Creating APIs over RDF

Leigh Dodds @ldodds http://kasabi.com Creating APIs over RDF Data Sources 8 th June 2011. SemTech 2011

The Trouble with SPARQL

Try our SPARQL Endpoint! Run Query

http://dbpedia.org/sparql?default-graph-uri=http%3A%2F%2Fdbpedia.org&query=SELECT+%3Fresult+FROM+%3Chttp%3A%2F%2Fdbpedia.org%2Fresource%2FBMW_Z8%3E+WHERE+{+%3Chttp%3A%2F%2Fdbpedia.org%2Fresource%2FBMW_Z8%3E+dbpprop%3Aname+%3Fresult+.+FILTER+%28LANG%28%3Fresult%29+%3D+%27en%27%29+.}&debug=on&timeout=&format=text%2Fhtml&save=display&fname= Trial and error experiments aren't easy

APIs have Affordance They're like pulling a thread The URLs unravel into useful data

http://api.simplegeo.com/1.0/context/{lat},{lon}.json Trial and error experiments are easy

SPARQL is for Power Users

How can we give RDF datasets more affordance?

Linked Data? Typically not expressive enough to support all useful interactions Expensive to materialize all useful data partitions E.g. by date-time, location, top ranking, etc

Build Custom APIs? Write custom code against your graph store Can be expensive to build & maintain Undermines power of web of data

OData? Standard data API from Microsoft Very similar to RDF & Linked Data approach Can be implemented over SPARQL endpoint

SPARQL Stored Procedures & The Linked Data API

SPARQL “Stored Procedures”

Bind SPARQL query to a URL http://api.kasabi.com/api/find-recipe-ingredient?ingredient={x}

More than just a URL shortener 1. Identify query bound to URL 2. Inject parameters from URL & config 3. Apply paging/sorting options 4. Execute query 5. Optionally, apply post-processing transformations

Parameter Passing Optional and Required Parameters Mapping to a particular type of RDF Term, e.g. literal, URI May have a default value May have a default prefix, e.g. base URL

Post-Processing Named transformations that generate a specific mime-type Supports generating custom output formats Currently just XSLT. Could support JS

Stored Procedure Benefits Declarative approach Offers simpler interface to developers Retains RDF output, but supports custom formats

The Linked Data API

Linked Data API http://purl.org/linked-data/api/spec Jeni Tennison (TSO), Dave Reynolds (Epimorphics), Leigh Dodds

Linked Data API Map parameterised URLs to graph queries to extract data views

Linked Data API Optimize for common query patterns Prioritize simple RESTful interactions not complex queries Provide pathway to exploring RDF and SPARQL

Processing Model
Select
View
Request
Response
Format
SPARQL Endpoint
SELECT ?item WHERE { … }
Endpoint
DESCRIBE <x> <y>
API Config

Selecting Resources URL mapped to query patterns by API configuration Query variables injected from URL path or query string Simple path expressions for traversing graph Support for sorting and paging

Viewing Resources Default “view” is Concise Bounded Description Can specify arbitrary collections of properties in config or URL

Output Formats Simple JSON and XML formats Standard RDF serializations Custom formats, including HTML, via XSLT

@prefix spec: <http://api.kasabi.com/api/foodista-lda/> . @prefix api: <http://purl.org/linked-data/api/vocab#> . @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> . @prefix recipe: <http://linkedrecipes.org/schema/> . spec:api a api:API ; api:defaultPageSize "10" ; api:endpoint spec:foods . spec:foods a api:ListEndpoint ; api:uriTemplate "/foods" ; api:selector [ api:filter "type=Food" ]; api:defaultViewer api:describeViewer . rdf:type api:label "type" . recipe:Food api:label "Food" .

{ "format": "linked-data-api", "version": "0.2" "result": { "items": [ { "label": "Balsamic Vinegar", "isPrimaryTopicOf": "http://www.foodista.com/food/23HWFLXY "type": "http://linkedrecipes.org/schema/Food", "_about": "http://data.kasabi.com/dataset/foodista/food/23HWFLXY", "description": "Balsamic Vinegar is..." }, ... ], "modified": "Monday, 06-Jun-11 19:15:43 UTC", "itemsPerPage": 10, "startIndex": 0, "type": "http://purl.org/linked-data/api/vocab#Page", "first": "/api/foodista-linked-data-api/foods.json?_page=1", "_about": "/api/foodista-linked-data-api/foods.json?_page=1", "next": "/api/foodista-linked-data-api/foods.json?_page=2", ... }

Open Source Implementations Puelia (PHP) http://code.google.com/p/puelia-php/ Elda (Java) http://code.google.com/p/elda/

Linked Data API Summary Declarative configuration-driven approach More sophisticated than SSPs Integrates well with Linked Data publishing Slightly higher learning curve

Summary

Adopt hybrid approach to encourage adoption Combine Linked Data with SPARQL powered APIs Recognise need for bridging, adapting between tools/systems/communities

Support a “view source” approach Enable Crowd-Sourcing of APIs by sharing configuration & queries

Try It For Yourself! http://beta.kasabi.com Linked Data, RDF and API hosting Demo: 9.30 tomorrow

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Creating APIs over RDF

by Leigh Dodds on Jun 08, 2011

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