
inverse Movie::casts;
attribute int age;
attribute set<string> directed;
};
© Leonidas Fegaras
XML
21
In XML …
<db>
<movie id=“m1” casts=“a1 a3”>
<title>Waking Ned Divine</title>
<director>Kirk Jones III</director>
<budget>100,000</budget>
</movie>
<movie id=“m2” casts=“a2 a9 a21”>
<title>Dragonheart</title>
<director>Rob Cohen</director>
<budget>110,000</budget>
</movie>
<movie id=“m3” casts=“a1 a8”>
<title>Moondance</title>
<director>Dagmar Hirtz</director>
<budget>90,000</budget>
</movie>
CSE 6331
© Leonidas Fegaras
XML
<actor id=“a1” acted_in=“m1 m3 m78”>
<name>David Kelly</name>
<age>55</age>
</actor>
<actor id=“a2” acted_in=“m2 m9 m11”>
<name>Sean Connery</name>
<age>68</age>
</actor>
<actor id=“a3” acted_in=“m1 m35”>
<name>Ian Bannen</name>
<age>45</age>
</actor>
:
</db>
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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
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CSE 6331
firstname
book*
year?
firstname,lastname
book | journal
© Leonidas Fegaras
XML
an element with tag name firstname
zero or more books
an optional year
a firstname followed by lastname
either a book or a journal
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Example of XML Data
<bib>
<vendor id="id0_1">
<name>Amazon</name>
<email>webmaster@amazon.com</email>
<phone>1-800-555-9999</phone>
<book>
<title>Unix Network Programming</title>
<publisher>Addison Wesley</publisher>
<year>1995</year>
<author>
<firstname>Richard</firstname>
<lastname>Stevens</lastname>
</author>
<price>38.68</price>
</book>
...
</vendor>
</bib>
CSE 6331
© Leonidas Fegaras
XML
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DTD Example
<?xml encoding="ISO-8859-1"?>
<!ELEMENT bib (vendor)*>
<!ELEMENT vendor (name, email, book*)>
<!ATTLIST vendor id ID #REQUIRED>
<!ELEMENT book (title, publisher?, year?, author+, price)>
<!ELEMENT author (firstname?, lastname)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT email (#PCDATA)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT publisher (#PCDATA)>
<!ELEMENT year (#PCDATA)>
<!ELEMENT firstname (#PCDATA)>
<!ELEMENT lastname (#PCDATA)>
<!ELEMENT price (#PCDATA)>
CSE 6331
© Leonidas Fegaras
XML
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Summary of the DTD Syntax
• A tagged element in a DTD is defined by
<!ELEMENT name e>
where e is a DTD expression
• If e, e1, e2 are DTD expressions, then so are:
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EMPTY
#PCDATA
A
e1,e2
e1 | e2
e*
e+
e?
(e)
empty content
any text
an element with tag name A
e1 followed by e2
either e1 or e2
zero or more occurrences of e
one or more occurrences of e
optional e (zero or one occurrences)
• Note: tagged elements are global
– must be defined once in a DTD
CSE 6331
© Leonidas Fegaras
XML
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DTD Syntax (cont.)
• Attribute specification:
<!ATTLIST name (attribute-name type accuracy?)+>
type is:
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ID
IDREF
IDREFS
CDATA
must be unique within the document
a reference to an existing ID
multiple IDREFs
any string
accuracy is #REQUIRED, #IMPLIED, #FIXED 'value', value 'v1 ... vn'
• ID, IDref, and IDrefs attributes are not typed!
• Example:
<!ELEMENT person (#PCDATA)>
<!ATTLIST person
id ID #REQUIRED
children IDrefs #IMPLIED >
the id attribute is required while the children attribute is optional
CSE 6331
© Leonidas Fegaras
XML
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Connecting an XML document to a DTD
• In-line the DTD into the XML file:
<?xml version=”1.0”?>
<!DOCTYPE db [
<!ELEMENT person ...>
...
]>
<db>
<person> ... </person>
...
</db>
DTD
XML data
• Better: put the DTD in a separate file and reference it by URL:
<!DOCTYPE db SYSTEM “http://lambda.uta.edu/person.dtd”>
• Documents are validated against their DTD before they are used
CSE 6331
© Leonidas Fegaras
XML
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Recursive DTDs
We want to capture a person with a mother and a father
• First attempt:
<!ELEMENT person (name, address, person, person)>
where the first person is the mother while the second is the father
• Second attempt:
<!ELEMENT person (name, address, person?, person?)>
• Third attempt:
<!ELEMENT person (name, address)>
<!ATTLIST person
id ID #REQUIRED
mother IDREF #IMPLIED
father IDREF #IMPLIED>
CSE 6331
© Leonidas Fegaras
XML
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Back to the OODB Schema
class Movie
( extent Movies, key title )
class Actor
( extent Actors, key name )
{
{
attribute string title;
attribute string director;
relationship set<Actor> casts
inverse Actor::acted_In;
attribute int budget;
};
CSE 6331
attribute string name;
relationship set<Movie> acted_In
inverse Movie::casts;
attribute int age;
attribute set<string> directed;
};
© Leonidas Fegaras
XML
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DTD
<!ELEMENT db (movie+, actor+)>
<!ELEMENT movie (title, director, budget)>
<!ATTLIST movie id ID #REQUIRED
casts IDREFS #REQUIRED>
<!ELEMENT title (#PCDATA)>
<!ELEMENT director (#PCDATA)>
<!ELEMENT budget (#PCDATA)>
<!ELEMENT actor (name, age, directed*)>
<!ATTLIST actor id ID #REQUIRED
acted_in IDREFS #REQUIRED>
<!ELEMENT name (#PCDATA)>
<!ELEMENT age (#PCDATA)>
<!ELEMENT directed (#PCDATA)>
CSE 6331
© Leonidas Fegaras
XML
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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: bib:author
• 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:
xmlns:bib=“http://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
xmlns=“URL”
If not defined, it is the global namespace
CSE 6331
© Leonidas Fegaras
XML
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Example
<item xmlns=“http://www.acme.com/jp#supplies”
xmlns:toy= “http://www.acme.com/jp#toys”>
<name>backpack</name>
<feature>
<toy:item>
<toy:name>cyberpet</toy:name>
</toy:item>
</feature>
</item>
CSE 6331
© Leonidas Fegaras
XML
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Query Languages for XML
• Need a language for XML data for
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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
CSE 6331
© Leonidas Fegaras
XML
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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 (which is the preorder of the
XML tree: every node occurs before its children)
• Main construct: axis navigation
• Consists of one or more navigation steps separated by /
• A navigation step is a triplet
axis :: node-test predicate*
• Each navigation path is evaluated relative to a context node
• Examples:
– child::bib /descendant::author
– descendant::book [ child::author/child::name = “Smith” ] /child::title
• Most people use shorthands
– bib//author
– //book[author/name=“Smith”]/title
CSE 6331
© Leonidas Fegaras
XML
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Axis
• Forward Axis
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child
descendant
attribute
self
descendant-or-self
following-sibling
following
• Reverse Axis
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–
–
–
CSE 6331
parent
ancestor
preceding-sibling
preceding
ancestor-or-self
© Leonidas Fegaras
XML
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Node Test
•
•
•
•
•
•
•
•
•
person
*
@price
@*
node()
text()
element()
element(person)
element(person, surgeon)
• element(*, surgeon)
• attribute()
• attribute(price)
• attribute(*, xs:decimal)
CSE 6331
© Leonidas Fegaras
XML
any element node whose name is person
any element node regardless of its name
any attribute whose name is price
any attribute, regardless of its name
any node
any text node
any element node
any element node whose name is person
any element node whose name is person, and
whose type annotation is surgeon
any element node whose type annotation is
surgeon, regardless of its name
any attribute node
any attribute whose name is price
any attribute whose type annotation is xs:decimal,
regardless of its name.
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Abbreviated Syntax
• The attribute axis child:: can be omitted
– section/para is an abbreviation for child::section/child::para,
– section/@id is an abbreviation for child::section/attribute::id
• … unless the axis step contains an attribute test, … then the
default axis is attribute
– section/attribute(id) is short for child::section/attribute::attribute(id)
• The attribute axis attribute:: can be abbreviated by @
– para[@type="warning"] is short for child::para[attribute::type="warning"]
• // is replaced by /descendant-or-self::node()/
– div//para is short for child::div/descendant-or-self::node()/child::para
• .. is short for parent::node()
– ../title is short for parent::node()/child::title
CSE 6331
© Leonidas Fegaras
XML
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Most Common Steps
XPath step
/author
/*
//book
//*
/@ssn
//@ssn
/..
/text()
new context nodes
all the children of a context node with tagname author
all the children of the context node
the context node and all its descendants with tagname book
the context node and all its descendants
the attribute value of the attribute name ssn of the context node
like /@ssn but for all descendants
parent of context node
the text of the context node
• Examples:
– /book/chapter/section
– //chapter/*
– //book/author/@*
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© Leonidas Fegaras
XML
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Example
<a>
For example, /a/b
returns nodes 2
and 4:
<b>
a
1
<c>text1</c>
<b>text2</b>
2
</b>
b
3
d
4
b
<b>
<d>
<c>text1</c>
<c>text3</c>
</d>
c
b
c
d
5
6
7
8
<b>
</b>
<b>
/./a
/./a./b
/a/c
/a/*/c
//b
//b/c
/a//c
<d>text4</d>
</b>
</a>
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<b>text2</b>
© Leonidas Fegaras
XML
or /a --> [1]
or /a/b --> [2,4]
--> []
--> [5,7]
--> [2,6,4]
--> [5]
--> [5,7]
<d>text4</d>
</b>
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Predicates
• Many variations
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–
–
–
–
//book[10]
the tenth child node of the context node (tenth book)
//book[last()]
the last child node of the context node (last book)
//book[author]
true, if the book has at least one author
//book[author/name]
true, if there is an author/name in the book
//book[author/name=“Smith”]
true if the author name is Smith
• Examples
/bib/book[@price < 100]/title
/bib/book[author/text()]
//author[name/firstname=“John”][name/lastname=“Smith”]/title
/bib/book/author[name/firstname][address[zip=1234][city]]/name/lastname
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© Leonidas Fegaras
XML
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