7 september 2005nvo summer school 2005 - aspen1 advanced but ever-so-useful xml technologies:...

7 September 2005NVO Summer School 2005 - Aspen 1

Advanced but ever-so-useful

XML Technologies:Schema, XPath, XQuery, XSL

an incomplete introduction

Ray Plante

THE US NATIONAL VIRTUAL OBSERVATORY


XML Schema• What it is:

– A W3C* standard for defining and verifying an XML grammar• An XML Schema document describes…

– a set of legal XML tags and attributes,– what order they go in, and – what values are allowed.

• An XML Schema-aware XML parser can tell you if an XML document follows the rules of the grammar

*World Wide Web Consortium

• Why you might care:– VO uses XML to encode metadata and service messages– XML Schema is used to define metadata encoding and message

syntax– Ability to read XML Schema will help you understand what metadata

is needed by an app and how to encode it• VO end users and many developers will never need to know about Schema• Can be helpful for debugging XML documents and messages

• What you’ll get from this session:– Rudimentary skills for reading an XML Schema document to discern

the specified XML syntax– Understand the role of namespaces in supporting multiple schemas– A tool for validating an instance document


Schema uses XML to describe syntax

http://www.w3.org/TR/xmlschema-0/

• Contains a list of definitions– Elements and types – Attributes, groups, attribute-groups

• Types– Simple types: string, integer, dateTime, etc.

<title>The Astronomy Digital Image Library</title>

<maxRecords>10000</maxRecords>

<date role=”completed”>2002-09-30T07:45:00</date>

– Complex types: contain other elements– Defining types

• Anonymous type – directly inside the definition of an element• Global type – “top-level” definition, can be reused


Namespaces• Namespace: a schema’s unique identifier

– Set with the targetNamespace attribute– Used by an XML document to indicate which schema it is

compliant with– URI format (URL or URN)

• Using a schema– Instance document: XML document that follows the

grammar defined by the schema– xmlns attribute used to identify the default namespace that

elements belong to– Tagging elements with namespace prefixes

• Prefix defined (anywhere) with xmlns:prefixxmlns:res="http://nvoss.org/Resource"

• Prefixes attached to element/attribute name denotes it belongs to the associated schema

<res:title>…</res:title>

– Unqualified elements: a special technique for namespaces• Tag the root element (or xsi:type) only• Do not use xmlns • Can make using multiple schemas easier


Validating an instance document

• A validating parser can test whether an instance document is compliant with its schema

• xsi:schemaLocation attribute tells parser where to find Schema document(s)– Look-up list made up of namespace-location pairs

xsi:schemaLocation=”namespace file-or-URL …”

• xsi:schemaLocation is just a recommendation– Parser may have local copies cached for more

efficient parsing

validate: a tool for validating XML documents against schemas


Simple schema<?xml version="1.0" encoding="UTF-8"?><xs:schema targetNamespace="http://nvoss.org/VOResource" xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">

<xs:element name="resource"> <xs:complexType> <xs:sequence>

<xs:element name="title" type="xs:string" />

<xs:element name="referenceURL" type="xs:anyURI" minOccurs="0"/>

<xs:element name="type" minOccurs="0" maxOccurs="unbounded"> <xs:simpleType> <xs:restriction base="xs:string"> <xs:enumeration value="Archive" /> <xs:enumeration value="Catalog" /> <xs:enumeration value="Organisation" /> </xs:restriction> </xs:simpleType> </xs:element>

</xs:sequence> <xs:attribute name="created" type="xs:dateTime" /> </xs:complexType> </xs:element></xs:schema>

xmltech-simple.xsd


<?xml version="1.0" encoding="UTF-8"?><xs:schema targetNamespace="http://nvoss.org/VOResource" xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">






