Table of Contents

1 Introduction

The core of the SWI-Prolog package semweb is an efficient main-memory RDF store written in C that is tightly integrated with Prolog. It provides a fully logical predicate rdf/3 to query the RDF store efficiently by using multiple (currently 9) indexes. In addition, SWI-Prolog provides libraries for reading and writing XML/RDF and Turtle and a library that provides persistency using a combination of efficient binary snapshots and journals.

Below, we describe a few usage scenarios that guides the current design of this Prolog-based RDF store.

Application prototyping platform

Bundled with ClioPatria, the store is an efficient platform for prototyping a wide range of semantic web applications. Prolog, connected to the main-memory based store is a productive platform for writing application logic that can be made available through the SPARQL endpoint of ClioPatria, using an application specific API (typically based on JSON or XML) or as an HTML based end-user application. Prolog is more versatile than SPARQL, allows composing of the logic from small building blocks and does not suffer from the Object-relational impedance mismatch.

Data integration

The SWI-Prolog store is optimized for entailment on the rdfs:subPropertyOf relation. The rdfs:subPropertyOf relation is crucial for integrating data from multiple sources while preserving the original richness of the sources because integration can be achieved by defining the native properties as sub-properties of properties from a unifying schema such as Dublin Core.

Dynamic data

This RDF store is one of the few stores that is primarily based on backward reasoning. The big advantage of backward reasoning is that it can much easier deal with changes to the database because it does not have to take care of propagating the consequences. Backward reasoning reduces storage requirements. The price is more reasoning during querying. In many scenarios the extra reasoning using a main memory will outperform the fetching the precomputed results from external storage.

Prototyping reasoning systems

Reasoning systems, not necessarily limited to entailment reasoning, can be prototyped efficiently on the Prolog based store. This includes‘what-if’reasoning, which is supported by snapshot and transaction isolation. These features, together with the concurrent loading capabilities, make the platform well equiped to collect relevant data from large external stores for intensive reasoning. Finally, the TIPC package can be used to create networks of cooperating RDF based agents.

Streaming RDF

Transactions, snapshots, concurrent modifications and the database monitoring facilities (see rdf_monitor/2) make the platform well suited for prototyping systems that deal with streaming RDF data.

2 Scalability

Depending on the OS and further application restrictions, the SWI-Prolog RDF stores scales to about 15 million triples on 32-bit hardware. On 64-bit hardware, the scalability is limited by the amount of physical memory, allowing for approximately 4 million triples per gigabyte. The other limiting factor for practical use is the time required to load data and/or restore the database from the persistent file backup. Performance depends highly on hardware, concurrent performance and whether or not the data is spread over multiple (named) graphs that can be loaded in parallel. Restoring over 20 million triples per minute is feasible on medium hardware (Intel i7/2600 running Ubuntu 12.10).

3 Two RDF APIs

The current‘semweb’package provides two sets of interface predicates. The original set is described in section 3.3. The new API is described in section 3.1. The original API was designed when RDF was not yet standardised and did not yet support data types and language indicators. The new API is designed from the RDF 1.1 specification, introducing consistent naming and access to literals using the value space. The new API is currently defined on top of the old API, so both APIs can be mixed in a single application.

3.1 library(semweb/rdf11): The RDF database

The library(semweb/rdf11) provides a new interface to the SWI-Prolog RDF database based on the RDF 1.1 specification.

3.1.1 Query the RDF database

[nondet]rdf(?S, ?P, ?O)

[nondet]rdf(?S, ?P, ?O, ?G)

True if an RDF triple <S,P,O> exists, optionally in the graph G. The object O is either a resource (atom) or one of the terms listed below. The described types apply for the case where O is unbound. If O is instantiated it is converted according to the rules described with rdf_assert/3.

Triples consist of the following three terms:

• Blank nodes are encoded by atoms that start with‘_:`.
• IRIs appear in two notations:
  • Full IRIs are encoded by atoms that do not start with‘_:`. Specifically, an IRI term is not required to follow the IRI standard grammar.
  • Abbreviated IRI notation that allows IRI prefix aliases that are registered by rdf_register_prefix/[2,3] to be used. Their notation is Alias:Local, where Alias and Local are atoms. Each abbreviated IRI is expanded by the system to a full IRI.

• Literals appear in two notations:

  String @ Lang

  A language-tagged string, where String is a Prolog string and Lang is an atom.

  Value ^^ Type

  A type qualified literal. For unknown types, Value is a Prolog string. If type is known, the Prolog representations from the table below are used.

  Datatype IRI	Prolog term
  xsd:float	float
  xsd:double	float
  xsd:decimal	float (1)
  xsd:integer	integer
  XSD integer sub-types	integer
  xsd:boolean	true or false
  xsd:date	date(Y,M,D)
  xsd:dateTime	date_time(Y,M,D,HH,MM,SS) (2,3)
  xsd:gDay	integer
  xsd:gMonth	integer
  xsd:gMonthDay	month_day(M,D)
  xsd:gYear	integer
  xsd:gYearMonth	year_month(Y,M)
  xsd:time	time(HH,MM,SS) (2)

Notes:

(1) The current implementation of xsd:decimal values as floats is formally incorrect. Future versions of SWI-Prolog may introduce decimal as a subtype of rational.

(2) SS fields denote the number of seconds. This can either be an integer or a float.

(3) The date_time structure can have a 7th field that denotes the timezone offset in seconds as an integer.

In addition, a ground object value is translated into a properly typed RDF literal using rdf_canonical_literal/2.

There is a fine distinction in how duplicate statements are handled in rdf/[3,4]: backtracking over rdf/3 will never return duplicate triples that appear in multiple graphs. rdf/4 will return such duplicate triples, because their graph term differs.

S	is the subject term. It is either a blank node or IRI.
P	is the predicate term. It is always an IRI.
O	is the object term. It is either a literal, a blank node or IRI (except for true and false that denote the values of datatype XSD boolean).
G	is the graph term. It is always an IRI.

See also

- Triple pattern querying
- xsd_number_string/2 and xsd_time_string/3 are used to convert between lexical representations and Prolog terms.

[nondet]rdf_has(?S, +P, ?O)

[nondet]rdf_has(?S, +P, ?O, -RealP)

Similar to rdf/3 and rdf/4, but P matches all predicates that are defined as an rdfs:subPropertyOf of P. This predicate also recognises the predicate properties inverse_of and symmetric. See rdf_set_predicate/2.

[nondet]rdf_reachable(?S, +P, ?O)

[nondet]rdf_reachable(?S, +P, ?O, +MaxD, -D)

True when O can be reached from S using the transitive closure of P. The predicate uses (the internals of) rdf_has/3 and thus matches both rdfs:subPropertyOf and the inverse_of and symmetric predicate properties. The version rdf_reachable/5 maximizes the steps considered and returns the number of steps taken.

If both S and O are given, these predicates are semidet. The number of steps D is minimal because the implementation uses breadth first search.

Constraints on literal values

[semidet]{}(+Where)

[semidet]rdf_where(+Where)

Formulate constraints on RDF terms, notably literals. These are intended to be used as illustrated below. RDF constraints are pure: they may be placed before, after or inside a graph pattern and, provided the code contains no commit operations (!, ->), the semantics of the goal remains the same. Preferably, constraints are placed before the graph pattern as they often help the RDF database to exploit its literal indexes. In the example below, the database can choose between using the subject and/or predicate hash or the ordered literal table.

    { Date >= "2000-01-01"^^xsd:date },
    rdf(S, P, Date)

The following constraints are currently defined:

> , >=,==,=<,<

The comparison operators are defined between numbers (of any recognised type), typed literals of the same type and langStrings of the same language.

prefix(String, Pattern)

substring(String, Pattern)

word(String, Pattern)

like(String, Pattern)

icase(String, Pattern)

Text matching operators that act on both typed literals and langStrings.

lang_matches(Term, Pattern)

Demands a full RDF term (Text@Lang) or a plain Lang term to match the language pattern Pattern.

The predicates rdf_where/1 and {}/1 are identical. The rdf_where/1 variant is provided to avoid ambiguity in applications where {}/1 is used for other purposes. Note that it is also possible to write rdf11:{...}.

3.1.2 Enumerating and testing objects

Enumerating objects by role

[nondet]rdf_subject(?S)

True when S is a currently known subject, i.e. it appears in the subject position of some visible triple. The predicate is semidet if S is ground.

[nondet]rdf_predicate(?P)

True when P is a currently known predicate, i.e. it appears in the predicate position of some visible triple. The predicate is semidet if P is ground.

[nondet]rdf_object(?O)

True when O is a currently known object, i.e. it appears in the object position of some visible triple. If Term is ground, it is pre-processed as the object argument of rdf_assert/3 and the predicate is semidet.

[nondet]rdf_node(?T)

True when T appears in the subject or object position of a known triple, i.e., is a node in the RDF graph.

[nondet]rdf_graph(?Graph)

True when Graph is an existing graph.

Enumerating objects by type

[nondet]rdf_literal(?Term)

True if Term is a known literal. If Term is ground, it is pre-processed as the object argument of rdf_assert/3 and the predicate is semidet.

[nondet]rdf_bnode(?BNode)

True if BNode is a currently known blank node. The predicate is semidet if BNode is ground.

[nondet]rdf_iri(?IRI)

True if IRI is a current IRI. The predicate is semidet if IRI is ground.

[nondet]rdf_name(?Name)

True if Name is a current IRI or literal. The predicate is semidet if Name is ground.

[nondet]rdf_term(?Term)

True if Term appears in the RDF database. Term is either an IRI, literal or blank node and may appear in any position of any triple. If Term is ground, it is pre-processed as the object argument of rdf_assert/3 and the predicate is semidet.

Testing objects types

[semidet]rdf_is_iri(@IRI)

True if IRI is an RDF IRI term.

For performance reasons, this does not check for compliance to the syntax defined in RFC 3987. This checks whether the term is (1) an atom and (2) not a blank node identifier.

Success of this goal does not imply that the IRI is present in the database (see rdf_iri/1 for that).

[semidet]rdf_is_bnode(@Term)

True if Term is an RDF blank node identifier.

A blank node is represented by an atom that starts with _:.

Success of this goal does not imply that the blank node is present in the database (see rdf_bnode/1 for that).

For backwards compatibility, atoms that are represented with an atom that starts with __ are also considered to be a blank node.

[semidet]rdf_is_literal(@Term)

True if Term is an RDF literal term.

An RDF literal term is of the form String@LanguageTag or Value^^Datatype.

Success of this goal does not imply that the literal is well-formed or that it is present in the database (see rdf_literal/1 for that).

[semidet]rdf_is_name(@Term)

True if Term is an RDF Name, i.e., an IRI or literal.

Success of this goal does not imply that the name is well-formed or that it is present in the database (see rdf_name/1 for that).

[semidet]rdf_is_object(@Term)

True if Term can appear in the object position of a triple.

Success of this goal does not imply that the object term in well-formed or that it is present in the database (see rdf_object/1 for that).

Since any RDF term can appear in the object position, this is equaivalent to rdf_is_term/1.

[semidet]rdf_is_predicate(@Term)

True if Term can appear in the predicate position of a triple.

Success of this goal does not imply that the predicate term is present in the database (see rdf_predicate/1 for that).

Since only IRIs can appear in the predicate position, this is equivalent to rdf_is_iri/1.

[semidet]rdf_is_subject(@Term)

True if Term can appear in the subject position of a triple.

Only blank nodes and IRIs can appear in the subject position.

Success of this goal does not imply that the subject term is present in the database (see rdf_subject/1 for that).

Since blank nodes are represented by atoms that start with‘_:` and an IRIs are atoms as well, this is equivalent to atom(Term).

[semidet]rdf_is_term(@Term)

True if Term can be used as an RDF term, i.e., if Term is either an IRI, a blank node or an RDF literal.

Success of this goal does not imply that the RDF term is present in the database (see rdf_term/1 for that).

3.1.3 RDF literals

[det]rdf_canonical_literal(++In, -Literal)

Transform a relaxed literal specification as allowed for rdf_assert/3 into its canonical form. The following Prolog terms are translated:

Prolog Term	Datatype IRI
float	xsd:double
integer	xsd:integer
string	xsd:string
true or false	xsd:boolean
date(Y,M,D)	xsd:date
date_time(Y,M,D,HH,MM,SS)	xsd:dateTime
date_time(Y,M,D,HH,MM,SS,TZ)	xsd:dateTime
month_day(M,D)	xsd:gMonthDay
year_month(Y,M)	xsd:gYearMonth
time(HH,MM,SS)	xsd:time

For example:

?- rdf_canonical_literal(42, X).
X = 42^^'http://www.w3.org/2001/XMLSchema#integer'.

[det]rdf_lexical_form(++Literal, -Lexical:compound)

True when Lexical is the lexical form for the literal Literal. Lexical is of one of the forms below. The ntriples serialization is obtained by transforming String into a proper ntriples string using double quotes and escaping where needed and turning Type into a proper IRI reference.

• String^^Type
• String@Lang

[det]rdf_compare(-Diff, +Left, +Right)

True if the RDF terms Left and Right are ordered according to the comparison operator Diff. The ordering is defines as:

• Literal < BNode < IRI
• For literals
  • Numeric < non-numeric
  • Numeric literals are ordered by value. If both are equal, floats are ordered before integers.
  • Other data types are ordered lexicographically.

• BNodes and IRIs are ordered lexicographically.

Note that this ordering is a complete ordering of RDF terms that is consistent with the partial ordering defined by SPARQL.

Diff	is one of <, = or >

3.1.4 Accessing RDF graphs

[det]rdf_default_graph(-Graph)

[det]rdf_default_graph(-Old, +New)

Query/set the notion of the default graph. The notion of the default graph is local to a thread. Threads created inherit the default graph from their creator. See set_prolog_flag/2.

3.1.5 Modifying the RDF store

[det]rdf_assert(+S, +P, +O)

[det]rdf_assert(+S, +P, +O, +G)

Assert a new triple. If O is a literal, certain Prolog terms are translated to typed RDF literals. These conversions are described with rdf_canonical_literal/2.

If a type is provided using Value^^Type syntax, additional conversions are performed. All types accept either an atom or Prolog string holding a valid RDF lexical value for the type and xsd:float and xsd:double accept a Prolog integer.

[nondet]rdf_retractall(?S, ?P, ?O)

[nondet]rdf_retractall(?S, ?P, ?O, ?G)

Remove all matching triples from the database. Matching is performed using the same rules as rdf/3. The call does not instantiate any of its arguments.

rdf_create_bnode(--BNode)

Create a new BNode. A blank node is an atom starting with _:. Blank nodes generated by this predicate are of the form _:genid followed by a unique integer.

3.1.6 Accessing RDF collections

The following predicates are utilities to access RDF 1.1 collections. A collection is a linked list created from rdf:first and rdf:next triples, ending in rdf:nil.

[det]rdf_last(+RDFList, -Last)

True when Last is the last element of RDFList. Note that if the last cell has multiple rdf:first triples, this predicate becomes nondet.

[semidet]rdf_list(?RDFTerm)

True if RDFTerm is a proper RDF list. This implies that every node in the list has an rdf:first and rdf:rest property and the list ends in rdf:nil.

If RDFTerm is unbound, RDFTerm is bound to each maximal RDF list. An RDF list is maximal if there is no triple rdf(_, rdf:rest, RDFList).

[det]rdf_list(+RDFList, -PrologList)

True when PrologList represents the rdf:first objects for all cells in RDFList. Note that this can be non-deterministic if cells have multiple rdf:first or rdf:rest triples.

[nondet]rdf_length(+RDFList, -Length:nonneg)

True when Length is the number of cells in RDFList. Note that a list cell may have multiple rdf:rest triples, which makes this predicate non-deterministic. This predicate does not check whether the list cells have associated values (rdf:first). The list must end in rdf:nil.

[nondet]rdf_member(?Member, +RDFList)

True when Member is a member of RDFList

[nondet]rdf_nth0(?Index, +RDFList, ?X)

[nondet]rdf_nth1(?Index, +RDFList, ?X)

True when X is the Index-th element (0-based or 1-based) of RDFList. This predicate is deterministic if Index is given and the list has no multiple rdf:first or rdf:rest values.

[det]rdf_assert_list(+PrologList, ?RDFList)

[det]rdf_assert_list(+PrologList, ?RDFList, +Graph)

Create an RDF list from the given Prolog List. PrologList must be a proper Prolog list and all members of the list must be acceptable as object for rdf_assert/3. If RDFList is unbound and PrologList is not empty, rdf_create_bnode/1 is used to create RDFList.

[det]rdf_retract_list(+RDFList)

Retract the rdf:first, rdf:rest and rdf:type=rdf:'List’triples from all nodes reachable through rdf:rest. Note that other triples that exist on the nodes are left untouched.

3.2 library(semweb/rdf11_containers): RDF 1.1 Containers

author

- Wouter Beek
- Jan Wielemaker

version

2016/01

See also

http://www.w3.org/TR/2014/REC-rdf-schema-20140225/\#ch_containervocab

Compatibility

RDF 1.1

Implementation of the conventional human interpretation of RDF 1.1 containers.

RDF containers are open enumeration structures as opposed to RDF collections or RDF lists which are closed enumeration structures. The same resource may appear in a container more than once. A container may be contained in itself.

---

[nondet]rdf_alt(+Alt, ?Default, ?Others)

True when Alt is an instance of rdf:Alt with first member Default and remaining members Others.

Notice that this construct adds no machine-processable semantics but is conventionally used to indicate to a human reader that the numerical ordering of the container membership properties of Container is intended to only be relevant in distinguishing between the first and all non-first members.

Default denotes the default option to take when choosing one of the alternatives container in Container. Others denotes the non-default options that can be chosen from.

[det]rdf_assert_alt(?Alt, +Default, +Others:list)

[det]rdf_assert_alt(?Alt, +Default, +Others:list, +Graph)

Create an rdf:Alt with the given Default and Others. Default and the members of Others must be valid object terms for rdf_assert/3.

[nondet]rdf_bag(+Bag, -List:list)

True when Bag is an rdf:Bag and set is the set values related through container membership properties to Bag.

Notice that this construct adds no machine-processable semantics but is conventionally used to indicate to a human reader that the numerical ordering of the container membership properties of Container is intended to not be significant.

[det]rdf_assert_bag(?Bag, +Set:list)

[det]rdf_assert_bag(?Bag, +Set:list, +Graph)

Create an rdf:Bag from the given set of values. The members of Set must be valid object terms for rdf_assert/3.

[nondet]rdf_seq(+Seq, -List:list)

True when Seq is an instance of rdf:Seq and List is a list of associated values, ordered according to the container membership property used.

Notice that this construct adds no machine-processable semantics but is conventionally used to indicate to a human reader that the numerical ordering of the container membership properties of Container is intended to be significant.

[det]rdf_assert_seq(?Seq, +List)

[det]rdf_assert_seq(?Seq, +List, +Graph)

[nondet]rdfs_container(+Container, -List)

True when List is the list of objects attached to Container using a container membership property (rdf:_0, rdf:_1, ...). If multiple objects are connected to the Container using the same membership property, this predicate selects one value non-deterministically.

[nondet]rdfs_container_membership_property(?Property)

True when Property is a container membership property (rdf:_1, rdf:_2, ...).

[nondet]rdfs_container_membership_property(?Property, ?Number:nonneg)

True when Property is the Nth container membership property.

Success of this goal does not imply that Property is present in the database.

[nondet]rdfs_member(?Elem, ?Container)

True if rdf(Container, P, Elem) is true and P is a container membership property.

[nondet]rdfs_nth0(?N, ?Container, ?Elem)

True if rdf(Container, P, Elem) is true and P is the N-th (0-based) container membership property.

3.3 library(semweb/rdf_db): The RDF database

The central module of the RDF infrastructure is library(semweb/rdf_db). It provides storage and indexed querying of RDF triples. RDF data is stored as quintuples. The first three elements denote the RDF triple. The extra Graph and Line elements provide information about the origin of the triple.

The actual storage is provided by the foreign language (C) module. Using a dedicated C-based implementation we can reduce memory usage and improve indexing capabilities, for example by providing a dedicated index to support entailment over rdfs:subPropertyOf. Currently the following indexes are provided (S=subject, P=predicate, O=object, G=graph):

• S, P, O, SP, PO, SPO, G, SG, PG
• Predicates connected by rdfs:subPropertyOf are combined in a predicate cloud. The system causes multiple predicates in the cloud to share the same hash. The cloud maintains a 2-dimensional array that expresses the closure of all rdfs:subPropertyOf relations. This index supports rdf_has/3 to query a property and all its children efficiently.
• Additional indexes for predicates, resources and graphs allow enumerating these objects without duplicates. For example, using rdf_resource/1 we enumerate all resources in the database only once, while enumeration using e.g., (rdf(R,_,_);rdf(_,_,R)) normally produces many duplicate answers.
• Literal Objects are combined in a skip list after case normalization. This provides for efficient case-insensitive search, prefix and range search. The plugin library library(semweb/litindex) provides indexed search on tokens inside literals.

3.3.1 Query the RDF database

[nondet]rdf(?Subject, ?Predicate, ?Object)

Elementary query for triples. Subject and Predicate are atoms representing the fully qualified URL of the resource. Object is either an atom representing a resource or literal(Value) if the object is a literal value. If a value of the form NameSpaceID:LocalName is provided it is expanded to a ground atom using expand_goal/2. This implies you can use this construct in compiled code without paying a performance penalty. Literal values take one of the following forms:

Atom

If the value is a simple atom it is the textual representation of a string literal without explicit type or language qualifier.

lang(LangID, Atom)

Atom represents the text of a string literal qualified with the given language.

type(TypeID, Value)

Used for attributes qualified using the rdf:datatype TypeID. The Value is either the textual representation or a natural Prolog representation. See the option convert_typed_literal(:Convertor) of the parser. The storage layer provides efficient handling of atoms, integers (64-bit) and floats (native C-doubles). All other data is represented as a Prolog record.

