M ETA XPath

DC2001 Conference - Toyko

METAXPath

Curtis DyresonE.E. and Computer Science Washington State University

USA

Michael Böhlenand

Christian S. JensenComputer ScienceAalborg University

Denmark

Nykredit Center for Database ResearchAalborg University, Denmark


Outline

• Data Data model

XML Query language

XPath

• Metadata METAXPath

• Future work


An XML Database Architecture

XML data and metadata

Database

Client(HTTP browser)

HTTP server


Database Data Model Evolution60s - Hierarchical data model

70s - Network data model

80s - Relational data model

90s - Object-oriented data model

00s - Unstructured/semistructured/XML Innovators

Unstructured data models (UPenn) UnQL/Strudel (AT&T) OEM and Lore (Stanford) XML (W3C)


Object Exchange Model (OEM)• Heterogeneous OODBs

Exchange objects Text description

text (XML)

object 1

object 1

my database your database

object 2


<person id=&1 name=“Joe Doe” age=“25” />

<person id=&1> <name>Joe Doe</name> <age>25</age> </person>

Object Representation in XML• Use names and values• Ignore types• &X denotes object X

// A person classclass Person { String name; int age; }

// A person objectPerson joe = new Person(‘Joe Doe’, 25);

<!ATTLIST person id ID #REQUIRED><!ELEMENT person (name age)>


XML (XPath) Data Model• Each element or attribute is a node

• Edges indicate nesting

• Nodes contain information

• Tree is ordered

age

element

person

element

name=“Joe”

attribute

id=“&1”

attribute

/n

text

25

text

/n

text

root

XML

<person id=&1 name=“Joe”> <age>25</age> </person>

XPath


Semistructured Data Model• Each element or attribute is a node• Edges indicate nesting• Edges are labeled

Joe25

XML Semistructured

&1

person

nameage

<person id=&1 name=“Joe”> <age>25</age> </person>


Data Models Compared• Insensitive to

text order, whitespace attributes vs. elements

• Directed graph (many roots, can contain cycles)

• Captures text order, whitespace, attributes and elements

• A tree (single root, no cycles)

age

element

person

element

name=“Joe”

attribute

id=“&1”

attribute

/n

text

25

text

/n

text

root

Joe25Semistructured

&1

person

nameage

XPath


Outline

• Data Data model

XML Query language

XPath

• Metadata XML - METAXPath

• Future work


XPath

• W3C Recommendation – 1999 Used in XQuery, XSLT, and XPointer Language for selecting locations in an XML document

• Query Sequence of location steps separated by ‘/’ Location step

axis::node_test [predicate1]…[predicateN]

Evaluated with respect to a context node Results in a node-set (actually a list of nodes!) Step continues from nodes reached in previous step


Descendent Axis Example

name

element

person

element

dateOfBirth

element

last

elementmonth

element

year

element

Susan

text

Douglas

text

January

text

1981

text

This…

comment

root

initial=“S”

attribute

SSN=“99…”

attribute

first

element


• Ancestor, descendent, following, preceding, and self partition a tree.

Axes that Partition a Tree

preceding followingdescendent

ancestor

self


XPath Node Test and Predicates

• Each node in result-set must pass node test Is this an element node named person?

person Is this an element node?

*

• Predicates are further tests (about other nodes) Does node have a ssn attribute?

[attribute::ssn]


Example /child::person/child::*/child::last

name

element

person

element

dateOfBirth

element

last

elementmonth

element

year

element

Susan

text

Douglas

text

January

text

1981

text

This…

comment

root

initial=“S”

attribute

SSN=“99…”

attribute

first

element

root

person

element

name

element

This…

comment

dateOfBirth

element

last

element

last

element


XPath Examples

• The dateOfBirth children of person nodes

/descendent::person/child::dateOfBirth

• The last text node

/descendent::text()[position()=last()]


Abbreviated Syntax

• Think of file path specifications in Unix• Year child of dateOfBirth

child::dateOfBirth/child::year

dateOfBirth/year

• name siblings

parent::*/child::name

../name

• All year nodes

/descendent-or-self::*/child::year

//year


Outline

• Data Data model

XML Query language

XPath


• Future work


Metadata

• Database metadata Schema, security, transaction time (versions)

• Web metadata Author, language, subject, privacy

• Web metadata recommendations RDF, RDD, P3P

• Features Descriptive, but also exclusionary Irregular Multiple Ad-hoc


A Movie Database

• Movie data Bruce Willis stars in Colour of Night. Colour of Night premiered 1/Jul/1995.

• Publication meta-data language English

URL http://www.auc.dk

publication date 2/Apr/1997

privacy/security ‘over 18’

publication history v1.2, modified 31/Jul/1998

subject Film, Suspense, Thriller

namespace http://www.auc.dk/movieDataDTD.xml


Movie Database Queries

• Metadata only Retrieve information published at Danish web sites.

• Metadata compared to data Find reviews published in the first week of the movie’s release.

• Metadata and data, but independent Get suspense films starring Bruce Willis.