Simple schema

targetNamespace

xmltech-simple.xsd


Simple schema

global = Direct child of xs:schema

Only global elements can serve as a document’s root element

globally-defined element







xmltech-simple.xsd


Simple schema

Anonymous type definition







xmltech-simple.xsd


Simple schema

sequence: a list of elements that must appear in order

choice: one from a list of elements may appear

Other models: group, all, any

Content model







xmltech-simple.xsd


Simple schema

title: any string

referenceURL: URI format

type: restricted to a specified list of strings

Locally-defined elements







xmltech-simple.xsd


Simple schema

Default: minOccurs=”1” maxOccurs=”1”

minOccurs=”0”: optional

minOccurs=”1”: required

Occurance restrictions







xmltech-simple.xsd


The compliant instance document

<?xml version="1.0" encoding="UTF-8"?><resource xmlns="http://nvoss.org/VOResource" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://nvoss.org/VOResource xmltech-simple.xsd" >

<title>NCSA Astronomy Digital Image Library</title>

<referenceURL>http://adil.ncsa.uiuc.edu/</referenceURL>

<type>Archive</type>

</resource>

xmltech-simple.xml



Default namespace

xmltech-simple.xml





</resource>



xsi namespacePrefix defined

Default namespace

xmltech-simple.xml





</resource>




Default namespace

xsi:schemaLocation

xsi:schemaLocation says, “Load schema called

http://nvoss.org/VOResource

from local file, xmltech-simple.xsd

xmltech-simple.xml





</resource>


Global (Reusable) Types<?xml version="1.0" encoding="UTF-8"?><xs:schema targetNamespace="http://nvoss.org/VOResource" xmlns:res="http://nvoss.org/VOResource" xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">

<xs:simpleType name="Type"> <xs:restriction base="xs:string"> <xs:enumeration value="Archive" /> <xs:enumeration value="Catalog" /> <xs:enumeration value="Organisation" /> </xs:restriction> </xs:simpleType>

<xs:complexType name="Resource"> <xs:sequence> <xs:element name="title" type="xs:string" /> <xs:element name="referenceURL" type="xs:anyURI" minOccurs="0"/> <xs:element name="type" type="res:Type" minOccurs="0" maxOccurs="unbounded"/> </xs:sequence> <xs:attribute name="created" type="xs:dateTime" /> </xs:complexType>

<xs:element name="resource" type="res:Resource" />

</xs:schema>

Define a prefix forthe targetNamespace

xmltech-globaltypes.xml


Global (Reusable) Types

Type defined here…

<?xml version="1.0" encoding="UTF-8"?><xs:schema targetNamespace="http://nvoss.org/VOResource" xmlns:res="http://nvoss.org/VOResource" xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">




</xs:schema>




Global (Reusable) Types

…and used here

Type defined here…





</xs:schema>




Global (Reusable) Elements<?xml version="1.0" encoding="UTF-8"?><xs:schema targetNamespace="http://nvoss.org/VOResource" xmlns:res="http://nvoss.org/VOResource" xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">


<xs:element name="type" type="res:Type"/>

<xs:complexType name="Resource"> <xs:sequence> <xs:element name="title" type="xs:string" /> <xs:element name="referenceURL" type="xs:anyURI" minOccurs="0"/> <xs:element ref="res:type" minOccurs="0" maxOccurs="unbounded"/> </xs:sequence> <xs:attribute name="created" type="xs:dateTime" /> </xs:complexType>


</xs:schema>

Element defined here…

…and used here

xmltech-elrefs.xml


<xs:complexType name="Resource"> <xs:annotation> <xs:documentation> Any entity or component of a VO application that is describable and identifiable by a IVOA Identifier. </xs:documentation> </xs:annotation>

<xs:sequence> <xs:element name="title" type="xs:string" > <xs:annotation> <xs:documentation> the full name given to the resource </xs:documentation> </xs:annotation> </xs:element>

<xs:element name="referenceURL" type="xs:anyURI" minOccurs="0"> <xs:annotation> <xs:documentation> URL pointing to a human-readable document describing this resource. </xs:documentation> </xs:annotation> </xs:element> …

Schema Documentationxmltech-documented.xml

• Most schema components can have documentation attached

• Documentation is important for defining metadata schemas

• Carnivore registry extracts documentation directly from schema for display to users


<xs:complexType name="Service"> <xs:complexContent> <xs:extension base="res:Resource"> <xs:sequence> <xs:element name="accessURL" type="xs:anyURI" /> </xs:sequence> </xs:extension> </xs:complexContent></xs:complexType>