For literal querying purposes, Object can be of the form literal(+Query, -Value), where Query is one of the terms below. If the Query takes a literal argument and the value has a numeric type numerical comparison is performed.

plain(+Text)

Perform exact match and demand the language or type qualifiers to match. This query is fully indexed.

icase(+Text)

Perform a full but case-insensitive match. This query is fully indexed.

exact(+Text)

Same as icase(Text). Backward compatibility.

substring(+Text)

Match any literal that contains Text as a case-insensitive substring. The query is not indexed on Object.

word(+Text)

Match any literal that contains Text delimited by a non alpha-numeric character, the start or end of the string. The query is not indexed on Object.

prefix(+Text)

Match any literal that starts with Text. This call is intended for completion. The query is indexed using the skip list of literals.

ge(+Literal)

Match any literal that is equal or larger than Literal in the ordered set of literals.

gt(+Literal)

Match any literal that is larger than Literal in the ordered set of literals.

eq(+Literal)

Match any literal that is equal to Literal in the ordered set of literals.

le(+Literal)

Match any literal that is equal or smaller than Literal in the ordered set of literals.

lt(+Literal)

Match any literal that is smaller than Literal in the ordered set of literals.

between(+Literal1, +Literal2)

Match any literal that is between Literal1 and Literal2 in the ordered set of literals. This may include both Literal1 and Literal2.

like(+Pattern)

Match any literal that matches Pattern case insensitively, where the‘*’character in Pattern matches zero or more characters.

Backtracking never returns duplicate triples. Duplicates can be retrieved using rdf/4. The predicate rdf/3 raises a type-error if called with improper arguments. If rdf/3 is called with a term literal(_) as Subject or Predicate object it fails silently. This allows for graph matching goals like rdf(S,P,O),rdf(O,P2,O2) to proceed without errors.

[nondet]rdf(?Subject, ?Predicate, ?Object, ?Source)

As rdf/3 but in addition query the graph to which the triple belongs. Unlike rdf/3, this predicate does not remove duplicates from the result set.

Source

is a term Graph:Line. If Source is instatiated, passing an atom is the same as passing Atom:_.

[nondet]rdf_has(?Subject, +Predicate, ?Object)

Succeeds if the triple rdf(Subject, Predicate, Object) is true exploiting the rdfs:subPropertyOf predicate as well as inverse predicates declared using rdf_set_predicate/2 with the inverse_of property.

[nondet]rdf_has(?Subject, +Predicate, ?Object, -RealPredicate)

Same as rdf_has/3, but RealPredicate is unified to the actual predicate that makes this relation true. RealPredicate must be Predicate or an rdfs:subPropertyOf Predicate. If an inverse match is found, RealPredicate is the term inverse_of(Pred).

[nondet]rdf_reachable(?Subject, +Predicate, ?Object)

Is true if Object can be reached from Subject following the transitive predicate Predicate or a sub-property thereof, while repecting the symetric(true) or inverse_of(P2) properties.

If used with either Subject or Object unbound, it first returns the origin, followed by the reachable nodes in breadth-first search-order. The implementation internally looks one solution ahead and succeeds deterministically on the last solution. This predicate never generates the same node twice and is robust against cycles in the transitive relation.

With all arguments instantiated, it succeeds deterministically if a path can be found from Subject to Object. Searching starts at Subject, assuming the branching factor is normally lower. A call with both Subject and Object unbound raises an instantiation error. The following example generates all subclasses of rdfs:Resource:

?- rdf_reachable(X, rdfs:subClassOf, rdfs:'Resource').
X = 'http://www.w3.org/2000/01/rdf-schema#Resource' ;
X = 'http://www.w3.org/2000/01/rdf-schema#Class' ;
X = 'http://www.w3.org/1999/02/22-rdf-syntax-ns#Property' ;
...

[nondet]rdf_reachable(?Subject, +Predicate, ?Object, +MaxD, -D)

Same as rdf_reachable/3, but in addition, MaxD limits the number of edges expanded and D is unified with the‘distance’between Subject and Object. Distance 0 means Subject and Object are the same resource. MaxD can be the constant infinite to impose no distance-limit.

3.3.2 Enumerating objects

The predicates below enumerate the basic objects of the RDF store. Most of these predicates also enumerate objects that are not associated to any currently visible triple. Objects are retained as long as they are visible in active queries or snapshots. After that, some are reclaimed by the RDF garbage collector, while others are never reclaimed.

[nondet]rdf_subject(?Resource)

True if Resource appears as a subject. This query respects the visibility rules implied by the logical update view.

See also

rdf_resource/1.

[nondet]rdf_resource(?Resource)

True when Resource is a resource used as a subject or object in a triple.

This predicate is primarily intended as a way to process all resources without processing resources twice. The user must be aware that some of the returned resources may not appear in any visible triple.

[nondet]rdf_current_predicate(?Predicate)

True when Predicate is a currently known predicate. Predicates are created if a triples is created that uses this predicate or a property of the predicate is set using rdf_set_predicate/2. The predicate may (no longer) have triples associated with it.

Note that resources that have rdf:type rdf:Property are not automatically included in the result-set of this predicate, while all resources that appear as the second argument of a triple are included.

See also

rdf_predicate_property/2.

[nondet]rdf_current_literal(-Literal)

True when Literal is a currently known literal. Enumerates each unique literal exactly once. Note that it is possible that the literal only appears in already deleted triples. Deleted triples may be locked due to active queries, transactions or snapshots or may not yet be reclaimed by the garbage collector.

[nondet]rdf_graph(?Graph)

True when Graph is an existing graph.

[nondet]rdf_current_ns(:Prefix, ?URI)

deprecated

Use rdf_current_prefix/2.

3.3.3 Modifying the RDF database

The predicates below modify the RDF store directly. In addition, data may be loaded using rdf_load/2 or by restoring a persistent database using rdf_attach_db/2. Modifications follow the Prolog logical update view semantics, which implies that modifications remain invisible to already running queries. Further isolation can be achieved using rdf_transaction/3.

[det]rdf_assert(+Subject, +Predicate, +Object)

Assert a new triple into the database. This is equivalent to rdf_assert/4 using Graph user. Subject and Predicate are resources. Object is either a resource or a term literal(Value). See rdf/3 for an explanation of Value for typed and language qualified literals. All arguments are subject to name-space expansion. Complete duplicates (including the same graph and‘line’and with a compatible‘lifespan’) are not added to the database.

[det]rdf_assert(+Subject, +Predicate, +Object, +Graph)

As rdf_assert/3, adding the predicate to the indicated named graph.

Graph

is either the name of a graph (an atom) or a term Graph:Line, where Line is an integer that denotes a line number.

[det]rdf_retractall(?Subject, ?Predicate, ?Object)

Remove all matching triples from the database. As rdf_retractall/4 using an unbound graph. See also rdf_retractall/4 and rdf_unload/1.

[det]rdf_retractall(?Subject, ?Predicate, ?Object, ?Graph)

As rdf_retractall/3, also matching Graph. This is particulary useful to remove all triples coming from a loaded file. See also rdf_unload/1.

[det]rdf_update(+Subject, +Predicate, +Object, ++Action)

[det]rdf_update(+Subject, +Predicate, +Object, +Graph, ++Action)

Replaces one of the three (four) fields on the matching triples depending on Action:

subject(Resource)

Changes the first field of the triple.

predicate(Resource)

Changes the second field of the triple.

object(Object)

Changes the last field of the triple to the given resource or literal(Value).

graph(Graph)

Moves the triple from its current named graph to Graph. This only works with rdf_update/5 and throws an error when used with rdf_update/4.

3.3.4 Update view, transactions and snapshots

The update semantics of the RDF database follows the conventional Prolog logical update view. In addition, the RDF database supports transactions and snapshots.

[semidet]rdf_transaction(:Goal)

Same as rdf_transaction(Goal, user, []). See rdf_transaction/3.

[semidet]rdf_transaction(:Goal, +Id)

Same as rdf_transaction(Goal, Id, []). See rdf_transaction/3.

[semidet]rdf_transaction(:Goal, +Id, +Options)

Run Goal in an RDF transaction. Compared to the ACID model, RDF transactions have the following properties:

1. Modifications inside the transactions become all atomically visible to the outside world if Goal succeeds or remain invisible if Goal fails or throws an exception. I.e., the atomicy property is fully supported.
2. Consistency is not guaranteed. Later versions may implement consistency constraints that will be checked serialized just before the actual commit of a transaction.
3. Concurrently executing transactions do not infuence each other. I.e., the isolation property is fully supported.
4. Durability can be activated by loading library(semweb/rdf_persistency).

Processed options are:

snapshot(+Snapshot)

Execute Goal using the state of the RDF store as stored in Snapshot. See rdf_snapshot/1. Snapshot can also be the atom true, which implies that an anonymous snapshot is created at the current state of the store. Modifications due to executing Goal are only visible to Goal.

[det]rdf_snapshot(-Snapshot)

Take a snapshot of the current state of the RDF store. Later, goals may be executed in the context of the database at this moment using rdf_transaction/3 with the snapshot option. A snapshot created outside a transaction exists until it is deleted. Snapshots taken inside a transaction can only be used inside this transaction.

[det]rdf_delete_snapshot(+Snapshot)

Delete a snapshot as obtained from rdf_snapshot/1. After this call, resources used for maintaining the snapshot become subject to garbage collection.

[nondet]rdf_active_transaction(?Id)

True if Id is the identifier of a transaction in the context of which this call is executed. If Id is not instantiated, backtracking yields transaction identifiers starting with the innermost nested transaction. Transaction identifier terms are not copied, need not be ground and can be instantiated during the transaction.

[nondet]rdf_current_snapshot(?Term)

True when Term is a currently known snapshot.

bug

Enumeration of snapshots is slow.

3.3.5 Type checking predicates

[semidet]rdf_is_resource(@Term)

True if Term is an RDF resource. Note that this is merely a type-test; it does not mean this resource is involved in any triple. Blank nodes are also considered resources.

See also

rdf_is_bnode/1

rdf_is_bnode(+Id)

Tests if a resource is a blank node (i.e. is an anonymous resource). A blank node is represented as an atom that starts with _:. For backward compatibility reason, __ is also considered to be a blank node.

See also

rdf_bnode/1.

[semidet]rdf_is_literal(@Term)

True if Term is an RDF literal object. Currently only checks for groundness and the literal functor.

3.3.6 Loading and saving to file

The RDF library can read and write triples in RDF/XML and a proprietary binary format. There is a plugin interface defined to support additional formats. The library(semweb/turtle) uses this plugin API to support loading Turtle files using rdf_load/2.

[det]rdf_load(+FileOrList)

Same as rdf_load(FileOrList, []). See rdf_load/2.

[det]rdf_load(+FileOrList, :Options)

Load RDF data. If this predicate is called a second time for the same file, it is by default treated as a no-op. See option =if(changed)=.

Options provides additional processing options. Defined options are:

blank_nodes(+ShareMode)

How to handle equivalent blank nodes. If share (default), equivalent blank nodes are shared in the same resource.

base_uri(+URI)

URI that is used for rdf:about="" and other RDF constructs that are relative to the base uri. Default is the source URL.

concurrent(+Jobs)

If FileOrList is a list of files, process the input files using Jobs threads concurrently. Default is the mininum of the number of cores and the number of inputs. Higher values can be useful when loading inputs from (slow) network connections. Using 1 (one) does not use separate worker threads.

format(+Format)

Specify the source format explicitly. Normally this is deduced from the filename extension or the mime-type. The core library understands the formats xml (RDF/XML) and triples (internal quick load and cache format). Plugins, such as library(semweb/turtle) extend the set of recognised extensions.

graph(?Graph)

Named graph in which to load the data. It is not allowed to load two sources into the same named graph. If Graph is unbound, it is unified to the graph into which the data is loaded. The default graph is a file:// URL when loading a file or, if the specification is a URL, its normalized version without the optional #fragment.

if(Condition)

When to load the file. One of true, changed (default) or not_loaded.

modified(-Modified)

Unify Modified with one of not_modified, cached(File), last_modified(Stamp) or unknown.

cache(Bool)

If false, do not use or create a cache file.

register_namespaces(Bool)

If true (default false), register xmlns namespace declarations or Turtle @prefix prefixes using rdf_register_prefix/3 if there is no conflict.

silent(+Bool)

If true, the message reporting completion is printed using level silent. Otherwise the level is informational. See also print_message/2.

prefixes(-Prefixes)

Returns the prefixes defined in the source data file as a list of pairs.

multifile Boolean+

Indicate that the addressed graph may be populated with triples from multiple sources. This disables caching and avoids that an rdf_load/2 call affecting the specified graph cleans the graph.

Other options are forwarded to process_rdf/3. By default, rdf_load/2 only loads RDF/XML from files. It can be extended to load data from other formats and locations using plugins. The full set of plugins relevant to support different formats and locations is below:

:- use_module(library(semweb/turtle)).        % Turtle and TriG
:- use_module(library(semweb/rdf_ntriples)).
:- use_module(library(semweb/rdf_zlib_plugin)).
:- use_module(library(semweb/rdf_http_plugin)).
:- use_module(library(http/http_ssl_plugin)).

See also

rdf_db:rdf_open_hook/3, library(semweb/rdf_persistency) and library(semweb/rdf_cache)

[det]rdf_unload(+Source)

Identify the graph loaded from Source and use rdf_unload_graph/1 to erase this graph.

deprecated

For compatibility, this predicate also accepts a graph name instead of a source specification. Please update your code to use rdf_unload_graph/1.

[det]rdf_save(+Out)

Same as rdf_save(Out, []). See rdf_save/2 for details.

[det]rdf_save(+Out, :Options)

Write RDF data as RDF/XML. Options is a list of one or more of the following options:

graph(+Graph)

Save only triples associated to the given named Graph.

anon(Bool)

If false (default true) do not save blank nodes that do not appear (indirectly) as object of a named resource.

base_uri(URI)

BaseURI used. If present, all URIs that can be represented relative to this base are written using their shorthand. See also write_xml_base option.

convert_typed_literal(:Convertor)

Call Convertor(-Type, -Content, +RDFObject), providing the opposite for the convert_typed_literal option of the RDF parser.

document_language(+Lang)

Initial xml:lang saved with rdf:RDF element.

encoding(Encoding)

Encoding for the output. Either utf8 or iso_latin_1.

inline(+Bool)

If true (default false), inline resources when encountered for the first time. Normally, only bnodes are handled this way.

namespaces(+List)

Explicitly specify saved namespace declarations. See rdf_save_header/2 option namespaces for details.

sorted(+Boolean)

If true (default false), emit subjects sorted on the full URI. Useful to make file comparison easier.

write_xml_base(Bool)

If false, do not include the xml:base declaration that is written normally when using the base_uri option.

xml_attributes(+Bool)

If false (default true), never use xml attributes to save plain literal attributes, i.e., always used an XML element as in <name>Joe</name>.

Out	Location to save the data. This can also be a file-url (file://path) or a stream wrapped in a term stream(Out).

See also

rdf_save_db/1

rdf_make

Reload all loaded files that have been modified since the last time they were loaded.

Partial save

Sometimes it is necessary to make more arbitrary selections of material to be saved or exchange RDF descriptions over an open network link. The predicates in this section provide for this. Character encoding issues are derived from the encoding of the Stream, providing support for utf8, iso_latin_1 and ascii.

rdf_save_header(+Fd, +Options)

Save XML document header, doctype and open the RDF environment. This predicate also sets up the namespace notation.

Save an RDF header, with the XML header, DOCTYPE, ENTITY and opening the rdf:RDF element with appropriate namespace declarations. It uses the primitives from section 3.5 to generate the required namespaces and desired short-name. Options is one of:

graph(+URI)

Only search for namespaces used in triples that belong to the given named graph.

namespaces(+List)

Where List is a list of namespace abbreviations. With this option, the expensive search for all namespaces that may be used by your data is omitted. The namespaces rdf and rdfs are added to the provided List. If a namespace is not declared, the resource is emitted in non-abreviated form.

[det]rdf_save_footer(Out:stream)

Finish XML generation and write the document footer.

See also

rdf_save_header/2, rdf_save_subject/3.

[det]rdf_save_subject(+Out, +Subject:resource, +Options)

Save the triples associated to Subject to Out. Options:

graph(+Graph)

Only save properties from Graph.

base_uri(+URI)

convert_typed_literal(:Goal)

document_language(+XMLLang)

See also

rdf_save/2 for a description of these options.

Fast loading and saving

Loading and saving RDF format is relatively slow. For this reason we designed a binary format that is more compact, avoids the complications of the RDF parser and avoids repetitive lookup of (URL) identifiers. Especially the speed improvement of about 25 times is worth-while when loading large databases. These predicates are used for caching by rdf_load/2 under certain conditions as well as for maintaining persistent snapshots of the database using library(semweb/rdf_persistency).

[det]rdf_save_db(+File)

[det]rdf_save_db(+File, +Graph)

Save triples into File in a quick-to-load binary format. If Graph is supplied only triples flagged to originate from that database are added. Files created this way can be loaded using rdf_load_db/1.

[det]rdf_load_db(+File)

Load triples from a file created using rdf_save_db/2.

3.3.7 Graph manipulation

Many RDF stores turned triples into quadruples. This store is no exception, initially using the 4th argument to store the filename from which the triple was loaded. Currently, the 4th argument is the RDF named graph. A named graph maintains some properties, notably to track origin, changes and modified state.

[det]rdf_create_graph(+Graph)

Create an RDF graph without triples. Succeeds silently if the graph already exists.

[det]rdf_unload_graph(+Graph)

Remove Graph from the RDF store. Succeeds silently if the named graph does not exist.

[nondet]rdf_graph_property(?Graph, ?Property)

True when Property is a property of Graph. Defined properties are:

hash(Hash)

Hash is the (MD5-)hash for the content of Graph.

modified(Boolean)

True if the graph is modified since it was loaded or rdf_set_graph/2 was called with modified(false).

source(Source)

The graph is loaded from the Source (a URL)

source_last_modified(?Time)

Time is the last-modified timestamp of Source at the moment the graph was loaded from Source.

triples(Count)

True when Count is the number of triples in Graph.

Additional graph properties can be added by defining rules for the multifile predicate property_of_graph/2. Currently, the following extensions are defined:

• library(semweb/rdf_persistency)

  persistent(Boolean)

  Boolean is true if the graph is persistent.

[det]rdf_set_graph(+Graph, +Property)

Set properties of Graph. Defined properties are:

modified(false)

Set the modified state of Graph to false.

3.3.8 Literal matching and indexing

Literal values are ordered and indexed using a skip list. The aim of this index is threefold.

• Unlike hash-tables, binary trees allow for efficient prefix and range matching. Prefix matching is useful in interactive applications to provide feedback while typing such as auto-completion.
• Having a table of unique literals we generate creation and destruction events (see rdf_monitor/2). These events can be used to maintain additional indexing on literals, such as‘by word’. See library(semweb/litindex).

As string literal matching is most frequently used for searching purposes, the match is executed case-insensitive and after removal of diacritics. Case matching and diacritics removal is based on Unicode character properties and independent from the current locale. Case conversion is based on the‘simple uppercase mapping’defined by Unicode and diacritic removal on the‘decomposition type’. The approach is lightweight, but somewhat simpleminded for some languages. The tables are generated for Unicode characters upto 0x7fff. For more information, please check the source-code of the mapping-table generator unicode_map.pl available in the sources of this package.

Currently the total order of literals is first based on the type of literal using the ordering numeric < string < term Numeric values (integer and float) are ordered by value, integers preceed floats if they represent the same value. Strings are sorted alphabetically after case-mapping and diacritic removal as described above. If they match equal, uppercase preceeds lowercase and diacritics are ordered on their unicode value. If they still compare equal literals without any qualifier preceeds literals with a type qualifier which preceeds literals with a language qualifier. Same qualifiers (both type or both language) are sorted alphabetically.

The ordered tree is used for indexed execution of literal(prefix(Prefix), Literal) as well as literal(like(Like), Literal) if Like does not start with a‘*’. Note that results of queries that use the tree index are returned in alphabetical order.

3.3.9 Predicate properties

The predicates below form an experimental interface to provide more reasoning inside the kernel of the rdb_db engine. Note that symetric, inverse_of and transitive are not yet supported by the rest of the engine. Also note that there is no relation to defined RDF properties. Properties that have no triples are not reported by this predicate, while predicates that are involved in triples do not need to be defined as an instance of rdf:Property.

[det]rdf_set_predicate(+Predicate, +Property)

Define a property of the predicate. This predicate currently supports the following properties:

symmetric(+Boolean)

Set/unset the predicate as being symmetric. Using symmetric(true) is the same as inverse_of(Predicate), i.e., creating a predicate that is the inverse of itself.

transitive(+Boolean)

Sets the transitive property.

inverse_of(+Predicate2)

Define Predicate as the inverse of Predicate2. An inverse relation is deleted using inverse_of([]).

The transitive property is currently not used. The symmetric and inverse_of properties are considered by rdf_has/3,4 and rdf_reachable/3.

To be done

Maintain these properties based on OWL triples.

rdf_predicate_property(?Predicate, ?Property)

Query properties of a defined predicate. Currently defined properties are given below.

symmetric(Bool)

True if the predicate is defined to be symetric. I.e., {A} P {B} implies {B} P {A}. Setting symmetric is equivalent to inverse_of(Self).

inverse_of(Inverse)

True if this predicate is the inverse of Inverse. This property is used by rdf_has/3, rdf_has/4, rdf_reachable/3 and rdf_reachable/5.

transitive(Bool)

True if this predicate is transitive. This predicate is currently not used. It might be used to make rdf_has/3 imply rdf_reachable/3 for transitive predicates.

triples(Triples)

Unify Triples with the number of existing triples using this predicate as second argument. Reporting the number of triples is intended to support query optimization.

rdf_subject_branch_factor(-Float)

Unify Float with the average number of triples associated with each unique value for the subject-side of this relation. If there are no triples the value 0.0 is returned. This value is cached with the predicate and recomputed only after substantial changes to the triple set associated to this relation. This property is intended for path optimalisation when solving conjunctions of rdf/3 goals.

rdf_object_branch_factor(-Float)

Unify Float with the average number of triples associated with each unique value for the object-side of this relation. In addition to the comments with the rdf_subject_branch_factor property, uniqueness of the object value is computed from the hash key rather than the actual values.

rdfs_subject_branch_factor(-Float)

Same as rdf_subject_branch_factor, but also considering triples of‘subPropertyOf’this relation. See also rdf_has/3.

rdfs_object_branch_factor(-Float)

Same as rdf_object_branch_factor, but also considering triples of‘subPropertyOf’this relation. See also rdf_has/3.

