A presentation about our project to provide an OData gateway over a SPARQL query service to enable OData clients to consume linked data from a SPARQL query endpoint.

1. OData / SPARQL
Interop
Kal Ahmed (@kal_ahmed)
Graham Moore (@gra_moore)

2. Agenda
• Quick Overview of OData
• Motivation
• Implementation
• Questions

3. OData
“OData is a standardized protocol
for creating and consuming
data APIs”
odata.org

4. Entity-Centric
• Collection of Entity Sets
• Service metadata
– List of entity sets
– Schema for entities
• Primary Keys
• Properties & Associations
• Results are returned as entities by default

5. URL-based access
http://example.org/Films
http://example.org/Films(1234)
http://example.org/Films?$filter=Runtime lt 120
http://example.org/Films(1234)/Director
http://example.org/Films(1234)?$expand=Director,Actor

6. RESTful Update
• POST to entity set‟s URL
• PUT, PATCH, MERGE or DELETE to the
edit URL of an existing entity
• Associations can also be exposed as a
collection of link resources.

7. Other Features
• Media entities
– Combine metadata properties with a special
media source URL.
• Named Streams
– Entity properties of type Edm.Stream
– Represented in the entity as a URL
• Custom Actions and Functions
– Entity set or entity-specific operations
exposed as URLs

8. Reasons to Like OData
• Schema discovery
• Client tooling support (esp for .NET)
• Easy to experiment
• Easy to use from JavaScript
• Growing set of OData consumers
– Data browsers
– GUI controls
– Applications

9. Criticisms of OData
• Services tend to be siloes
• No shared ontologies
• Perceived as a vendor-specific protocol

10. ODATA / SPARQL INTEROP

11. Motivation
• We like the features of OData
• We like the flexibility of RDF / SPARQL
• Goals:
– Read access to open SPARQL endpoints via
OData
– Declarative configuration
– Automatic configuration via RDF schema /
SPARQL introspection

12. Previous Attempt
• LINQ – to – SPARQL
– Hand-crafted model as annotated C#
interfaces
– Generated implementation classes
– OData -> LINQ -> SPARQL and all the way
back
– Worked, but it was clunky

13. Implementation
• Proxy service for a SPARQL endpoint
– OData service description derived from the
schema of the SPARQL endpoint
– OData request converted to a SPARQL query
– SPARQL response converted to OData response
– Stateless
– Configured by an annotated OData schema
document
http://github.com/brightstardb/odata-sparql

14. Annotating OData Services
• Annotations are part of the OData spec.
• We define annotations for:
– URI namespace for entity primary keys
– URIs for entity types, properties and associations
– Direction of associations
• Annotations are made to the published
schema
• Allows consumer to reconstruct the source
triples

15. Implementation Issues
• DESCRIBE is not an option
• CONSTRUCTed graphs
– Sorting
– Identifying the entity resources
• Server-controlled paging for OData
• Paging requires sub-queries
• Entities require a common identifier prefix

16. Select a Single Film
OData
/Films(„Un_Chien_Andalou‟)
SPARQL
CONSTRUCT {
res:Un_Chien_Andalou ?p ?o .
} WHERE {
res:Un_Chien_Andalou ?p ?o .
}

17. Enumerate Films
OData
/Films
SPARQL
CONSTRUCT {
?v1 ?v1_p ?v1_o.
?v1 ods:variable-binding “v1”
} WHERE {
?v1 ?v1_p ?v1_o .
{
SELECT { ?v1 rdf:type o:Film . } LIMIT 100
}
}

18. Property Navigation
OData
/Films(„Un_Chien_Andalou‟)/Director
SPARQL
CONSTRUCT {
?v1 ?v1_p ?v1_o .
?v1 ods:variable-binding “v1” .
} WHERE {
?v1 ?v1_p ?v1_o .
SELECT {
?v1 WHERE {
res:Un_Chien_Andalou p:director ?v1 .
} LIMIT 100
}

19. Questions
• In discussing Open Data what is the
perceived importance of standards
interoperability ?
• If it is important, where does the
responsibility for interoperability lie ?

20. More Questions
• What could W3C Linked Data Platform
learn from OData and vice-versa ?
• Should these efforts be sharing use-cases ?

21. Final Question
• Is there a meta-model for an entity-
oriented view of data resources ?
One that both OData and LDP could share.