<xs:element name="service" type="res:Service">

Derived Types

• Two ways to derive a new type from an existing one– Extension

• Applicable only to complex types• Adds additional elements or attributes to the content model

<service xmlns="http://nvoss.org/VOResource" created="1994-11-01T12:00:00" > <title>ADIL Query Page</title> <referenceURL>http://adil.ncsa.uiuc.edu/help.html</referenceURL> <type>Archive</type> <accessURL>http://adil.ncsa.uiuc.edu/QueryPage.html</accessURL> </service>


Derived Types

• Two ways to derive a new type from an existing one– Restriction

• Simple types: restrict the legal values in some wayEx: integer: restrict range string: restrict to match a pattern

• Complex types: – Disallow optional elements, attributes– Restricting occurances– Setting default or fixed values where none were previously set


Extending Schemas I:plugging-in derived entities

• Suppose you want to define an element in terms of a base type but allow any type derived from it to be inserted in its place– a form of polymorphism

• Two techniques– xsi:type

• a label in the instance document)

– Substitution groups • a label in the schema document



• xsi:type techniqueFrom our example…– The resource element has the type Resource– The Service type is derived from the Resource type– Declaring a service element is not necessary

In the instance document…

<resourceList xmlns="http://nvoss.org/VOReso

<resource created="1994-11-01T12:00:00" > <title>NCSA Astronomy Digital Image Li <referenceURL>http://adil.ncsa.uiuc.ed <type>Archive</type> </resource>

<resource xsi:type="Service" created="1994-11-01T12:00:00" > <title>ADIL Query Page</title> <referenceURL>http://adil.ncsa.uiuc.ed <type>Archive</type> <accessURL>http://adil.ncsa.uiuc.edu/Q </resource>

</resourceList>

IVOA VOResource schema uses this technique

In the schema document…

<xs:element name="resourceList"> <xs:complexContent> <xs:sequence> <xs:element name="resource" type="res:Resource" maxOccurs="unbounded" /> </xs:sequence> </xs:complexContent></xs:element>



• Substitution group techniqueFrom our example…– The Service type is derived from the Resource type– Add substitutionGroup attribute to service element definition

• means we can substitute service element anywhere a resource is allowed

In the schema document…

<xs:element name="resource" type="res:Resource">

<xs:element name="service" type="res:Service" substitutionGroup="res:resource">

<xs:element name="resourceList"> <xs:complexContent> <xs:sequence> <xs:element ref="res:resource" maxOccurs="unbounded" /> </xs:sequence> </xs:complexContent></xs:element>

In the instance document…

<resourceList xmlns="http://nvoss.org/VOReso <resource created="1994-11-01T12:00:00" > <title>NCSA Astronomy Digital Image Li <referenceURL>http://adil.ncsa.uiuc.ed <type>Archive</type> </resource>

<service created="1994-11-01T12:00:00" > <title>ADIL Query Page</title> <referenceURL>http://adil.ncsa.uiuc.ed <type>Archive</type> <accessURL>http://adil.ncsa.uiuc.edu/Q </service></resourceList>

IVOA Space-Time Coordinates schema uses this technique


• separate schema file, separate namespace– Enables schema evolution in a backward compatible

way.

– Extension file uses xs:import to load schema being extended

– Instance documents generally must define prefixes for both the original schema namespace and the extension namespace.

• Example: VOResource metadata schemas– Core metadata schema: VOResource

– Extension schemas: VODataService, VORegistry, ConeSearch, SimpleImageAccess

Extending Schemas II:placing extensions into a separate namespace


A word about namespaces in instance documents…(FYI)

• An instance document must indicate what namespace(s) the elements belong to– So that the document can be validated– 3 ways to indicate this

• controlled by elementFormDefault attribute in Schema file


1. Default namespacexmlns tags a whole section

Advantages:• Great for simple ns useDisadvantages:• Can be confusing for authors

when drawing on multiple schema

2. Namespace prefixesInvidually tag elements

Advantages:• Visually explicit about ns

membership• Can mix in elements from

different ns with the same name.

