review of semantic enablement techniques used in geospatial and semantic standards for legacy and...

Review of semantic enablement techniques used in geospatial and semantic standards for legacy and opportunistic mashups Laurent Lefort, Australian Ontology Workshop 2009

Standards for Semantic Sensor Mashups

CSIRO. Standards for Semantic Sensor Mashups

Outline of the talk

• Which standards for which mashups? • Server-side/legacy or client-side/opportunistic

• Semantic-enabled?• Semantic enablement pathways

• Links and annotations

• Meshup “value pyramid”

• Review of specific standards• XLink, RDFa, SAWSDL/hRESTs

• Failure risk and validation issues• Conclusion


Web 2.0 & 3.0 (Sem Web) rocks XML and WSDL don’t (anymore)

Matt Jones http://www.flickr.com/photos/blackbeltjones/3150215637/

WHICH STANDARDS FOR WHICH MASHUPS?


Motivations: W3C Semantic Sensor Network incubator group

Registries & Dictionaries

Sensor and obs.

Sensors and Observations

Linking Open Data resources

OGC Services (SOS, SPS, SAS, SES)

Semanticaly-annotated OGC services

(SOS, SPS, SAS, …)

Enable semantic service integration

Semantic annotations

Ontology-enabled APIs

Ontology-enabled

reference datasets

Enable semantic mashups

To begin the formal process of producing ontologies that define the capabilities of sensors and sensor networks

To develop semantic annotations of a key language used by services based sensor networks

(especially the ones developed by the Open Geospatial Consortium)

Semantic annotations - for OGC services?- for Mashups?


Server-side mashups (Web 1.0 & 2.0)

• Server-side mashups• Server-side software component accessing XML files, Databases,

SOAPful or RESTful web services

• The result is generally packaged as a web service

• For legacy resources: • Complex APIs

• Workflow engine and wrappers

• Output in XML


Client-side mashups (Web 1.0 & 2.0)

• Client-side mashup: • Client-side scripts accessing mashable resources (RESTful

services mostly)

• The result is packaged into an interactive web application

• For opportunistic mashups: • Simpler APIs

• Scripting languages

• Output in HTML


Server-side semantic mashups (Web 3.0)

• Server side mashup:• Semantic enablement of XML files, Databases, SOAPful or

RESTful web services (SAWSDL)

• Integration with linking open data and ontologies services through triple stores (APIs or resources)


Example of semantic composition (server side)

• Composer’s Workbench• XML-RDF

• Wrap complex services using semantic annotations mapping WSDL/XML schema to DL ontology (also SQL DBs)

• New requirements: provenance XG

Cameron et al. (2009) Semantic Solutions for Integration of Federated Ocean Observations


Client-side semantic mashups (Web 3.0)

• Client side mashup:• Enrichment of HTML resources with RDFa markup allowing to “lift”

the content into RDF

• Reduction of number of APIs to handle by scripts (SPARQL or equivalent)


Example of semantic pipes (client side)

• Sensor masher (browser-based)• RDF-HTML (RESTful services, Javascript)

• Avoid the use of proprietary or product-specific APIs

• Leverage URI-based data integration (Linked Open Data)

• Lightweight pipes (user-defined) based on DERI Pipes

Danh Le Phuoc (2009): SensorMasher : publishing and building mashup of sensor data


Semantic enablement: where?


Four semantic enablement pathways: Server-side (1,2, 3) or client-side (3,4)

• 1. Include RDF (SKOS/OWL) resources in XML using XLink,• 2. Annotate SOAPful web services with SAWSDL• 3. Annotate RESTful web services with hRESTs

(SA-REST/MicroWSMO),• 4. Include RDF (SKOS/OWL) resources in HTML using RDFa.

1

4

2 33

1

1

1

XML HTML


A possible use case with all types of mashups bundled together

Model Mashups

Web mashups

Search

Legacy OGC

services

Legacy OGC aggregator

(e.g. ArcGIS)

RDF-ized OGC

service

Composed services

Semantic mashups

Geospatial mashups

Model pipes

Geo Mashups

Legacy services

(XML+REST/WSDL) Legacy mashups

(XML+REST/WSDL)

Web mashups

Search

Geo Mashups

Model Mashups

Web pages mashups (HTML)Geospatial mashups (XML/JSON+REST)

Semantic pipes

RDF-ized Composed

service

RDF-ized OGC

Aggregator

Model mashups (XML/JSON+REST)

Semantic mashups (RDF/JSON+SPARQL)

Semantic web page mashups (HTML+RDFa)

1 42 32 33


Meshup “value pyramid”

• Semantic mashups over• RDFa content embedded in web

pages• Linked Open Data resources• XML, database and web service

resources

• Meshup• A semantically mashable

semantic mashup• a mashup consuming and serving

SW content,

• RDFa standard is disruptive• New generation of SW apps• New “value pyramid” top

RDF-ization (Lifting layer)

Linked Open Data resources

SPARQL protocol

HTML/RDFa

Mesh ups

Legacy Resources (XML, Database, Web services)

Extension of Kingsley Idehen’s pyramid: “Getting The Linked Data Value Pyramid Layers Right (Updated)”

XML

RDF

HTML


Meshup standard “value pyramid” vs. TBL’s Cracks and Mortar



SPARQL protocol

HTML

Mashups


Tim Berners-Lee, Cracks and Mortar W3C TPAC 2007


Meshup standard “value pyramid”vs. new “Cracks and Mortar”

Ontology of objects

Virtual RDF data

XML Schema

Existing SQL DB

SQL Schema

Lifting script

XSLT or Xquery

HTML pages

Ontology of objects

Virtual RDF data

Ontology of objects Virtual

RDF data

RDB-RDF Mapping

RDFa markup

Existing XML

GRDDLmarkup

Lifting service

Lifting service

RDFa service

SparQL

SparQL SparQL

Users

Mashup site

Mashup site

Mashup site

Mashup site

HTTP + HTML (RDFa) + SVG + DOM + JS + Mashable APIs

XML or JSON + HTTP + JS + Mashable APIs

HTTP + HTML (RDFa) + SVG + DOM + JS + Mashable APIs



SPARQL protocol

HTML/RDFa

Mesh ups



Definitions: links, annotations, lifting operations

• Links specifies the inclusion of remotely managed resources.• Mechanisms used to extend available content from any type of

resources with information sourced from remotely managed content (type or instance).

• Possible between two documents of the same type or between documents of different types.

• Semantic annotations define how to map service capabilities to semantic definitions to enable the discovery or composition of web services.

• The transition from XML-based services to RDF-based services is called a lifting operation (Farrell and Lausen 2007) and the inverse one, from RDF to XML is called a lowering operation.


Semantic enablement pathways using differentlinking and annotation standards

• 1. Include RDF (SKOS/OWL) resources in XML using XLink,

• 2. Annotate SOAPful web services with SAWSDL

• 3. Annotate RESTful web services with hRESTs (SA-REST/MicroWSMO),

• 4. Include RDF (SKOS/OWL) resources in HTML using RDFa.



SPARQL protocol

HTML/RDFa

Mesh ups

Legacy Resources (XML, Database, Web services)1

2 3

4

XLink

SAWSDL

hRESTs

RDFa

Lifting operations

1

2

3

4


Semantically-enabled XML resources and XLink

HTTP + HTML + SVG + DOM + JS + RDF + OWL + SPARQL

Ontology of objects

Virtual RDF data

XML Schema

Lifting script

XSLT or Xquery Existing XML

GRDDLmarkup

Lifting service

SparQL

~1


Variants of XLink usage

Existing XML

XLinkmarkup

Existing XML

(instances)

Existing XML

XLinkmarkup

Existing XML

(types)

Existing XML

XLinkmarkup

Existing RDF

(instances)

Existing XML

XLinkmarkup

Existing OWL

(types)

Xlink hrefor URNs for ontologies (class or property)

Xlink hrefor URNs for

“data” (individuals)

Inclusion of remote resources

Inclusion of remote semantic resources

Model reference to ontological descriptionDescribed in GML spec. xlink

@href

Xlink @href and SWE/GML@definition

Model reference to ontological description

Xlink @role and @arcrole

Xlink @role, @arcrole and SWE/GML @definition


XLink and RDF

Attribute Description Intended RDF

xlink:href Identifier of the resource which is the target of the association, given as a URI

rdf:about of range resource

xlink:role Nature of the target resource, given as a URI

rdf:about of class of range resource

xlink:arcrole Role or purpose of the target resource in relation to the present resource, given as a URI

rdf:about of object property linking domain element to range resource

xlink:title Text describing the association or the target resource

rdfs:comment


Usage of XLink in GML – related to URNs

• Conventions defined by the GML standard (Portele 2007)• Portele C. (2007): OpenGIS® Geography Markup Language (GML)

Encoding Standard version 3.2.1 OGC 07-036 Open Geospatial Consortium 2007-08-27

• Reference to an object element in the same GML document <myProperty xlink:href="#o1"/>

• Reference to an object element in a remote XML document using the gml:id value of that object: <myProperty xlink:href="http://my.big.org/test.xml#o1"/>

• Reference to an object element with a uniform resource name may be encoded as follows (a URN resolver is required): <myProperty xlink:href="urn:x-ogc:def:crs:EPSG:6.3:4326"/>

• URN: Uniform Resource Name • May or may not correspond to Semantic Web resources

• http://en.wikipedia.org/wiki/Uniform_Resource_Name

• URN is a generic resource naming mechanism: the mapping of a URN to a class, property or individual is not normalised


Current XLink usage

• Sheth Semantic Sensor Markup of Data and Services SSN-XG briefing

• XLink @href pointing to individual

• Luis Bermudez Enriching SOS services with Ontologies - OOSTethys/OceansIE and MMI SSN-XG briefing

• XLink @href pointing to individual

• Janowicz et al. (2009; forthcoming): Semantic Enablement for Spatial Data Infrastructures. Transactions in GIS.

• XLink @href pointing to individual with @role pointing to sawsdl:modelReference (should be arcrole)

• Correct use of sawsdl:modelReference in XML schema but does not define the associated lifting script

• Compton et al. (2009) A Survey of the Semantic Specification of Sensors, in Proc. International Workshop on Semantic Sensor Networks SSN’09 CEUR-WS Vol. 552

• XLink @href pointing to undefined concepts (#AirTemperature)


Major issues with XLink (and its usage in OGC)

• ISSUE: URNs can point to an individual, a class or a property • No guidelines on these three types of URN

• <swe:Quantity definition="urn:ogc:def:property:SBE:batteryCurrent">

• Confusion between XLink @role vs. @arcrole • Ex of a property URN (here, @arcrole should be used): <swe:field

name="Battery Current“ xlink:role="urn:ogc:def:property:powerSupply">

• Same issue with the @definition attribute

• Usage of @href (to an individual) generally correct• Because the majority of the community developing and using OGC

standard plans to use SKOS to manage vocabulary elements


Semantically-enabled web pages (RDFa)

HTTP + HTML (RDFa) + DOM + JS + RDF + OWL + SPARQL

HTML pages

Ontology of objects

Virtual RDF data

RDFa markup

RDFa service

SparQL

4


Variants of RDFa usage comparable to XLink

Existing HTML

RDFamarkup

Existing RDF

(instances)

Existing HTML

RDFamarkup

Existing OWL

(types)

Linkage to remote individuals Mapping to ontology classesDescribed in RDFa spec.

Example: @about Described in RDFa spec.

Example: @typeOf

Existing HTML

RDFamarkup

Existing OWL

(properties)

Mapping to ontology propertiesDescribed in RDFa spec.

