A few thoughts on RDF-star, Reification, and Labeled Property Graphs

The Amazon Neptune team is committed to lowering the barriers to the adoption of graph databases and graph-based computing. Our customers benefit when we reduce the conceptual and technological gap between RDF graphs and Labeled Property Graphs (LPGs). Over the last several years we have seen LPGs increase their popularity thanks to easy-to-understand and easy-to-use features, even when RDF offers more sophisticated features such as (for example) easy graph merging, federated queries, and expressive schema languages. The importance and relevance of interoperability between RDF and LPG was established several years ago at the W3C workshop on Web Standardization for Graph Data (Creating Bridges: RDF, Property Graph and SQL) [1]. While its origins are much older, the RDF-star Community Group was established in the wake of this event. We believe that improving the ability for RDF and LPG graphs to interoperate will benefit the entire graph community.

As we see it, the most critical outcomes of the work of the RDF-star working group should include:

  *   Efficient RDF support for “edge properties”, including the ability to have different property sets for otherwise identical edges (LPGs do not have the restriction RDF has where triples are unique in a graph).
  *   Simple and clear RDF support for statements about statements (supporting provenance mechanisms and other identified use cases).
  *   Laying the groundwork for interoperability “in the data” between RDF and LPG languages (e.g., a single database that can expose both LPG and RDF based query languages over the same data).

The alignment of features and capabilities between RDF and LPGs is possible if there are no fundamental incompatibilities between the two graph models. The RDF-star Working Group’s original goal, an easy mechanism for making “statements about statements”, would make the gap between the two models significantly smaller; statements about statements are a feature similar to “edge properties” in LPGs, the lack of which in RDF we often hear cited as the reason users choose LPGs. On the other hand, the current proposal the WG is entertaining, the “single reifier multiple triples” model, has no clear counterpart in LPGs, renders the two graph models even more different than they are today, adds significant complexity (there are more expressive alternatives with simpler semantics), and makes it even more difficult to understand RDF reification rather than offering a conceptually simple framework.

Limiting reifiers to single statements – and classifying scenarios with a single reifier for multiple statements as “non-well formed” – will bring the greatest benefit to the graph community at large. On the other hand, allowing a single reifier for multiple statements will make it very difficult to align the LPG and RDF models. Please see the examples below.

We strongly believe that the continued relevance of RDF depends on establishing interoperability with LPGs. As stated above, RDF brings some tremendous advantages, and we are committed to bringing these advantages to the community of LPG users as well. We believe that this reflects the spirit of the W3C workshop on Web Standardization for Graph Data and resonates with inputs from some other members of the working group.

[1] https://www.w3.org/Data/events/data-ws-2019/


Examples:

# (A) An LPG edge with a single edge property.
(s) - [p {ep: 1}] → (o)

# (B) An interpretation of that in an SPOI model (where I is a statement identifier).
# The OneGraph model is based on such SPOI tuples.
s p o :sid1
:sid1 ep 1 :sid2

# (C) An RDF-star expression consisting of an asserted triple and a statement about
# that.
:s :p :o {| :ep 1 |} # with an anonymous identifier for the (s p o) statement.

# (D) The RDF interpretation of that RDF-star expression.
:s :p :o . # The asserted triple.
_:b rdf:reifies <<( :s :p :o )>> . # A reifier for that triple.
_:b :ep 1 . # Using that reifier to make an assertion about a triple occurrence.

# Note that the LPG example (A), the SPOI interpretation (B), and the RDF model (D)
# can be handled as exactly the same data within possible database implementations
# such as proposed by Souri or by a OneGraph implementation.  The case where _:b is
# replaced by an IRI can also be handled under LPG, 1G, etc.

# Now, let us look at the case where different statements are assigned the same
# reifier:

# (E) Same reifier used in two expressions about different triples.
:s1 :p :o {| :b | :ep 1 |} # a statement about a statement with reifier ":b".
:s2 :p :o {| :b | :ep 2 |} # a statement about a different statement, same reifier.

# This last case (E) has no sensible intepretation under LPG.

# If we accept a constraint that using the same reifiers for different TripleTerms
# is not well-formed, then we can maintain a consistent interpretation with LPG edge
# properties.  Further, we can use explicit modeling to group statements and retain
# transparency about the functional or semantic roles in such groupings.

# (F) Two statements are being grouped by an explicit semantic relationship (:partOf).
:s1 :p :o {| :b1 | :ep 1 |}
:s2 :p :o {| :b2 | :ep 2 |}
:b1 :partOf :b
:b2 :partOf :b

# We submit that this explicit modeling is more useful and preserves the alignment
# with LPG and RDF which has such great value to the world wide graph community.


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Dr. Ora Lassila
Principal Technologist, Amazon Neptune