Disadvantages:• Error-prone, ugly when

drawing multiple schema

3. Unqualified: tag (global) root element

Advantages:• Single prefix tag needed at the

top of the document; no other tags needed

• Good for when using multiple schemas together

• Least error prone• Makes XPaths simplerDisadvantage:• Can’t have 2 elements from

different ns w/same name

elementFormDefault="qualified" elementFormDefault="unqualified"

<resource xmlns="http://nvoss.org/VOResource"> <title>…</title> <referenceURL>…</referenceURL> <type>Archive</type></resource>

<vr:resource xmlns:vr="http://nvoss.org/VOResource" xmlns:cv="http://nvoss.org/Coverage"> <vr:title>…</vr:title> <cv:coverage>…<cv:coverage></vr:resource>

<vr:resource xmlns:vr="http://nvoss.org/VOResource"> <title>…</title> <coverage>…<coverage></vr:resource>

IVOA VOResource uses this technique


XPath

• What it is:– A W3C standard syntax for pointing to elements,

attributes, and/or their values in an XML file

• Why you might care:– XPath is used in two other important XML

technologies: XQuery and XSL

– XPath is used in ADQL to query a registry via the standard Registry Interface

• What you’ll get from this session:– Ability to form simple XPath queries


Can you tell me how to get to Sesame Street?

• An XPath is a set of directions from one point in an XML document to another

<NewYork>

<Borough name="Brooklyn">

<light/>

<light/>

<light>

<SesameStreet>

<weather>sunny</weather>

</SesameStreet>

</light>

</Borough>

<Borough name="Queens"/>

</NewYork>

/NewYork/Burough[@name="Brooklyn“]/light[3]/SesameStreet

Begin at the start of the document




<NewYork>


<light/>

<light/>

<light>

<SesameStreet>


</SesameStreet>

</light>

</Borough>


</NewYork>

/NewYork/Burough[@name="Brooklyn“]/light[3]/SesameStreet

Go to NewYork




<NewYork>


<light/>

<light/>

<light>

<SesameStreet>


</SesameStreet>

</light>

</Borough>


</NewYork>

/NewYork/Burough[@name="Brooklyn"]/light[3]/SesameStreet

Find the Borough named Brooklyn




<NewYork>


<light/>

<light/>

<light>

<SesameStreet>


</SesameStreet>

</light>

</Borough>


</NewYork>


Go to the 3rd light




<NewYork>


<light/>

<light/>

<light>

<SesameStreet>


</SesameStreet>

</light>

</Borough>


</NewYork>


And there’s SesameStreet


XPath syntax

• XPath fields (between the /s)– Each represents a descent in the XML hierarchy

• “//” means drop any number of levels– Points to an XML node

• element name or, if preceeded with an @, an attribute name– Other useful node-matching symbols

* = wildcard (any name) . = current node .. = parent node

• Context node = starting point– If it does not start with a /, XPath is relative to a context-specific

starting point. /NewYork/Burough[@name="Brooklyn"] • @name is an XPath relative to /NewYork/Burough

• Predicates: [ ]– Read “[…]” as “where …”– XPaths inside resolve to string value inside element/attribute pointed

to– Operators: = != < > <= >= and or– Many Useful Functions: contains(string, string), position(),

count(), last(), local-name()– [3] is short-hand for [position()=3]


XPath as a Query

• An XPath returns matched XML nodes/NewYork/Burough[@name="Brooklyn“]/light[3]/SesameStreet

On…<NewYork>


<light/>

<light/>

<light>

<SesameStreet>


</SesameStreet>

</light>

</Borough>


</NewYork>

Returns…<SesameStreet>


</SesameStreet>


XPath as a Query

• An XPath returns matched XML nodes/NewYork/Burough/light

On…<NewYork>


<light/>

<light/>

<light>

<SesameStreet>


</SesameStreet>

</light>

</Borough>


</NewYork>

Returns…<light/>

<light/>

<light>

<SesameStreet>


</SesameStreet>

</light>


XPath as a Query

• An XPath returns matched XML nodes/NewYork/Burough/light/SesameStreet/weather

On…<NewYork>


<light/>

<light/>

<light>

<SesameStreet>


</SesameStreet>

</light>

</Borough>


</NewYork>

Returns…<weather>sunny</weather>


XPath as a Query

• An XPath returns matched XML nodesstring(/NewYork/Burough/light/SesameStreet/weather)