See also

rdf_set_predicate/2.

3.3.10 Prefix Handling

Prolog code often contains references to constant resources with a known prefix (also known as XML namespaces). For example, http://www.w3.org/2000/01/rdf-schema#Class refers to the most general notion of an RDFS class. Readability and maintability concerns require for abstraction here. The RDF database maintains a table of known prefixes. This table can be queried using rdf_current_ns/2 and can be extended using rdf_register_ns/3. The prefix database is used to expand prefix:local terms that appear as arguments to calls which are known to accept a resource. This expansion is achieved by Prolog preprocessor using expand_goal/2.

[nondet]rdf_current_prefix(:Alias, ?URI)

Query predefined prefixes and prefixes defined with rdf_register_prefix/2 and local prefixes defined with rdf_prefix/2. If Alias is unbound and one URI is the prefix of another, the longest is returned first. This allows turning a resource into a prefix/local couple using the simple enumeration below. See rdf_global_id/2.

rdf_current_prefix(Prefix, Expansion),
atom_concat(Expansion, Local, URI),

[det]rdf_register_prefix(+Prefix, +URI)

[det]rdf_register_prefix(+Prefix, +URI, +Options)

Register Prefix as an abbreviation for URI. Options:

force(Boolean)

If true, replace existing namespace alias. Please note that replacing a namespace is dangerous as namespaces affect preprocessing. Make sure all code that depends on a namespace is compiled after changing the registration.

keep(Boolean)

If true and Alias is already defined, keep the original binding for Prefix and succeed silently.

Without options, an attempt to redefine an alias raises a permission error.

Predefined prefixes are:

Alias	IRI prefix
dc	http://purl.org/dc/elements/1.1/
dcterms	http://purl.org/dc/terms/
eor	http://dublincore.org/2000/03/13/eor\#
foaf	http://xmlns.com/foaf/0.1/
owl	http://www.w3.org/2002/07/owl\#
rdf	http://www.w3.org/1999/02/22-rdf-syntax-ns\#
rdfs	http://www.w3.org/2000/01/rdf-schema\#
serql	http://www.openrdf.org/schema/serql\#
skos	http://www.w3.org/2004/02/skos/core\#
void	http://rdfs.org/ns/void\#
xsd	http://www.w3.org/2001/XMLSchema\#

Explicit expansion is achieved using the predicates below. The predicate rdf_equal/2 performs this expansion at compile time, while the other predicates do it at runtime.

rdf_equal(?Resource1, ?Resource2)

Simple equality test to exploit goal-expansion.

[semidet]rdf_global_id(?IRISpec, :IRI)

Convert between Prefix:Local and full IRI (an atom). If IRISpec is an atom, it is simply unified with IRI. This predicate fails silently if IRI is an RDF literal.

Note that this predicate is a meta-predicate on its output argument. This is necessary to get the module context while the first argument may be of the form (:)/2. The above mode description is correct, but should be interpreted as (?,?).

Errors

existence_error(rdf_prefix, Prefix)

See also

- rdf_equal/2 provides a compile time alternative
- The rdf_meta/1 directive asks for compile time expansion of arguments.

bug

Error handling is incomplete. In its current implementation the same code is used for compile-time expansion and to facilitate runtime conversion and checking. These use cases have different requirements.

[semidet]rdf_global_object(+Object, :GlobalObject)

[semidet]rdf_global_object(-Object, :GlobalObject)

Same as rdf_global_id/2, but intended for dealing with the object part of a triple, in particular the type for typed literals. Note that the predicate is a meta-predicate on the output argument. This is necessary to get the module context while the first argument may be of the form (:)/2.

Errors

existence_error(rdf_prefix, Prefix)

[det]rdf_global_term(+TermIn, :GlobalTerm)

Performs rdf_global_id/2 on predixed IRIs and rdf_global_object/2 on RDF literals, by recursively analysing the term. Note that the predicate is a meta-predicate on the output argument. This is necessary to get the module context while the first argument may be of the form (:)/2.

Terms of the form Prefix:Local that appear in TermIn for which Prefix is not defined are not replaced. Unlike rdf_global_id/2 and rdf_global_object/2, no error is raised.

Namespace handling for custom predicates

If we implement a new predicate based on one of the predicates of the semweb libraries that expands namespaces, namespace expansion is not automatically available to it. Consider the following code computing the number of distinct objects for a certain property on a certain object.

cardinality(S, P, C) :-
      (   setof(O, rdf_has(S, P, O), Os)
      ->  length(Os, C)
      ;   C = 0
      ).

Now assume we want to write labels/2 that returns the number of distict labels of a resource:

labels(S, C) :-
      cardinality(S, rdfs:label, C).

This code will not work because rdfs:label is not expanded at compile time. To make this work, we need to add an rdf_meta/1 declaration.

:- rdf_meta
      cardinality(r,r,-).

• [rdf_meta/1]
  

The example below defines the rule concept/1.

:- use_module(library(semweb/rdf_db)).  % for rdf_meta
:- use_module(library(semweb/rdfs)).    % for rdfs_individual_of

:- rdf_meta
        concept(r).

%%      concept(?C) is nondet.
%
%       True if C is a concept.

concept(C) :-
        rdfs_individual_of(C, skos:'Concept').

In addition to expanding calls, rdf_meta/1 also causes expansion of clause heads for predicates that match a declaration. This is typically used write Prolog statements about resources. The following example produces three clauses with expanded (single-atom) arguments:

:- use_module(library(semweb/rdf_db)).

:- rdf_meta
        label_predicate(r).

label_predicate(rdfs:label).
label_predicate(skos:prefLabel).
label_predicate(skos:altLabel).

3.3.11 Miscellaneous predicates

This section describes the remaining predicates of the library(semweb/rdf_db) module.

rdf_bnode(-Id)

Generate a unique anonymous identifier for a subject.

[nondet]rdf_source_location(+Subject, -Location)

True when triples for Subject are loaded from Location.

Location

is a term File:Line.

[det]rdf_generation(-Generation)

True when Generation is the current generation of the database. Each modification to the database increments the generation. It can be used to check the validity of cached results deduced from the database. Committing a non-empty transaction increments the generation by one.

When inside a transaction, Generation is unified to a term TransactionStartGen + InsideTransactionGen. E.g., 4+3 means that the transaction was started at generation 4 of the global database and we have created 3 new generations inside the transaction. Note that this choice of representation allows for comparing generations using Prolog arithmetic. Comparing a generation in one transaction with a generation in another transaction is meaningless.

rdf_estimate_complexity(?Subject, ?Predicate, ?Object, -Complexity)

Return the number of alternatives as indicated by the database internal hashed indexing. This is a rough measure for the number of alternatives we can expect for an rdf_has/3 call using the given three arguments. When called with three variables, the total number of triples is returned. This estimate is used in query optimisation. See also rdf_predicate_property/2 and rdf_statistics/1 for additional information to help optimizers.

[nondet]rdf_statistics(?KeyValue)

Obtain statistics on the RDF database. Defined statistics are:

graphs(-Count)

Number of named graphs.

triples(-Count)

Total number of triples in the database. This is the number of asserted triples minus the number of retracted ones. The number of visible triples in a particular context may be different due to visibility rules defined by the logical update view and transaction isolation.

resources(-Count)

Number of resources that appear as subject or object in a triple. See rdf_resource/1.

properties(-Count)

Number of current predicates. See rdf_current_predicate/1.

literals(-Count)

Number of current literals. See rdf_current_literal/1.

gc(GCCount, ReclaimedTriples, ReindexedTriples, Time)

Information about the garbage collector.

searched_nodes(-Count)

Number of nodes expanded by rdf_reachable/3 and rdf_reachable/5.

lookup(rdf(S,P,O,G), Count)

Number of queries that have been performed for this particular instantiation pattern. Each of S,P,O,G is either + or -. Fails in case the number of performed queries is zero.

hash_quality(rdf(S,P,O,G), Buckets, Quality, PendingResize)

Statistics on the index for this pattern. Indices are created lazily on the first relevant query.

triples_by_graph(Graph, Count)

This statistics is produced for each named graph. See triples for the interpretation of this value.

[semidet]rdf_match_label(+How, +Pattern, +Label)

True if Label matches Pattern according to How. How is one of icase, substring, word, prefix or like. For backward compatibility, exact is a synonym for icase.

[semidet]lang_matches(+Lang, +Pattern)

True if Lang matches Pattern. This implements XML language matching conform RFC 4647. Both Lang and Pattern are dash-separated strings of identifiers or (for Pattern) the wildcard *. Identifiers are matched case-insensitive and a * matches any number of identifiers. A short pattern is the same as *.

[semidet]lang_equal(+Lang1, +Lang2)

True if two RFC language specifiers denote the same language

See also

lang_matches/2.

rdf_reset_db

Remove all triples from the RDF database and reset all its statistics.

bug

This predicate checks for active queries, but this check is not properly synchronized and therefore the use of this predicate is unsafe in multi-threaded contexts. It is mainly used to run functionality tests that need to start with an empty database.

[det]rdf_version(-Version)

True when Version is the numerical version-id of this library. The version is computed as

Major*10000 + Minor*100 + Patch.

3.3.12 Memory management considerations

Storing RDF triples in main memory provides much better performance than using external databases. Unfortunately, although memory is fairly cheap these days, main memory is severely limited when compared to disks. Memory usage breaks down to the following categories. Rough estimates of the memory usage is given for 64-bit systems. 32-bit system use slightly more than half these amounts.

• Actually storing the triples. A triple is stored in a C struct of 144 bytes. This struct both holds the quintuple, some bookkeeping information and the 10 next-pointers for the (max) to hash tables.
• The bucket array for the hashes. Each bucket maintains a head, and tail pointer, as well as a count for the number of entries. The bucket array is allocated if a particular index is created, which implies the first query that requires the index. Each bucket requires 24 bytes.

  Bucket arrays are resized if necessary. Old triples remain at their original location. This implies that a query may need to scan multiple buckets. The garbage collector may relocate old indexed triples. It does so by copying the old triple. The old triple is later reclaimed by GC. Reindexed triples will be reused, but many reindexed triples may result in a significant memory fragmentation.

• Resources are maintained in a separate table to support rdf_resource/1. A resources requires approximately 32 bytes.
• Identical literals are shared (see rdf_current_literal/1) and stored in a skip list. A literal requires approximately 40 bytes, excluding the atom used for the lexical representation.
• Resources are stored in the Prolog atom-table. Atoms with the average length of a resource require approximately 88 bytes.

The hash parameters can be controlled with rdf_set/1. Applications that are tight on memory and for which the query characteristics are more or less known can optimize performance and memory by fixing the hash-tables. By fixing the hash-tables we can tailor them to the frequent query patterns, we avoid the need for to check multiple hash buckets (see above) and we avoid memory fragmentation due to optimizing triples for resized hashes.

set_hash_parameters :-
      rdf_set(hash(s,   size, 1048576)),
      rdf_set(hash(p,   size, 1024)),
      rdf_set(hash(sp,  size, 2097152)),
      rdf_set(hash(o,   size, 1048576)),
      rdf_set(hash(po,  size, 2097152)),
      rdf_set(hash(spo, size, 2097152)),
      rdf_set(hash(g,   size, 1024)),
      rdf_set(hash(sg,  size, 1048576)),
      rdf_set(hash(pg,  size, 2048)).

[det]rdf_set(+Term)

Set properties of the RDF store. Currently defines:

hash(+Hash, +Parameter, +Value)

Set properties for a triple index. Hash is one of s, p, sp, o, po, spo, g, sg or pg. Parameter is one of:

size

Value defines the number of entries in the hash-table. Value is rounded down to a power of 2. After setting the size explicitly, auto-sizing for this table is disabled. Setting the size smaller than the current size results in a permission_error exception.

average_chain_len

Set maximum average collision number for the hash.

optimize_threshold

Related to resizing hash-tables. If 0, all triples are moved to the new size by the garbage collector. If more then zero, those of the last Value resize steps remain at their current location. Leaving cells at their current location reduces memory fragmentation and slows down access.

The garbage collector

The RDF store has a garbage collector that runs in a separate thread named =__rdf_GC=. The garbage collector removes the following objects:

• Triples that have died before the the generation of last still active query.
• Entailment matrices for rdfs:subPropertyOf relations that are related to old queries.

In addition, the garbage collector reindexes triples associated to the hash-tables before the table was resized. The most recent resize operation leads to the largest number of triples that require reindexing, while the oldest resize operation causes the largest slowdown. The parameter optimize_threshold controlled by rdf_set/1 can be used to determine the number of most recent resize operations for which triples will not be reindexed. The default is 2.

Normally, the garbage collector does it job in the background at a low priority. The predicate rdf_gc/0 can be used to reclaim all garbage and optimize all indexes.Warming up the database

The RDF store performs many operations lazily or in background threads. For maximum performance, perform the following steps:

• Load all the data without doing queries or retracting data in between. This avoids creating the indexes and therefore the need to resize them.
• Perform each of the indexed queries. The following call performs this. Note that it is irrelevant whether or not the query succeeds.

  warm_indexes :-
      ignore(rdf(s, _, _)),
      ignore(rdf(_, p, _)),
      ignore(rdf(_, _, o)),
      ignore(rdf(s, p, _)),
      ignore(rdf(_, p, o)),
      ignore(rdf(s, p, o)),
      ignore(rdf(_, _, _, g)),
      ignore(rdf(s, _, _, g)),
      ignore(rdf(_, p, _, g)).

• Duplicate adminstration is initialized in the background after the first call that returns a significant amount of duplicates. Creating the adminstration can be forced by calling rdf_update_duplicates/0.

Predicates:

[det]rdf_gc

Run the RDF-DB garbage collector until no garbage is left and all tables are fully optimized. Under normal operation a separate thread with identifier __rdf_GC performs garbage collection as long as it is considered‘useful’.

Using rdf_gc/0 should only be needed to ensure a fully clean database for analysis purposes such as leak detection.

[det]rdf_update_duplicates

Update the duplicate administration of the RDF store. This marks every triple that is potentionally a duplicate of another as duplicate. Being potentially a duplicate means that subject, predicate and object are equivalent and the life-times of the two triples overlap.

The duplicates marks are used to reduce the administrative load of avoiding duplicate answers. Normally, the duplicates are marked using a background thread that is started on the first query that produces a substantial amount of duplicates.

3.4 Monitoring the database

The predicate rdf_monitor/2 allows registrations of call-backs with the RDF store. These call-backs are typically used to keep other databases in sync with the RDF store. For example, library(library(semweb/rdf_persistency)) monitors the RDF store for maintaining a persistent copy in a set of files and library(library(semweb/rdf_litindex)) uses added and deleted literal values to maintain a fulltext index of literals.

rdf_monitor(:Goal, +Mask)

Goal is called for modifications of the database. It is called with a single argument that describes the modification. Defined events are:

assert(+S, +P, +O, +DB)

A triple has been asserted.

retract(+S, +P, +O, +DB)

A triple has been deleted.

update(+S, +P, +O, +DB, +Action)

A triple has been updated.

new_literal(+Literal)

A new literal has been created. Literal is the argument of literal(Arg) of the triple's object. This event is introduced in version 2.5.0 of this library.

old_literal(+Literal)

The literal Literal is no longer used by any triple.

transaction(+BeginOrEnd, +Id)

Mark begin or end of the commit of a transaction started by rdf_transaction/2. BeginOrEnd is begin(Nesting) or end(Nesting). Nesting expresses the nesting level of transactions, starting at‘0’for a toplevel transaction. Id is the second argument of rdf_transaction/2. The following transaction Ids are pre-defined by the library:

parse(Id)

A file is loaded using rdf_load/2. Id is one of file(Path) or stream(Stream).

unload(DB)

All triples with source DB are being unloaded using rdf_unload/1.

reset

Issued by rdf_reset_db/0.

load(+BeginOrEnd, +Spec)

Mark begin or end of rdf_load_db/1 or load through rdf_load/2 from a cached file. Spec is currently defined as file(Path).

rehash(+BeginOrEnd)

Marks begin/end of a re-hash due to required re-indexing or garbage collection.

Mask is a list of events this monitor is interested in. Default (empty list) is to report all events. Otherwise each element is of the form +Event or -Event to include or exclude monitoring for certain events. The event-names are the functor names of the events described above. The special name all refers to all events and assert(load) to assert events originating from rdf_load_db/1. As loading triples using rdf_load_db/1 is very fast, monitoring this at the triple level may seriously harm performance.

This predicate is intended to maintain derived data, such as a journal, information for undo, additional indexing in literals, etc. There is no way to remove registered monitors. If this is required one should register a monitor that maintains a dynamic list of subscribers like the XPCE broadcast library. A second subscription of the same hook predicate only re-assignes the mask.

The monitor hooks are called in the order of registration and in the same thread that issued the database manipulation. To process all changes in one thread they should be send to a thread message queue. For all updating events, the monitor is called while the calling thread has a write lock on the RDF store. This implies that these events are processed strickly synchronous, even if modifications originate from multiple threads. In particular, the transaction begin, ... updates ... end sequence is never interleaved with other events. Same for load and parse.

3.5 Issues with rdf_db

This RDF low-level module has been created after two year experimenting with a plain Prolog based module and a brief evaluation of a second generation pure Prolog implementation. The aim was to be able to handle upto about 5 million triples on standard (notebook) hardware and deal efficiently with subPropertyOf which was identified as a crucial feature of RDFS to realise fusion of different data-sets.

The following issues are identified and not solved in suitable manner.

subPropertyOf of subPropertyOf

is not supported.

Equivalence

Similar to subPropertyOf, it is likely to be profitable to handle resource identity efficient. The current system has no support for it.

4 Plugin modules for rdf_db

The library(rdf_db) module provides several hooks for extending its functionality. Database updates can be monitored and acted upon through the features described in section 3.4. The predicate rdf_load/2 can be hooked to deal with different formats such as rdfturtle, different input sources (e.g. http) and different strategies for caching results.

4.1 Hooks into the RDF library

The hooks below are used to add new RDF file formats and sources from which to load data to the library. They are used by the modules described below and distributed with the package. Please examine the source-code if you want to add new formats or locations.

library(library(semweb/turtle))

Load files in the Turtle format. See section 5.

library(library(semweb/rdf_zlib_plugin))

Load gzip compressed files transparently. See section 4.2.

library(library(semweb/rdf_http_plugin))

Load RDF documents from HTTP servers. See section 4.3.

library(library(http/http_ssl_plugin))

May be combined with library(library(semweb/rdf_http_plugin)) to load RDF from HTTPS servers.

library(library(semweb/rdf_persistency))

Provide persistent backup of the triple store.

library(library(semweb/rdf_cache))

Provide caching RDF sources using fast load/safe files to speedup restarting an application.

rdf_db:rdf_open_hook(+Input, -Stream, -Format)

Open an input. Input is one of file(+Name), stream(+Stream) or url(Protocol, URL). If this hook succeeds, the RDF will be read from Stream using rdf_load_stream/3. Otherwise the default open functionality for file and stream are used.

rdf_db:rdf_load_stream(+Format, +Stream, +Options)

Actually load the RDF from Stream into the RDF database. Format describes the format and is produced either by rdf_input_info/3 or rdf_file_type/2.

rdf_db:rdf_input_info(+Input, -Modified, -Format)

Gather information on Input. Modified is the last modification time of the source as a POSIX time-stamp (see time_file/2). Format is the RDF format of the file. See rdf_file_type/2 for details. It is allowed to leave the output variables unbound. Ultimately the default modified time is‘0’and the format is assumed to be xml.

rdf_db:rdf_file_type(?Extension, ?Format)

True if Format is the default RDF file format for files with the given extension. Extension is lowercase and without a’.’. E.g. owl. Format is either a built-in format (xml or triples) or a format understood by the rdf_load_stream/3 hook.

rdf_db:url_protocol(?Protocol)

True if Protocol is a URL protocol recognised by rdf_load/2.

4.2 library(semweb/rdf_zlib_plugin): Reading compressed RDF

This module uses the library(zlib) library to load compressed files on the fly. The extension of the file must be .gz. The file format is deduced by the extension after stripping the .gz extension. E.g. rdf_load('file.rdf.gz’.

4.3 library(semweb/rdf_http_plugin): Reading RDF from a HTTP server

This module allows for rdf_load('http://...’. It exploits the library library(http/http_open.pl). The format of the URL is determined from the mime-type returned by the server if this is one of text/rdf+xml, application/x-turtle or application/turtle. As RDF mime-types are not yet widely supported, the plugin uses the extension of the URL if the claimed mime-type is not one of the above. In addition, it recognises text/html and application/xhtml+xml, scanning the XML content for embedded RDF.

4.4 library(semweb/rdf_cache): Cache RDF triples

The library library(semweb/rdf_cache) defines the caching strategy for triples sources. When using large RDF sources, caching triples greatly speedup loading RDF documents. The cache library implements two caching strategies that are controlled by rdf_set_cache_options/1.

Local caching This approach applies to files only. Triples are cached in a sub-directory of the directory holding the source. This directory is called .cache (_cache on Windows). If the cache option create_local_directory is true, a cache directory is created if posible.

Global caching This approach applies to all sources, except for unnamed streams. Triples are cached in directory defined by the cache option global_directory.

When loading an RDF file, the system scans the configured cache files unless cache(false) is specified as option to rdf_load/2 or caching is disabled. If caching is enabled but no cache exists, the system will try to create a cache file. First it will try to do this locally. On failure it will try to configured global cache.

rdf_set_cache_options(+Options)

Change the cache policy. Provided options are:

• enabled(Boolean) If true, caching is enabled.
• local_directory(Name). Plain name of local directory. Default .cache (_cache on Windows).
• create_local_directory(Bool) If true, try to create local cache directories
• global_directory(Dir) Writeable directory for storing cached parsed files.
• create_global_directory(Bool) If true, try to create the global cache directory.

