facet: building web pages with sparql

a division of Publishing Technology

FacetBuilding Web Pages With SPARQL

SWIG-UK Event, HP LabsNovember 23rd 2007

Leigh DoddsChief 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


Journal

Article

Article

ArticleIssue

Issue

Issue

2007 Index

2006 Index

New Article Index


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…

1. What lens are we going to apply?

2. What data model are we going to apply it to?

3. 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>


SummaryPros• 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”