Inference in Semantic Web

Inferencing in Semantic Web

In Semantic Web, using RDFS and OWL many things can be inferred based on facts that are stored in the RDF triple store

• This is used for Knowledge Discovery in Semantic Web
• All RDF statements or RDFS and OWL can be translated to First Order Logic and Descriptive Logic to facilitate inferencing
  • see Semantic Web Logics

Inferencing - a systematic process of adding new tuples to an RDF graph based on some patterns (rules)

• '’asserted triples’’ - RDF triples provided by some data source
• '’inferred triples’’ - new triples added by inference rules
• '’inference rules’’ - systematic patterns that define how and what to infer
• '’inference engine’’ - engine that does the inference

Motivating Example

Suppose you have a SPARQL query on your RDF graph

• you look for :RedDelicious apples
• you are doing it in the :Fruit section
• but the result is empty, because :RedDelicious is an :Apple, not a :Fruit
  • i.e. :RedDelicious a :Apple
  • and :Apple :subClassOf :Fruit

Possible solution:

• use SPARQL#Transitive Queries

SELECT ?item 
WHERE {
  ?class :subClassOf* :Fruit . 
  ?item a ?class . 
}

But users will have to do it each time

• alternatives?
• can have rules if $X$ subclass of $Y$, then $\forall x \in X: x \in Y$
• '’inferencing’’ - given some information we can determine related information - and consider that it’s also stored in our database
• so here we’d infer that if :RedDelicious is an :Apple, it’s also a :Fruit
• :Fruit is broader than :Apple, so :Fruit is a subclass of :Apple

Inferencing

The motivating example illustrates the ‘‘type propagation rule’’

• this is a part of the RDFS language: rdfs:subClassOf relation
• rule: $X$ rdfs:subClassOf $Y \Rightarrow $ every member of $X$ is also a member of $Y$

Reference rules of RDFS/OWL can be expressed using SPARQL Construct queries:

• this is a good way of describing rules

CONSTRUCT { ?r rdf:type ?B } 
WHERE {
  ?A rdfs:subClassOf ?B 
  ?r rdf:type ?A
}

Implementation Details

When the inference happens?

• Cached Inference:
  • inferred triples are stored along with asserted
  • risk an explosion of the triple store
  • also, change management is important - how to propagate changes and deletes
  • for deletes - same inferred tuple can be due to several facts, so need to be careful when deleting
• Just-In-Time Inference
  • To respond to queries only
  • no inferred triples retained

Sources

• Semantic Web for the Working Ontologist (book)
• XML and Web Technologies (UFRT)