Skip to content

Latest commit

 

History

History
69 lines (56 loc) · 4.47 KB

README.md

File metadata and controls

69 lines (56 loc) · 4.47 KB
Raw

\page triples Triples

Triples are the basic building blocks of the RDF data model. A triple is a statement that consists of three parts: a subject, a predicate, and an object. The subject is the resource being described, the predicate is the property of the resource, and the object is the value of the property. For example, the triple ex:John ex:hasAge "25"^^xsd:integer states that the resource ex:John has the age of 25. A collection of triples forms an RDF graph, which is a set of subject-predicate-object triples.

While the term "triple" is widely used, most of the triple stores in fact support an additional fourth element, usually the "graph" name or "origin" of the triple, which is used to distinguish between different named graphs. In such a case, the triple is actually a quadruple and often referred to as a "quad".

However, in KnowRob, we consider additional context parameters as part of the triple itself. Such a triple with context parameters is referred to as FramedTriple within KnowRob. This is done to cope with uncertain statements, and statements that are only valid in a certain context (or frame). Nevertheless, KnowRob can make use of any quad store by internally reifying the contextualized triple into a set of triples without context parameters (see StorageInterface).

Context Parameters of Triples

The following table lists the context parameters that can be used in a FramedTriple:

Parameter Description
graph The graph or origin in which this triple is loaded (default: user).
perspective The perspective from which this triple is valid (default: ego).
uncertain A flag indicating if this triple is uncertain (default: false).
occasional A flag indicating if this triple is only occasionally true (default: false).
confidence The (normalized) confidence that this triple is true (default: 1.0).
begin The begin time of the validity of this triple (default: -inf).
end The end time of the validity of this triple (default: +inf).

Each of these parameters is optional in queries. Some additional comments on the context parameters:

  • A triple can be asserted into multiple origins.
  • The "perspective" parameter can be e.g. the IRI of another agent. The default case is the "ego perspective" which is the perspective of the agent that runs the knowledge base.

Triple Patterns

A triple pattern is a triple with some parts replaced by variables. For example, the triple pattern ?x ex:hasAge ?y matches all triples where the subject is a variable ?x and the predicate is ex:hasAge. Triple patterns are used in queries to match triples in the knowledge base. The query engine then binds the variables in the triple pattern to the actual values in the triples.

In KnowRob, triple patterns are represented by the TriplePattern class. It uses terms to represent the subject, predicate, object, and context parameters of the triple. Each of the terms can either be a variable or a constant term. In addition, a few comparison filters can be specified to restrict matching triples. Once a triple pattern is created, it can be used to query the knowledge base for matching triples. Given the Bindings obtained from the query, the triple pattern can be instantiated to a FramedTriple.

Graph Queries

More complex graph queries can be formed over triple patterns by introducing additional operators. Triple stores usually support connective operators UNION and PATH to combine multiple triple patterns. In KnowRob, the GraphQuery class is used to represent such more complex graph queries. It has the subtypes GraphUnion and GraphSequence to represent the UNION and PATH operators, respectively. In addition, the GraphBuiltin class is used to represent built-in operators that are not part of the RDF standard. The builtin operators that must be supported by backends include the BIND operator for variable assignment and the FILTER operator for filtering results. In addition, a MAX and MIN operator is needed that binds a variable to the maximum or minimum value of a set of values, respectively.

The operators listed above are mainly required to support the context parameters of FramedTriple with storage backends that cannot store them directly.