KiWi Triplestore

KiWi is a triplestore developed as part of the Apache Marmotta project, an Open Platform for Linked Data.

KiWi stores RDF as relational tables in a PostgreSQL database (can also use H2 or MySQL databases as backend). However, it supports the SPARQL query language, semantic reasoning, quads/named graphs, standard RDF serializations (e.g. Turtle), and transactions.

Relational structure

As stated above, in KiWi the RDF graph is represented via relational tables. For representing nodes in the graph (i.e. resources), it uses the following table:

nodes(
 id bigint PRIMARY KEY, 
 ntype nodetype, -- uri, bnode, string, int, double, timestamp, boolean
 svalue text, -- lexical value (e.g. "1" for 1)
 dvalue double precision,
 ivalue bigint,
 tvalue timestamp,
 tzoffset int,
 bvalue boolean,
 ltype bigint REFERENCES nodes(id), -- IRI denoting its type (xsd:int)
 lang varchar(5), -- language tag for strings
 createdat timestamp
)

All nodes are assigned a unique ID. If the node is a URI/IRI or a blank node, ntype equals uri or bnode, respectively. If the node is a literal, ntype is either string, int, double, timestamp or boolean, depending on the literals type. E.g. if it is an integer, ntype is int. Note that if a literal is non of these types, it will have ntype equal to string. The more concrete literal type (as added to a literal value in RDF, such as xsd:int in "1"^^xsd:int) is represented by the column ltype, which references a node with the corresponding types URI.

The column svalue contains the lexical/string representation of the node. The following columns, dvalue, ivalue, tvalue, and bvalue contain the value cast to the columns type, if it preserved the value and NULL otherwise. E.g. an integer 1 would have svalue equal to '1', ivalue equal to 1, a dvalue equal to 1.0, and NULL for all the other coumns. A double value 1.2 would have svalue equal to '1.2', dvalue equal to 1.2, and the NULL for the other columns (also ivalue equal to NULL, as 1 is not equal to the original value).

A timestamp value can store the timezone as an offset in the column tzoffset.

If the literal is a string with a language tag, this tag is put into the lang column.

Finally, the column createdat simply contains the timestamp for when the node was inserted into the database.

Triples/quads are represented in KiWi with the following table:

triples(
  id bigint PRIMARY KEY,
  subject bigint REFERENCES nodes(id),
  predicate bigint REFERENCES nodes(id),
  object bigint REFERENCES nodes(id),
  context bigint REFERENCES nodes(id),
  creator bigint REFERENCES nodes(id),
  inferred boolean,
  deleted boolean,
  createdat timestamp,
  deletedat timestamp
)

Each triple/quad is assigned a unique ID. The columns subject, predicate, and object contains the ID of the nodes that is the subject, predicate and object, respectively. If the triple is part of a named graph (called a context in KiWi), the column context contains the ID of the node with the named graph’s IRI.

The column creator contains a possible ID for a node denoting the creator of the triple/quad.

As stated above, KiWi supports reasoning, and the column inferred contains a boolean to state whether the triple/quad is inferred or not. The following column, deleted states whether the triple/quad should be considered deleted or not. If it is deleted, the column deletedat tracks the timestamp of the deletion. Similarly as nodes, the triple-table also contains a createdat column containing the timestamp the triple/quad was inseted.

If the triplestore is initialized via Toko, there will also be a view named triples_str that contains all triples in the triplestore with the svalue (lexical value) of the elements. This is convenient if one simply wants to see (as in read) all triples in the triplestore. The view contains the following columns:

triples_str (
    id bigint,
    subject text, 
    predicate text,
    object text,
    context text
)

Example

If we load the following RDF graph:

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
@prefix ex: <http://example.org/> .

ex:earth rdf:type ex:Planet ;
    rdfs:label "The Earth"@en ;
    ex:radius "6371.1"^^xsd:decimal ;
    ex:orbits [ rdfs:label "The sun" ] .

into a KiWi triplestore, we would get the following nodes-table:

 id | ntype  |                     svalue                      | dvalue | ivalue | tvalue | tzoffset | bvalue | ltype | lang |        createdat        
----+--------+-------------------------------------------------+--------+--------+--------+----------+--------+-------+------+-------------------------
  1 | uri    | http://www.w3.org/1999/02/22-rdf-syntax-ns#     |        |        |        |          |        |       |      | 2021-02-08 19:04:40.494
  2 | uri    | http://www.w3.org/2000/01/rdf-schema#           |        |        |        |          |        |       |      | 2021-02-08 19:04:40.503
  3 | uri    | http://www.w3.org/2001/XMLSchema#               |        |        |        |          |        |       |      | 2021-02-08 19:04:40.507
  5 | uri    | http://example.org/                             |        |        |        |          |        |       |      | 2021-02-08 19:04:40.511
  6 | uri    | http://example.org/earth                        |        |        |        |          |        |       |      | 2021-02-08 19:04:40.515
  7 | uri    | http://www.w3.org/1999/02/22-rdf-syntax-ns#type |        |        |        |          |        |       |      | 2021-02-08 19:04:40.518
  8 | uri    | http://example.org/Planet                       |        |        |        |          |        |       |      | 2021-02-08 19:04:40.521
  9 | uri    | http://localhost/context/default                |        |        |        |          |        |       |      | 2021-02-08 19:04:40.524
 10 | uri    | http://www.w3.org/2000/01/rdf-schema#label      |        |        |        |          |        |       |      | 2021-02-08 19:04:40.536
 11 | string | The Earth                                       |        |        |        |          |        |       | en   | 2021-02-08 19:04:40.565
 12 | uri    | http://example.org/radius                       |        |        |        |          |        |       |      | 2021-02-08 19:04:40.572
 13 | uri    | http://www.w3.org/2001/XMLSchema#decimal        |        |        |        |          |        |       |      | 2021-02-08 19:04:40.575
 14 | double | 6371.1                                          | 6371.1 |        |        |          |        |    13 |      | 2021-02-08 19:04:40.582
 15 | uri    | http://example.org/orbits                       |        |        |        |          |        |       |      | 2021-02-08 19:04:40.587
 16 | bnode  | 1778308138d26b                                  |        |        |        |          |        |       |      | 2021-02-08 19:04:40.589
 17 | string | The sun                                         |        |        |        |          |        |       |      | 2021-02-08 19:04:40.592

with the following triples-table:

 id | subject | predicate | object | context | creator | inferred | deleted |        createdat        | deletedat 
----+---------+-----------+--------+---------+---------+----------+---------+-------------------------+-----------
 18 |       6 |         7 |      8 |       9 |         | f        | f       | 2021-02-08 20:04:40.529 | 
 19 |       6 |        10 |     11 |       9 |         | f        | f       | 2021-02-08 20:04:40.57  | 
 20 |       6 |        12 |     14 |       9 |         | f        | f       | 2021-02-08 20:04:40.585 | 
 21 |      16 |        10 |     17 |       9 |         | f        | f       | 2021-02-08 20:04:40.594 | 
 22 |       6 |        15 |     16 |       9 |         | f        | f       | 2021-02-08 20:04:40.595 |

Reasoning

KiWi supports reasoning using a datalog-like rule language over triples, with truth maintenance and justifications. For more information, see the KiWi Reasoner page.

Known Bugs

The following is a list of bugs encountered while using the KiWi triplestore in IN5800: