Facet: Building Web Pages with SPARQL

Facet Building Web Pages With SPARQL SWIG-UK Event, HP Labs November 23 rd 2007 Leigh Dodds Chief Technology Officer, Ingenta

Problem Statement

Where’s my RDF-native Web Framework?! There is no good system for integrating RDF repositories with existing an web framework (in Java)

Design Constraints

Design Constraints
A web page presents data that is a sub-graph (i.e. a view) over a larger RDF store, (the data model)
The extent of the sub-graph may vary for different presentations of the data, and may contain arbitrary properties
The description of the sub-graph (a lens) should be declarative
That sub-graph is “rooted” on a single primary resource (e.g. a Journal)
The identifier of the primary resource can be derived from the request URL, e.g. by rewriting the URI. And vice versa
Therefore, we don’t support blank nodes as primary resources
Or fragment identifiers in URIs!
The sub-graph should be serializable into an object graph for presentation to the templating system

Facet Request Handling
To return a response we need to answer three questions…
What lens are we going to apply?
What data model are we going to apply it to?
What’s the identifier of the primary resource ?

Lenses Describing views of RDF data

A Simple Lens
PREFIX dc: <http://purl.org/dc/elements/1.1/>
CONSTRUCT {
?item dc:title ?title .
?item dc:language ?language .
}
WHERE {
?item dc:title ?title .
OPTIONAL { ?item dc:language ?language . }
}

Configuring Lenses
<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:ja="http://jena.hpl.hp.com/2005/11/Assembler#"
xmlns:view="http://metastore.ingenta.com/facet/lens/">
<rdf:Description
rdf:about="http://metastore.ingenta.com/facet/lens/Sparql"> <ja:assembler>com.ingenta.facet….</ja:assembler>
</rdf:Description>
<view:Sparql
rdf:about=" http://oecd.metastore.ingenta.com/views/just-the-title ">
<view:query> sparql/just-the-title.rq </view:query>
</view:Sparql>
</rdf:RDF>

Data Model Configuring RDF graphs

Data Model Configuration
Jena Assembler API
Add notion of application level default data model
Uses well-known URI
Lenses may be configured to apply to a specific data model
Allows “sharding” of data models
<view:Sparql
rdf:about=" http://oecd.metastore.ingenta.com/views/just-the-title ">
<view:appliesTo rdf:resource=“…”/>
</view:Sparql>

Resource Identifiers

Mapping Resource Identifiers
In a RESTful application, each resource should have a single primary location
Allows resource identifiers to be derived using URL rewriting
http://test.sourceoecd.org/oecd/content/journal/18168116
http://oecd.metastore.ingenta.com/content/journal/18168116

Serialization Mapping an RDF sub-graph to a Java object model

Serialization
Primary resource is a ContentItem
Has an identifier and Map of properties
Walk through graph, beginning at “root” resource, mapping RDF statements to Map entries
Mapping of property names is configurable.
Default based on namespace prefix, E.g. dc_title
Mapping of objects of each statement to a suitable Java object
ContentItem, Map, List, Integer , etc

Serialization (special cases)
Multilingual properties
Special casing (i.e. a hack!) to modify naming, e.g. dc_title_fr
Repeated properties, e.g. dc:subject
Use schema annotation to indicate these, and then Serialize to a List
XML Literals & Multi-lingual data
E.g. multi-lingual abstracts ( dc:description ) that contain XHTML markup
Use schema annotation, parse and create separate Map entries

Additional Features
“ MultiLens”, applying multiple queries in series to build results
Automatic availability of URL parameters as SPARQL query parameters
Integral API support
RDF output for free; JSON output trivial
Simple content lifecycle, mapping to HTTP resource statuses
E.g. Content Not Found, Moved, Gone
Add type ( life:Deleted ) and properties ( life:newLocation ) to data
Support for URL Aliasing based on property values
/content/issn/1234-5678 -> /content/journal/abcdef
<prism:issn>

Summary
Pros
By embracing a few limitations on RDF modelling, e.g. identifiers provides a very flexible means of building web pages from an RDF repository
Reliance on SPARQL and Jena API features provides great deal of configuration options
Good integration with existing web templating environments
Quick to learn
Cons
Model limitations mean its not suited to all RDF “in the wild”

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Facet: Building Web Pages with SPARQL

by Leigh Dodds on Nov 26, 2007

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Facet: Building Web Pages with SPARQL — Presentation Transcript