On…<NewYork>


<light/>

<light/>

<light>

<SesameStreet>


</SesameStreet>

</light>

</Borough>


</NewYork>

Returns…sunny


XPath as a Query

• An XPath returns matched XML nodes/NewYork/Burough/light/SesameStreet[weather='sunny']

On…<NewYork>


<light/>

<light/>

<light>

<SesameStreet>


</SesameStreet>

</light>

</Borough>


</NewYork>

Returns…<SesameStreet>


</SesameStreet>

weather was automatically convertedto a string before comparison operatorwas applied.


XPath as a Query

• An XPath “returns” matched XML nodes– If path is ambiguous, all matching nodes are returned– If path does not resolve to an existing node, the empty set is returned– Predicates, […], provide constraints

• In some contexts, XPath is automatically converted to string value inside the matched element or attribute.

• Examples querying a set of VOResource documents /Resource[contains(content/description, 'cluster')]– Return all resource elements where the description contains the word ‘cluster’

/Resource[facility]– Return all resources that have a facility element

/Resource/capability[@xsi:type='cs:ConeSearch']/interface/accessURL– Return the interface URLs of all ConeSearch services

/Resource[@xsi:type='vs:DataCollection']/coverage/stc:STCResourceProfile//stc:AllSky

– Return all data collections that purport to have data distributed overall


XQuery (a.k.a. XML Query)

• What it is:– A W3C standard for querying XML documents

• An analogue to SQL for tables

• Why you might care:– It is one of the supported query languages in

the standard VO Registry Interface. – It can handle certain complex registry

queries that ADQL cannot

• What you’ll get from this session:– Ability to query XML documents by modifying

existing an XQuery


XQuery: an analogy to SQL

• SQL queries tables– Result of an SQL statement is a table– Columns of result table controlled by the

SELECT clause– Rows controlled by the WHERE clause

• XQuery queries XML documents– Result of an XQuery is an XML document– The form of the XML document is set by the return clause

– The contents of the result is controlled by the for, let, and where clauses


XQuery syntax: think FLWOR

• XQuery supports several types of expressions that can return XML results – XPath, Constructors, …– FLWOR = for let where orderby return

• FLWOR clauses– for/let clause selects data from source XML

documents



• Searching for Cone Search services suitable for getting data about galaxy clusters

• To be used with the Carnivore Registry

declare namespace vr= "http://www.ivoa.net/xml/VOResource/v0.10";

declare namespace vs= "http://www.ivoa.net/xml/VODataService/v0.5";

for $vr in //Resource/capability[@xsi:type="cs:ConeSearch"] where contains($vr//description, "quasar") return <conesearch> <title>{string(title)}</title> <url>{string($vr/capability/interface/accessURL)}</url> </conesearch>



• Searching for Cone Search services suitable for getting data about galaxy clusters

• To be used with the Carnivore Registry

Declare namespaceprefixes to use in query






• for clause sets up a loop around matching occurrences– XPath both selects the Resource element node to put into

the variable and constrains which Resources are included

• let clause (not used here) can also set variables. – Used to join across documents, self-joins– If $vr is used in the variable definition, the new value

would be different in each pass of the loop.

Loop over all ConeSearch resources







• where clause further restricts the output– Optional, often don’t need it:

for $vr in //vr:Resource[@xsi:type="cs:ConeSearch“ and contains($vr//vr:description, "quasar")]

Restrict output to ConeSearch servicesabout quasars








• return clause sets the output template – { } used to denote non-literal output– Applied to each value of $vr

• XQuery supports several other expression types not shown here– Conditionals, function definition, etc.– Many more predefined funtions

XQuery: a full XML processing language


Extract and displayDesired information

Restrict output to ConeSearch servicesabout quasars

Declare namespaceprefixes to use in query declare namespace vr=

"http://www.ivoa.net/xml/VOResource/v0.10"; declare namespace vs=

"http://www.ivoa.net/xml/VODataService/v0.5"; for $vr in //Resource/capability[@xsi:type="cs:ConeSearch"] where contains($vr//description, "quasar") return <conesearch> <title>{string(title)}</title> <url>{string($vr/capability/interface/accessURL)}</url> </conesearch>


