Unfortunately, RDF, the data model underlying Linked Data and the Semantic Web, has no built-in mechanism to attach data to its source. To some extent, this is a deliberate choice in the design of the model, and also a deep one. True facts can’t really have sources, so a knowledge representation system that includes connections of facts to their sources is, in a way, polluted. Instead, RDF takes the point of view that statements are asserted, and if you want to deal with assertions and how they are asserted in a clean logic system, the assertions should be reified. — https://blogs.library.duke.edu/dcthree/2013/07/27/the-trouble-with-triples/
Let’s say we have following RDF graph:
:Alice → foaf:knows → :Bob
Or as graph (not a real notation, just for visualisation purposes)
And we want to add metadata to this relation, for example to specify that Alice knows Bob since 2002.
It would be straightforward task in labeled property Graph: ,
:Alice - {label=foaf:knows, ex:since="2002"} → :Bob
But in RDF you need to use reification for this task. And there is more than one approach how to do reification
Standard reification
:id1 → ex:subject → :Alice
:id1 → ex:predicate → foaf:knows
:id1 → ex:object → :Bob
:id1 → ex:since → "2002"
Singleton properties
:Alice → ex:knows → :Bob
ex:knows → ex:singletonPropertyOf → foaf:knows
ex:knows → ex:since → "2002"
Companion properties (cpprop)
It suppose to be similar to singleton property, but with lower number of unique properties
:Alice → ex:knows.1 → :Bob
:Alice → ex:knows.1.SID → :id1
ex:knows.1.SID → rdf:idPropertyOf → ex:knows.1
ex:knows.1.SID → rdf:companionPropertyOf → foaf:knows
:id1 → ex:since → "2002"
Universal property graph
Read: A novel property graph model for knowledge representation on the Web
:Alice → ex:knows → :Bob
ex:knows → za:datatype → foaf:knows
ex:knows → ppi:1 → properties[ex:since → 2002]
N-ary relation
:Alice → foaf:knows → :id1
:id1 → ex:naryValue → :Bob
:id1 → ex:since → "2002"
Named graphs
Named graphs is extension of RDF. Each triple additionaly contains one more IRI value (which make it quad) - identifier of the graph. This allows to put more than one graph in one document. Can be used to preserve source (for provenance), to version graphs, etc. But as well can be used for reification.
In different notations graph id can be in head or tail position. If in the head position:
graph:id1 → :Alice → foaf:knows → :Bob
ex:meta → graph:id1 → ex:since → "2002"
If in the tail position:
:Alice → foaf:knows → :Bob → graph:id1
graph:id1 → ex:since → "2002" → ex:meta
Multilayer graph
Multilayer graph is extension of RDF-graph by adding unique id to each edge. This id can be used as node in the “next layer”. Neptune (1G) is implementation of multilayer graph
Neptune (1G)
Neptune is quad-store - triple store where each has unique id. The main saling point is that it can be used as RDF store and as property graph store.
:Alice → foaf:knows → :Bob → :id1
:id1 → ex:since → "2002" → :id2
RDF-star
RDF-star is an extension of RDF
(:Alice → ex:knows → :Bob) → ex:since → 2002
If we would assign blank node (as opaque id) to each triple it would look the same as approach above.
Links
- Reifying RDF: What Works Well With Wikidata?
- standard reification (sr) whereby an RDF resource is used to denote the triple itself, denoting its subject, predicate and object as attributes and allowing additional meta-information to be added.
- n-ary relations (nr) whereby an intermediate resource is used to denote the relationship, allowing it to be annotated with meta-information.
- singleton properties (sp) whereby a predicate unique to the statement is created, which can be linked to the high-level predicate indicating the relationship, and onto which can be added additional meta-information.
- Named Graphs (ng) whereby triples (or sets thereof) can be identified in a fourth field using, e.g., an IRI, onto which meta-information is added.
- Evaluation of metadata representations in RDF stores
- companion property (cpprop)
- n-ary relations (naryrel) 👆nr
- named graphs (ngraphs) 👆ng
- singleton properties (sgprop) 👆sp
- standard reification (stdreif) 👆sr
- the Blazegraph-specific Reification Done Right (rdr). Blazegraph implementation of RDF-star
- Benchmarking RDF Metadata Representations: Reification, Singleton Property and RDF-star
- RDF data model and its multi reification approaches: A comprehensive comparitive analysis
- Singleton properties
- Named Graphs
- RDF-star and SPARQL-star