Properties of a Metadata Data Model

• Goal: Same query language for data and metadata User learns “one” language Compiler/optimization reuse

• Challenges: Data and metadata in different dataspaces Query on data should not accidently query metadata Meta-metadata

Metadata for metadata Metadata has semantics Data with/without metadata


METAXPath Data Model

• Data model Reuse XPath data model Meta attribute points to metadata tree “Right angle” data model

• Features Minimal extension of XPath Backwards-compatible


Example

• Data<?xml version="1.0">

<person ssn="234">

<name>Ichiro</name>

</person>

• URL metadata<source URL=“www.wsu.edu/p.htm”>

• Language metadata of person element<language>English</language>

• Author meta-metadata - language metadata author<author name="Suzuki"/>

Type element

Value person

Attributes {(ssn, 223)}

Type element

Value name

Attributes {}

Type text

Value Ichiro

Type root

Type text

Value \n

Type text

Value \n\t

<?xml version="1.0"><person ssn="234"> <name>Ichiro</name></person>

Type element

Value name

Attributes {}

Type text

Value \n

Type text

Value \n\t

Type element

Value person


Type text

Value Ichiro

Type root

Meta

Type element

Value source

Attributes {(URL, www.wsu.edu/p.htm)}

Type root

<source URL=“www.wsu.edu/p.htm”>

Type element

Value name

Attributes {}

Type text

Value \n

Type text

Value \n\t

Type element

Value person


Meta

Type element

Value language

Attributes {}

Type text

Value English

Type text

Value Ichiro

Type root

Meta

Type element

Value source


Type root

Type root

<language>English</language>

Type element

Value name

Attributes {}

Type text

Value \n

Type text

Value \n\t

Type element

Value person


Meta

Type element

Value language

Attributes {}

Type text

Value English

Type text

Value Ichiro

Type root

Meta

Type element

Value source


Type root

Type root

Meta Type root

Type element

Value author

Attributes {(name, Suzuki)}

<author name="Suzuki"/>


Sharing and Excluding Metadata

• Meta property points to metadata for a node Shared pointers ==> shared metadata

• To share with child Copy pointer

• To exclude from child Duplicate excluded portion Copy remaining shared pointers

Type text

Value Ichiro

Meta

Type element

Value person


Meta

Type text

Value English

Meta

Type root

Meta

Type element

Value source


Type root

Type root

Meta

Type text

Value \n\t

Meta

Type element

Value name

Attributes {}

Meta

Type text

Value \n

Meta

Type root

Type element

Value language

Attributes {}

Meta

Type element

Value author


Share metadata with descendents

Type element

Value person


Meta

Type root

Meta

Type element

Value source


Type root

Type root

Meta

Type text

Value \n\t

Meta

Type element

Value name

Attributes {}

Meta

Type text

Value \n

Meta

Type root

Type root

Meta

Type text

Value English

Meta

Type element

Value language

Attributes {}

Meta

Type element

Value author


Type text

Value Ichiro

Meta

Ichiro text not

authored by

Suzuki


METAXPath Queries

• XPath plus level shift operation meta axis ^ in abbreviated syntax

• Example - Locate data nodes with URL metadata of p.htm /descendent-or-self::*

[meta::*/child::source[attribute::URL="p.htm"]] In abbreviated syntax

//*[^source[@URL="p.htm"]]

• Example - Locate the URL metadata //*^source/@URL

• Example - Locate data that has metadata authored by Suzuki (meta-metadata)//*[^//*^author[@name="Suzuki"]]


Outline

• Data Data model

XML Query language

XPath


• Future work


Metadata Semantics

• Transaction time example

Color of Night

&2

&3

Colour of Night

name: title

trans. time: [1/Aug/1998 - uc]

&1

name: reviewed

trans. time: [1/Sep/1999 - uc]

name: movie

name: title

trans. time: [2/Apr/1997 - 31/Jul/1998]

&1

&2

&3

Not a path!


AUCQL Collapse Example

• PropertyCollapse for name is concatenation, for trans. time it is temporal intersection.

Color of Night

&1

Colour of Night

name: reviewed


&2

&3

name: title

trans. time: [2/Apr/1997 - 31/Jul/1998]

name: title

trans. time: [1/Aug/1998 - uc]

name: movie

name: reviewed.movie.title


name: reviewed.movie.title

trans. time: undefined


AUCQL Additional Operations

• Coalesce - compute a distributed property value

&1

&2

name: review

security! developer

trans. time: [1/Jul/1999 - 15/Jul/1999]

name: review

security! subscriber

trans. time: [16/Jul/1999 - uc]

trans. time: [1/Jul/1999 - uc]


Thin Layer Impementation

METAXPath query

METAXPath CompilerMetadata

encoding

DB

XPath Compiler

XPath query

result


Prototype Implementation

METAXPath query

METAXPath Compiler

DBM

Query Evaluation Engine

Evaluation Tree

result

Database API

Perl

Perl

XML

Parser

XML

RDF

Indexing


Summary

• METAXPath website http://www.eecs.wsu.edu/~cdyreson/pub/MetaXPath

• AUCQL website VLDB ‘99 Implemented research prototype Free, downloadable, Unix environment http://www.eecs.wsu.edu/~cdyreson/pub/AUCQL Interactive query engine Tutorials

M ETA XPath

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