XSL – XML Stylesheet Language

• What it is– A language for describing the transformation of XML

data from one form to another• XML -> HTML• XML -> XML• XML -> Plain text

• Why you might care– XSL can be used to create human readable

renderings of XML data from the VO– XSL is used by the Java Skynode toolkit for

converting ADQL to local SQL

• What you’ll get from this session:– Ability to modify XML transformations via simple changes

to a stylesheet


XML Stylesheet Language

• An XML document that describes a transformation

• Contains a list of templates – Each describes the transformation for one type of

node (e.g. an element with a particular name)– Node is identified by an XPath

• Relative to current context node– Usually one template for /, the root of the document– A template can call other templates

• XSL provides a number of programming structures– Conditionals, looping, user-defined functions, built-in

functions, extensibilty– Variables are immutable!

• XSLT = XSL Transformation


A tour through a stylesheetxmltech-VOResource.xsl to transform xmltech-adil.xml into plain text

<?xml version="1.0" encoding="UTF-8"?>

<xsl:stylesheet xmlns="http://www.ivoa.net/xml/VOResource/v1.0"

xmlns:vr="http://www.ivoa.net/xml/VOResource/v1.0"

xmlns:vs="http://www.ivoa.net/xml/VODataService/v1.0"

xmlns:xsl="http://www.w3.org/1999/XSL/Transform"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

version="1.0">

<xsl:output method="text"/>

<xsl:template match="/">

Resource Description Record

<xsl:apply-templates select="Resource" />

</xsl:template>



Define prefixes for all namespaces we’llbe using







version="1.0">





</xsl:template>



• Three output types– xml, html, text


Our output formatwill be plain text







version="1.0">





</xsl:template>



• When raw text appears, XSLT engine will preserve spacing (like carriage returns) around text

• Resource is the root of our data document


Our output formatwill be plain text

Our root documenttemplate sets up the output document andcalls next template







version="1.0">





</xsl:template>



• When a template “runs”, it changes the context node to the node matched by the template

• Subsequent XPaths within template are relative to that context node

<xsl:template match="Resource" > <xsl:value-of select="substring-after(@xsi:type,':')"/> <xsl:text> </xsl:text> <xsl:value-of select="title"/> <xsl:text> (</xsl:text> <xsl:value-of select="shortName"/> <xsl:text>) IVOA Identifier: </xsl:text> <xsl:value-of select="identifier"/>

<xsl:apply-templates select="content" /> <xsl:apply-templates select="curation" /> </xsl:template>

<xsl:template match="content"> <xsl:apply-templates select="description" /> <xsl:text> </xsl:text>

<xsl:text>Target Communities: </xsl:text> <xsl:for-each select="contentLevel"> <xsl:value-of select="."/> <xsl:if test="position()!=last()"> <xsl:text>, </xsl:text> </xsl:if>

</xsl:for-each> </xsl:template>

Resource template



text tags can be used totake explicit control ofspacing

Resource template <xsl:template match="Resource" > <xsl:value-of select="substring-after(@xsi:type,':')"/> <xsl:text> </xsl:text> <xsl:value-of select="title"/> <xsl:text> (</xsl:text> <xsl:value-of select="shortName"/> <xsl:text>) IVOA Identifier: </xsl:text> <xsl:value-of select="identifier"/>







• our XPaths point to elements– Relative to vr:Resource!

• value-of will convert it to a string


value-of will print thestring values of nodes










Pass control to other templates







• Normally, apply-templates will automatically loop over multiple occurances

• Here, we need to insert commas

• for-each also changes the context node





Resource template

Loop over all occurancesof contentLevel







• Use choose/when for if-then-else blocks





Resource template

Loop over all occurancesof contentLevel

If block







XSL for metadata

• Transformation is a powerful paradigm for metadata processing– Consider all uses of metadata as a

transformation to another form…• User display• An SQL statement• A workflow script• Compile-able code

– XSL stylesheet somewhere between configuration file and script

• Rapid prototyping and adaptation• xsl:import: ability to extend & override other

stylesheets

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