[semidet]rdf_cache_file(+URL, +ReadWrite, -File)

File is the cache file for URL. If ReadWrite is read, it returns the name of an existing file. If write it returns where a new cache file can be overwritten or created.

4.5 library(semweb/rdf_litindex): Indexing words in literals

The library library(semweb/rdf_litindex.pl) exploits the primitives of section 4.5.1 and the NLP package to provide indexing on words inside literal constants. It also allows for fuzzy matching using stemming and‘sounds-like’based on the double metaphone algorithm of the NLP package.

rdf_find_literals(+Spec, -ListOfLiterals)

Find literals (without type or language specification) that satisfy Spec. The required indices are created as needed and kept up-to-date using hooks registered with rdf_monitor/2. Numerical indexing is currently limited to integers in the range ±2^30 (±2^62 on 64-bit platforms). Spec is defined as:

and(Spec1, Spec2)

Intersection of both specifications.

or(Spec1, Spec2)

Union of both specifications.

not(Spec)

Negation of Spec. After translation of the full specification to Disjunctive Normal Form (DNF), negations are only allowed inside a conjunction with at least one positive literal.

case(Word)

Matches all literals containing the word Word, doing the match case insensitive and after removing diacritics.

stem(Like)

Matches all literals containing at least one word that has the same stem as Like using the Porter stem algorithm. See NLP package for details.

sounds(Like)

Matches all literals containing at least one word that‘sounds like’ Like using the double metaphone algorithm. See NLP package for details.

prefix(Prefix)

Matches all literals containing at least one word that starts with Prefix, discarding diacritics and case.

between(Low, High)

Matches all literals containing an integer token in the range Low..High, including the boundaries.

ge(Low)

Matches all literals containing an integer token with value Low or higher.

le(High)

Matches all literals containing an integer token with value High or lower.

Token

Matches all literals containing the given token. See tokenize_atom/2 of the NLP package for details.

rdf_token_expansions(+Spec, -Expansions)

Uses the same database as rdf_find_literals/2 to find possible expansions of Spec, i.e. which words‘sound like’,‘have prefix’, etc. Spec is a compound expression as in rdf_find_literals/2. Expansions is unified to a list of terms sounds(Like, Words), stem(Like, Words) or prefix(Prefix, Words). On compound expressions, only combinations that provide literals are returned. Below is an example after loading the ULAN^{2Unified List of Artist Names from the Getty Foundation.} database and showing all words that sounds like‘rembrandt’and appear together in a literal with the word‘Rijn’. Finding this result from the 228,710 literals contained in ULAN requires 0.54 milliseconds (AMD 1600+).

?- rdf_token_expansions(and('Rijn', sounds(rembrandt)), L).

L = [sounds(rembrandt, ['Rambrandt', 'Reimbrant', 'Rembradt',
                        'Rembrand', 'Rembrandt', 'Rembrandtsz',
                        'Rembrant', 'Rembrants', 'Rijmbrand'])]

Here is another example, illustrating handling of diacritics:

?- rdf_token_expansions(case(cafe), L).

L = [case(cafe, [cafe, caf\'e])]

rdf_tokenize_literal(+Literal, -Tokens)

Tokenize a literal, returning a list of atoms and integers in the range -1073741824 ... 1073741823. As tokenization is in general domain and task-dependent this predicate first calls the hook rdf_litindex:tokenization(Literal, -Tokens). On failure it calls tokenize_atom/2 from the NLP package and deletes the following: atoms of length 1, floats, integers that are out of range and the english words and, an, or, of, on, in, this and the. Deletion first calls the hook rdf_litindex:exclude_from_index(token, X). This hook is called as follows:

no_index_token(X) :-
        exclude_from_index(token, X), !.
no_index_token(X) :-
        ...

4.5.1 Literal maps: Creating additional indices on literals

‘Literal maps’provide a relation between literal values, intended to create additional indexes on literals. The current implementation can only deal with integers and atoms (string literals). A literal map maintains an ordered set of keys. The ordering uses the same rules as described in section 4.5. Each key is associated with an ordered set of values. Literal map objects can be shared between threads, using a locking strategy that allows for multiple concurrent readers.

Typically, this module is used together with rdf_monitor/2 on the channals new_literal and old_literal to maintain an index of words that appear in a literal. Further abstraction using Porter stemming or Metaphone can be used to create additional search indices. These can map either directly to the literal values, or indirectly to the plain word-map. The SWI-Prolog NLP package provides complimentary building blocks, such as a tokenizer, Porter stem and Double Metaphone.

rdf_new_literal_map(-Map)

Create a new literal map, returning an opaque handle.

rdf_destroy_literal_map(+Map)

Destroy a literal map. After this call, further use of the Map handle is illegal. Additional synchronisation is needed if maps that are shared between threads are destroyed to guarantee the handle is no longer used. In some scenarios rdf_reset_literal_map/1 provides a safe alternative.

rdf_reset_literal_map(+Map)

Delete all content from the literal map.

rdf_insert_literal_map(+Map, +Key, +Value)

Add a relation between Key and Value to the map. If this relation already exists no action is performed.

rdf_insert_literal_map(+Map, +Key, +Value, -KeyCount)

As rdf_insert_literal_map/3. In addition, if Key is a new key in Map, unify KeyCount with the number of keys in Map. This serves two purposes. Derived maps, such as the stem and metaphone maps need to know about new keys and it avoids additional foreign calls for doing the progress in rdf_litindex.pl.

rdf_delete_literal_map(+Map, +Key)

Delete Key and all associated values from the map. Succeeds always.

rdf_delete_literal_map(+Map, +Key, +Value)

Delete the association between Key and Value from the map. Succeeds always.

[det]rdf_find_literal_map(+Map, +KeyList, -ValueList)

Unify ValueList with an ordered set of values associated to all keys from KeyList. Each key in KeyList is either an atom, an integer or a term not(Key). If not-terms are provided, there must be at least one positive keywords. The negations are tested after establishing the positive matches.

rdf_keys_in_literal_map(+Map, +Spec, -Answer)

Realises various queries on the key-set:

all

Unify Answer with an ordered list of all keys.

key(+Key)

Succeeds if Key is a key in the map and unify Answer with the number of values associated with the key. This provides a fast test of existence without fetching the possibly large associated value set as with rdf_find_literal_map/3.

prefix(+Prefix)

Unify Answer with an ordered set of all keys that have the given prefix. Prefix must be an atom. This call is intended for auto-completion in user interfaces.

ge(+Min)

Unify Answer with all keys that are larger or equal to the integer Min.

le(+Max)

Unify Answer with all keys that are smaller or equal to the integer Max.

between(+Min, +Max)

Unify Answer with all keys between Min and Max (including).

rdf_statistics_literal_map(+Map, +Key(-Arg...))

Query some statistics of the map. Provides keys are:

size(-Keys, -Relations)

Unify Keys with the total key-count of the index and Relation with the total Key-Value count.

4.6 library(semweb/rdf_persistency): Providing persistent storage

The library(semweb/rdf_persistency) provides reliable persistent storage for the RDF data. The store uses a directory with files for each source (see rdf_source/1) present in the database. Each source is represented by two files, one in binary format (see rdf_save_db/2) representing the base state and one represented as Prolog terms representing the changes made since the base state. The latter is called the journal.

rdf_attach_db(+Directory, +Options)

Attach Directory as the persistent database. If Directory does not exist it is created. Otherwise all sources defined in the directory are loaded into the RDF database. Loading a source means loading the base state (if any) and replaying the journal (if any). The current implementation does not synchronise triples that are in the store before attaching a database. They are not removed from the database, nor added to the presistent store. Different merging options may be supported through the Options argument later. Currently defined options are:

concurrency(+PosInt)

Number of threads used to reload databased and journals from the files in Directory. Default is the number of physical CPUs determined by the Prolog flag cpu_count or 1 (one) on systems where this number is unknown. See also concurrent/3.

max_open_journals(+PosInt)

The library maintains a pool of open journal files. This option specifies the size of this pool. The default is 10. Raising the option can make sense if many writes occur on many different named graphs. The value can be lowered for scenarios where write operations are very infrequent.

silent(Boolean)

If true, supress loading messages from rdf_attach_db/2.

log_nested_transactions(Boolean)

If true, nested log transactions are added to the journal information. By default (false), no log-term is added for nested transactions.

The database is locked against concurrent access using a file lock in Directory. An attempt to attach to a locked database raises a permission_error exception. The error context contains a term rdf_locked(Args), where args is a list containing time(Stamp) and pid(PID). The error can be caught by the application. Otherwise it prints:

ERROR: No permission to lock rdf_db `/home/jan/src/pl/packages/semweb/DB'
ERROR: locked at Wed Jun 27 15:37:35 2007 by process id 1748

rdf_detach_db

Detaches the persistent store. No triples are removed from the RDF triple store.

rdf_current_db(-Directory)

Unify Directory with the current database directory. Fails if no persistent database is attached.

rdf_persistency(+DB, +Bool)

Change presistency of named database (4th argument of rdf/4). By default all databases are presistent. Using false, the journal and snapshot for the database are deleted and further changes to triples associated with DB are not recorded. If Bool is true a snapshot is created for the current state and further modifications are monitored. Switching persistency does not affect the triples in the in-memory RDF database.

rdf_flush_journals(+Options)

Flush dirty journals. With the option min_size(KB) only journals larger than KB Kbytes are merged with the base state. Flushing a journal takes the following steps, ensuring a stable state can be recovered at any moment.

1. Save the current database in a new file using the extension .new.
2. On success, delete the journal
3. On success, atomically move the .new file over the base state.

Note that journals are not merged automatically for two reasons. First of all, some applications may decide never to merge as the journal contains a complete changelog of the database. Second, merging large databases can be slow and the application may wish to schedule such actions at quiet times or scheduled maintenance periods.

4.6.1 Enriching the journals

The above predicates suffice for most applications. The predicates in this section provide access to the journal files and the base state files and are intented to provide additional services, such as reasoning about the journals, loaded files, etc.^{3A library library(rdf_history) is under development exploiting these features supporting wiki style editing of RDF.}

Using rdf_transaction(Goal, log(Message)), we can add additional records to enrich the journal of affected databases with Term and some additional bookkeeping information. Such a transaction adds a term begin(Id, Nest, Time, Message) before the change operations on each affected database and end(Id, Nest, Affected) after the change operations. Here is an example call and content of the journal file mydb.jrn. A full explanation of the terms that appear in the journal is in the description of rdf_journal_file/2.

?- rdf_transaction(rdf_assert(s,p,o,mydb), log(by(jan))).

start([time(1183540570)]).
begin(1, 0, 1183540570.36, by(jan)).
assert(s, p, o).
end(1, 0, []).
end([time(1183540578)]).

Using rdf_transaction(Goal, log(Message, DB)), where DB is an atom denoting a (possibly empty) named graph, the system guarantees that a non-empty transaction will leave a possibly empty transaction record in DB. This feature assumes named graphs are named after the user making the changes. If a user action does not affect the user's graph, such as deleting a triple from another graph, we still find record of all actions performed by some user in the journal of that user.

rdf_journal_file(?DB, ?JournalFile)

True if File is the absolute file name of an existing named graph DB. A journal file contains a sequence of Prolog terms of the following format.^{4Future versions of this library may use an XML based language neutral format.}

start(Attributes)

Journal has been opened. Currently Attributes contains a term time(Stamp).

end(Attributes)

Journal was closed. Currently Attributes contains a term time(Stamp).

assert(Subject, Predicate, Object)

A triple {Subject, Predicate, Object} was added to the database.

assert(Subject, Predicate, Object, Line)

A triple {Subject, Predicate, Object} was added to the database with given Line context.

retract(Subject, Predicate, Object)

A triple {Subject, Predicate, Object} was deleted from the database. Note that an rdf_retractall/3 call can retract multiple triples. Each of them have a record in the journal. This allows for‘undo’.

retract(Subject, Predicate, Object, Line)

Same as above, for a triple with associated line info.

update(Subject, Predicate, Object, Action)

See rdf_update/4.

begin(Id, Nest, Time, Message)

Added before the changes in each database affected by a transaction with transaction identifier log(Message). Id is an integer counting the logged transactions to this database. Numbers are increasing and designed for binary search within the journal file. Nest is the nesting level, where‘0’is a toplevel transaction. Time is a time-stamp, currently using float notation with two fractional digits. Message is the term provided by the user as argument of the log(Message) transaction.

end(Id, Nest, Others)

Added after the changes in each database affected by a transaction with transaction identifier log(Message). Id and Nest match the begin-term. Others gives a list of other databases affected by this transaction and the Id of these records. The terms in this list have the format DB:Id.

rdf_db_to_file(?DB, ?FileBase)

Convert between DB (see rdf_source/1) and file base-file used for storing information on this database. The full file is located in the directory described by rdf_current_db/1 and has the extension .trp for the base state and .jrn for the journal.

5 library(semweb/turtle): Turtle: Terse RDF Triple Language

See also

http://www.w3.org/TR/turtle/ (used W3C Recommendation 25 February 2014)

This module implements the Turtle language for representing the RDF triple model as defined by Dave Beckett from the Institute for Learning and Research Technology University of Bristol and later standardized by the W3C RDF working group.

This module acts as a plugin to rdf_load/2, for processing files with one of the extensions .ttl or .n3.

rdf_read_turtle(+Input, -Triples, +Options)

Read a stream or file into a set of triples or quadruples (if faced with TriG input) of the format

rdf(Subject, Predicate, Object [, Graph])

The representation is consistent with the SWI-Prolog RDF/XML and ntriples parsers. Provided options are:

base_uri(+BaseURI)

Initial base URI. Defaults to file://<file> for loading files.

anon_prefix(+Prefix)

Blank nodes are generated as <Prefix>1, <Prefix>2, etc. If Prefix is not an atom blank nodes are generated as node(1), node(2), ...

format(+Format)

One of auto (default), turtle or trig. The auto mode switches to TriG format of there is a { before the first triple. Finally, of the format is explicitly stated as turtle and the file appears to be a TriG file, a warning is printed and the data is loaded while ignoring the graphs.

resources(URIorIRI)

Officially, Turtle resources are IRIs. Quite a few applications however send URIs. By default we do URI->IRI mapping because this rarely causes errors. To force strictly conforming mode, pass iri.

prefixes(-Pairs)

Return encountered prefix declarations as a list of Alias-URI

namespaces(-Pairs)

Same as prefixes(Pairs). Compatibility to rdf_load/2.

base_used(-Base)

Base URI used for processing the data. Unified to [] if there is no base-uri.

on_error(+ErrorMode)

In warning (default), print the error and continue parsing the remainder of the file. If error, abort with an exception on the first error encountered.

error_count(-Count)

If on_error(warning) is active, this option cane be used to retrieve the number of generated errors.

Input

is one of stream(Stream), atom(Atom), a http, https or file url or a filename specification as accepted by absolute_file_name/3.

rdf_load_turtle(+Input, -Triples, +Options)

deprecated

Use rdf_read_turtle/3

[det]rdf_process_turtle(+Input, :OnObject, +Options)

Streaming Turtle parser. The predicate rdf_process_turtle/3 processes Turtle data from Input, calling OnObject with a list of triples for every Turtle statement found in Input. OnObject is called as below, where ListOfTriples is a list of rdf(S,P,O) terms for a normal Turtle file or rdf(S,P,O,G) terms if the GRAPH keyword is used to associate a set of triples in the document with a particular graph. The Graph argument provides the default graph for storing the triples and Line is the line number where the statement started.

call(OnObject, ListOfTriples, Graph:Line)

This predicate supports the same Options as rdf_load_turtle/3.

Errors encountered are sent to print_message/2, after which the parser tries to recover and parse the remainder of the data.

See also

This predicate is normally used by load_rdf/2 for processing RDF data.

[semidet]turtle_pn_local(+Atom:atom)

True if Atom is a valid Turtle PN_LOCAL name. The PN_LOCAL name is what can follow the : in a resource. In the new Turtle, this can be anything and this function becomes meaningless. In the old turtle, PN_LOCAL is defined similar (but not equal) to an XML name. This predicate is used by rdf_save_turtle/2 to write files such that can be read by old parsers.

See also

xml_name/2.

[det]turtle_write_quoted_string(+Out, +Value)

Same as turtle_write_quoted_string(Out, Value, false), writing a string with only a single ". Embedded newlines are escapes as \n.

[det]turtle_write_uri(+Out, +Value)

Write a URI as <...>

[multifile]rdf_db:rdf_load_stream(+Format, +Stream, :Options)

(Turtle clauses)

[det]rdf_save_turtle(+Out, :Options)

Save an RDF graph as Turtle. Options processed are:

a(+Boolean)

If true (default), use a for the predicate rdf:type. Otherwise use the full resource.

align_prefixes(+Boolean)

Nicely align the @prefix declarations

base(+Base)

Save relative to the given Base

canonize_numbers(+Boolean)

If true (default false), emit numeric datatypes using Prolog's write to achieve canonical output.

comment(+Boolean)

It true (default), write some informative comments between the output segments

encoding(+Encoding)

Encoding used for the output stream. Default is UTF-8.

expand(:Goal)

Query an alternative graph-representation. See below.

indent(+Column)

Indentation for ; -lists.‘0’does not indent, but writes on the same line. Default is 8.

graph(+Graph)

Save only the named graph

group(+Boolean)

If true (default), using P-O and O-grouping.

inline_bnodes(+Boolean)

if true (default), inline bnodes that are used once.

abbreviate_literals(+Boolean)

if true (default), omit the type if allowed by turtle.

only_known_prefixes(+Boolean)

Only use prefix notation for known prefixes. Without, some documents produce huge amounts of prefixes.

prefixes(+List)

If provided, uses exactly these prefixes. List is a list of prefix specifications, where each specification is either a term Prefix_-_URI or a prefix that is known to rdf_current_prefix/2.

silent(+Boolean)

If true (default false), do not print the final informational message.

single_line_bnodes(+Bool)

If true (default false), write [...] and (...) on a single line.

subject_white_lines(+Count)

Extra white lines to insert between statements about a different subject. Default is 1.

tab_distance(+Tab)

Distance between tab-stops.‘0’forces the library to use only spaces for layout. Default is 8.

user_prefixes(+Boolean)

If true (default), use prefixes from rdf_current_prefix/2.

The option expand allows for serializing alternative graph representations. It is called through call/5, where the first argument is the expand-option, followed by S,P,O,G. G is the graph-option (which is by default a variable). This notably allows for writing RDF graphs represented as rdf(S,P,O) using the following code fragment:

triple_in(RDF, S,P,O,_G) :-
    member(rdf(S,P,O), RDF).

    ...,
    rdf_save_turtle(Out, [ expand(triple_in(RDF)) ]),

Out	is one of stream(Stream), a stream handle, a file-URL or an atom that denotes a filename.

[det]rdf_save_ntriples(+Spec, :Options)

Save RDF using ntriples format. The ntriples format is a subset of Turtle, writing each triple fully qualified on its own line.

[det]rdf_save_canonical_trig(+Spec, :Options)

Save triples in a canonical format. See rdf_save_canonical_turtle/2 foir details.

[det]rdf_save_trig(+Spec, :Options)

Save multiple RDF graphs into a TriG file. Options are the same as for rdf_save_turtle/2. rdf_save_trig/2 ignores the graph(+Graph) option and instead processes one additional option:

graphs(+ListOfGraphs)

List of graphs to save. When omitted, all graphs in the RDF store are stored in the TriG file.

[det]rdf_save_canonical_turtle(+Spec, :Options)

Save triples in a canonical format. This is the same as rdf_save_turtle/2, but using different defaults. In particular:

• encoding(utf8),
• indent(0),
• tab_distance(0),
• subject_white_lines(1),
• align_prefixes(false),
• user_prefixes(false)
• comment(false),
• group(false),
• single_line_bnodes(true)

To be done

Work in progress. Notably blank-node handling is incomplete.

6 library(semweb/rdf_ntriples): Process files in the RDF N-Triples format

See also

http://www.w3.org/TR/n-triples/

To be done

Sync with RDF 1.1. specification.

The library(semweb/rdf_ntriples) provides a fast reader for the RDF N-Triples and N-Quads format. N-Triples is a simple format, originally used to support the W3C RDF test suites. The current format has been extended and is a subset of the Turtle format (see library(semweb/turtle)).

The API of this library is almost identical to library(semweb/turtle). This module provides a plugin into rdf_load/2, making this predicate support the format ntriples and nquads.

[det]read_ntriple(+Stream, -Triple)

Read the next triple from Stream as Triple. Stream must have UTF-8 encoding.

Triple

is a term triple(Subject,Predicate,Object). Arguments follow the normal conventions of the RDF libraries. NodeID elements are mapped to node(Id). If end-of-file is reached, Triple is unified with end_of_file.

Errors

syntax_error(Message) on syntax errors

[det]read_nquad(+Stream, -Quad)

Read the next quad from Stream as Quad. Stream must have UTF-8 encoding.

Quad	is a term quad(Subject,Predicate,Object,Graph). Arguments follow the normal conventions of the RDF libraries. NodeID elements are mapped to node(Id). If end-of-file is reached, Quad is unified with end_of_file.

Errors

syntax_error(Message) on syntax errors

[det]read_ntuple(+Stream, -Tuple)

Read the next triple or quad from Stream as Tuple. Tuple is one of the terms below. See read_ntriple/2 and read_nquad/2 for details.

• triple(Subject,Predicate,Object)
• quad(Subject,Predicate,Object,Graph).

[det]rdf_read_ntriples(+Input, -Triples, +Options)

[det]rdf_read_nquads(+Input, -Quads, +Options)

True when Triples/Quads is a list of triples/quads from Input. Options:

anon_prefix(+AtomOrNode)

Prefix nodeIDs with this atom. If AtomOrNode is the term node(_), bnodes are returned as node(Id).

base_uri(+Atom)

Defines the default anon_prefix as _:<baseuri>_

on_error(Action)

One of warning (default) or error

error_count(-Count)

If on_error is warning, unify Count with th number of errors.

graph(+Graph)

For rdf_read_nquads/3, this defines the graph associated to triples loaded from the input. For rdf_read_ntriples/3 this opion is ignored.

Triples	is a list of rdf(Subject, Predicate, Object)
Quads	is a list of rdf(Subject, Predicate, