Copyright © 2008 Model Driven Solutions EKB XML Interface Jim Logan September 2008 Formerly Data Access Technologies

Page 2 EKB XML Interface Rationale Business systems are increasingly using XML interfaces It would be nice to have one XML interface for systems and UI Challenges with this approach are: –Our EKB data is represented as a virtual graph of RDF triples –How do we get from triples to XML and back again? –Popular UI technologies require lots of code to bridge browsers and XML –Is there a UI technology where the widgets are tied directly to XML data? Copyright © 2008 Model Driven Solutions.March 2008

Page 3 From RDF Graph to XML: Overview Our EKB data is represented as a virtual graph We need to somehow transform that to XML and back again There is a W3C recommendation called RDF/XML that helps, but: –The graph has no real “root”, so it walks the graph in an unpredictable order –If an instance has multiple types, the type used as the XML element name is unpredictable Copyright © 2008 Model Driven Solutions.March 2008

Page 4 From Triples to XML: Virtual Graph Our EKB data is represented as RDF triples in a special repository Each triple represents {subject, predicate, object} The subject and predicate is always a URI The object can be either a URI or a literal value These triples form a virtual graph Copyright © 2008 Model Driven Solutions.March 2008 #p1 #p2 “John” “Jeff” name knows name #Person type

Page 5 From Triples to XML: RDF/XML John Jeff Copyright © 2008 Model Driven Solutions.March 2008 Striped RDF/XML is a W3C recommendation that almost works This is a bit oversimplified*, but roughly: The black-on-tan elements represent subjects and objects The blue elements represent properties (kinds of predicates) Person “John” “Jeff” name knows name * This is oversimplified because each Person node actually represents an additional node and an arc: This turns into an XML element for the type name and an XML attribute for the ID / URI #p1#Person type

Page 6 From Triples to XML: Solution RDF/XML is really close, but: –It walks the graph in an unpredictable order, since there is no “root” –It uses unpredictable XML element names when several types exist –We only use it as a guide The EKB provides a REST service that produces XML instance documents, given: –A “viewpoint” –The “root” (i.e., subject) of the graph, so the structure is predictable Copyright © 2008 Model Driven Solutions.March 2008

Page 7 From Triples to XML: Viewpoint A “viewpoint” is expressed in an ontology and specifies: –What types of properties to traverse –How deep to traverse –Which type name to use for polymorphic traversals The UI tells the service: –Which viewpoint to use –The subject, as either: A specific URI, or A query-by-example in XML –The context (i.e., ontology file), as either: The REST URL, or An attribute in the XML query-by-example Copyright © 2008 Model Driven Solutions.March 2008

Page 8 Both examples will return an XML instance document: –HTTP POST to –HTTP GET from viewpoint-uri&id=&23p1 viewpoint-uri&id=&23p1 From Triples to XML: HTTP Examples Copyright © 2008 Model Driven Solutions.March 2008 Actors.owl Viewpoint Property / predicate Type

Page 9 From Triples to XML: Instance Example Copyright © 2008 Model Driven Solutions.March 2008 <Versioning:Data_asset rdf:about=" <Authority:Authority rdf:about=" Cory Casanave Actors.owl Viewpoint Subject Property / predicate Type Object

Page 10 From Triples to XML: Interface Specifics The service requires: –The name of the structure, called the viewpoint, either as: A URL query parameter The outermost XML element in a POSTed query –The name of the containing ontology file, called the context, either as: The URL of the GET or POST An asset attribute in the outermost XML element in a POSTed query A GET from the special “global asset URL”: “.../asset/head/root/asserted/RDFS/*” –The subject of interest within that ontology file or one of the files it includes, either as: A full URI (e.g., “about= An abbreviated URI (e.g., “id=#p1”) A query-by example Copyright © 2008 Model Driven Solutions.March 2008

Page 11 MVC In one sense: –The model is the RDF triples –The view is what the service generates algorithmically from the viewpoint –The controller is the service In another sense: –The model is the XML instance document –The view is the UI controls –The controller is the UI In a unified sense: –The model is the RDF triples –The view is the UI controls –The controller is the UI and the service Copyright © 2008 Model Driven Solutions.March 2008

Page 12 From XML to RDF Triples: Overview The outside world interacts with the EKB using XML The EKB data is represented as a virtual graph We need to somehow transform the XML into RDF triples We need concurrency: –No locking –No stomping –Minimal rollback We need value-level provenance (e.g., who set this value when?) Copyright © 2008 Model Driven Solutions.March 2008

Page 13 From XML to RDF Triples: The UI The UI requests an XML instance document from the EKB The user changes the XML instance document, as the UI marks up the XML instance The UI sends the marked-up XML instance document back to the EKB EKB uses this XML instance document to change the RDF triples Copyright © 2008 Model Driven Solutions.March 2008

Page 14 From XML to RDF Triples: Edited XML Copyright © 2008 Model Driven Solutions.September 2008 <Versioning:Data_asset rdf:about=" action="edit"> <Authority:Authority rdf:about=" Cory Casanave <Authority:Authority rdf:about=" Jim Logan Actors.owl Changed Actors.owl Viewpoint Subject

Page 15 From XML to RDF Triples: EKB Service Provides a REST service that consumes a marked-up XML instance document containing: –The “viewpoint” –The subject –Actions Starts a transaction Ensures that the removed values are still the current values Deletes the corresponding triples Adds the new triples Ends the transaction Unimplemented: if any rules are violated, the transaction should fail and the service should send back a report Copyright © 2008 Model Driven Solutions.March 2008