Example: @property


XLink – RDFa comparison

RDF mapping Xlink RDFa

Domain instance about or src

Domain class typeof

Object property arc role rel

Inverse object property rev

Range instance href href or resource

Range class role typeof

Datatype property property

Datatype property type role datatype

Range value content or element content


Tentative use of RDFa instead of XLink

• Barnaghi et al. Sense and Sensíability: Semantic Data Modelling for Sensor Networks, in Proc. of the ICT Mobile Summit 2009, June 2009.

• SWE’s @definition mapped to class

• RDFa-inspired (to fix): • OWL-like attribute namespaces to clear • @about mapped to individual, • @datatype mapped to xsd type, • @resource used but without corresponding @property, • @ID used, • URI conventions?

• It is important to note that RDFa obeys to a rigorous specification which allows the development and usage of generic lifting scripts


Variants of RDFa usage in relation to hRESTs

• Two possibilities to do semantic markup of HTML files• Microformats

• RDFa


Semantically-enabled RESTful web services (hREST-microformat)


HTML description

Dynamic XML

hRESTs-micro-

formatsmarkup

Ontology of objects

Lifting script

RESTful service

Service ontology

RDF description of service

Lifting service for data

XSLT or Xquery SA-REST or MicroWSMO

?

Semantically-enabled output

Semantically-enabled service

Lifting operation

and service ontology

hard-coded

Lifting script

3 ~4


Semantically-enabled RESTful web services (hREST-RDFa)


HTML description

Dynamic XML

hRESTs-in-RDFamarkup

Ontology of objects

Lifting script

RESTful service

Service ontology


Lifting service for data

XSLT or Xquery SA-REST or MicroWSMO

?



Lifting operation following

RDFa spec.

3 ~4


hRESTs-microformat vs. hRESTs-RDFa

RDF mapping hRESTs-microformat hRESTs-RDFa

Domain instance id (URL-prefixed) about

Domain class class (closed list) typeof

Object property ref=”model” rel

Inverse object property rev

Range instance href or resource

rdf:about of range class href typeof

Datatype property property

Datatype property type datatype

Range value content or element content


hRESTs-RDFa preferred to hRESTs-microformat

• hRESTs-microformat forces the user to pick the service ontology and have access to the corresponding lifting script

• SAREST ontology ~ what’s used in SAWSDL• http://knoesis.wright.edu/research/srl/standards/sa-rest/

• MicroWSMO ontology: WSMO-Lite: • http://www.wsmo.org/ns/wsmo-lite/

• hRESTs-RDFa allows to specify the service ontology the mapping definitions will be lifted to

• e.g. one adapted to a specific platform • sensor networks, grid computing, …

• It should be possible to have a similar freedom of choice with SAWSDL

• It’s not the case right now (next slide)

http://www.wsmo.org/ns/wsmo-lite/

http://www.wsmo.org/ns/wsmo-lite/


Semantically-enabled SOAPful web services


WSDL

XML Schema

Dynamic XML

SAWSDLmarkup

Lifting script

WSDL Web service


XSLT or Xquery

Service ontology

Lifting operation

and service ontology

hard-coded

Lifting script

Lifting service for description

Ontology of objects

Lifting service for data ?



2


Failure risk analysis

• Opportunistic mashups depends on external resources which may disappear or evolve without notice,

• especially mashable services and semantic resources,

• The risks of failure are greater and more diverse than in other environments.

• Question: where to start

Semantic Services

Legacy mashups

Mashable web pages

Web services

Ontologies

Triple stores

SPARQL

Linked Open Data

Web pages

RDFa

XML RDF HTML

Opportunistic mashupsSem

anti

c en

able

men

t


Validator mashup framework: Unicorn (Universal Conformance Observation and Report Notation)

• Unicorn (2006-2008)• Validator Mashup project at W3C

• http://www.w3.org/QA/Tools/Unicorn/

• HTML-only• Markup Validator,

• CSS Validator,

• Link Checker


Extend Unicorn to build a complete top-down validator mashup pyramid

• Mashable validators• HTML validators

• HTML + RDFa http://validator.w3.org/ • HTML http://validator.nu/

• SPARQL• SPARQL* http://www.sparql.org/validator.html • Linked Data (URIs)* http://vapour.sourceforge.net/

• Linked Open Data• OWL http://owl.cs.manchester.ac.uk/validator/• RDF http://www.w3.org/RDF/Validator/

• RDF-ization • SAWSDL, …: ?• GRDDL (service) http://www.w3.org/2007/08/grddl/

• XML validators• WSDL http://www.validwsdl.com/ (via Wikipedia)• OGC valdiators• XLink SXLink?

• Full list of W3C list validators: http://www.w3.org/QA/TheMatrix



SPARQL protocol

HTML/RDFa

Mesh ups



Identification of area of future work

• Semantic annotation standards for both WSDL and REST services• Ontologies for different types of services • Lifting scripts for services • Guidelines on the part of HTML to be annotated for RESTful services

• Controlled upgrade of legacy standards: need at least better guidelines (and validation tools)

• XLink @role and @arcrole are easy to confuse• URNs mappings to individuals, class or properties should be specified

unambiguously in OGC specifications (and elsewhere?) • Develop a RDFa style for XLink may help to separate the current usage of

XLink (intra-XML) to new usages where XLink would be used in conjunction with semantic web resources

• Validators and validator mashups• Higher risk of errors with mashups• Golden opportunity to re-engineer and mash existing validators• Some missing validators especially at the lower levels (e.g. XLink, URNs)


Conclusions

• Semantic mashups complete existing semantic integration approaches but don’t replace them

• Lightweight composition by end users with semantic pipes to explore opportunities

• Transition to more stable infrastructure built on top of legacy services if the proof of concept phase is successful

• Mashups require hybrid combination of XML, RDF and HTML standards

• Some standards like XLink or RDFa are adaptable at different levels of the pyramid

• Special care must be taken for the semantic upgrades of existing standards

• Mashups requires new validation approaches• Which may also be based on mashups (Unicorn-like)

Contact UsPhone: 1300 363 400 or +61 3 9545 2176

Email: [email protected] Web: www.csiro.au

Thank you

CSIRO ICT CentreLaurent LefortSenior Software Engineer and W3C Office manager

Phone: +61 2 6216 7046Email: [email protected]: www.ict.csiro.au

Backup slides


Memo

• GRDDL - A markup format for Gleaning Resource Descriptions from Dialects of Languages. It is a W3C Recommendation, and enables users to obtain RDF triples out of XML documents, including XHTML. It defines the syntax to include a reference to a lifting script in a source document - the lifting script can then be used to transform the document to RDF

• Microdata - Allows nested groups of name-value pairs to be added to documents, in parallel with the existing content. A non-semantic alternatibe to RDFa

• SAWSDL - A set of extension attributes for the Web Services Description Language and XML Schema definition language that allows description of additional semantics of WSDL components. Allows the user to record the mapping of WSDL elements to concepts defined in a reference ontology and to specify the lifting scripts which can be applied to the output of a service to transform it into a RDF file using the reference ontology concepts

• hRESTs - A microformat to add additional meta-data to REST API descriptions in HTML and XHTML. Developers can directly embed meta-data from various models such an ontology, taxonomy or a tag cloud into their API descriptions. The embedded meta-data can be used to improve search (for example: perform faceted search for APIs), data mediation (in conjunction with XML annotation) as well as help in easier integration of services to create mashups.

• SA-REST and Micro-WSMO: two similar methods to semantically annotate REST services using the same microformat (hRESTs) and a different target ontology. Similar basis than SAWSDL (including the possibility to include a reference to a lifting script) but applicable to an HTML-based description of a service).

review of semantic enablement techniques used in geospatial and semantic standards for legacy and...

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