RDF rank

What is RDF Rank

RDF Rank is an algorithm that identifies the more important or more popular entities in the repository by examining their interconnectedness. The popularity of entities can then be used to order the query results in a similar way to the internet search engines, the way Google orders search results using PageRank.

The RDF Rank component computes a numerical weighting for all nodes in the entire RDF graph stored in the repository, including URIs, blank nodes and literals. The weights are floating point numbers with values between 0 and 1 that can be interpreted as a measure of a node’s relevance/popularity.

_images/RDF_rank.png

Since the values range from 0 to 1, the weights can be used for sorting a result set (the lexicographical order works fine even if the rank literals are interpreted as plain strings).

Here is an example SPARQL query that uses the RDF rank for sorting results by their popularity:

PREFIX rank: <http://www.ontotext.com/owlim/RDFRank#>
PREFIX opencyc-en: <http://sw.opencyc.org/2008/06/10/concept/en/>
SELECT * WHERE {
  ?Person a opencyc-en:Entertainer .
  ?Person rank:hasRDFRank ?rank .
}
ORDER BY DESC(?rank) LIMIT 100

As seen in the example query, RDF Rank weights are made available via a special system predicate. GraphDB handles triple patterns with the predicate http://www.ontotext.com/owlim/RDFRank#hasRDFRank in a special way, where the object of the statement pattern is bound to a literal containing the RDF Rank of the subject.

In order to use this mechanism, the RDF ranks for the whole repository must be computed in advance. This is done by committing a series of SPARQL updates that use special vocabulary to parameterise the weighting algorithm, followed by an update that triggers the computation itself.

Parameters

Parameter Maximum iterations
Predicate http://www.ontotext.com/owlim/RDFRank#maxIterations
Description Sets the maximum number of iterations of the algorithm over all entities in the repository.
Default 20
Example
PREFIX rank: <http://www.ontotext.com/owlim/RDFRank#>
INSERT DATA { rank:maxIterations rank:setParam “16” . }
Parameter Epsilon
Predicate http://www.ontotext.com/owlim/RDFRank#epsilon
Description Terminates the weighting algorithm early when the total change of all RDF Rank scores has fallen below this value.
Default 0.01
Example
PREFIX rank: <http://www.ontotext.com/owlim/RDFRank#>
INSERT DATA { rank:epsilon rank:setParam “0.05” . }

Full computation

To trigger the computation of the RDF Rank values for all resources, use the following update:

PREFIX rank: <http://www.ontotext.com/owlim/RDFRank#>
INSERT DATA { _:b1 rank:compute _:b2. }

Incremental updates

The full computation of RDF Rank values for all resources can be relatively expensive. When new resources have been added to the repository after a previous full computation of the RDF Rank values, you can either have a full re-computation for all resources (see above) or compute only the RDF Rank values for the new resources (an incremental update).

The following control update:

PREFIX rank: <http://www.ontotext.com/owlim/RDFRank#>
INSERT DATA {_:b1 rank:computeIncremental "true"}

computes RDF Rank values for the resources that do not have an associated value, i.e., the ones that have been added to the repository since the last full RDF Rank computation.

Note

The incremental computation uses a different algorithm, which is lightweight (in order to be fast), but is not as accurate as the proper ranking algorithm. As a result, ranks assigned by the proper and the lightweight algorithms will be slightly different.

Exporting RDF Rank values

The computed weights can be exported to an external file using an update of this form:

PREFIX rank: <http://www.ontotext.com/owlim/RDFRank#>
INSERT DATA { _:b1 rank:export "/home/user1/rdf_ranks.txt" . }

If the export fails, the update throws an exception and an error message is recorded in the log file.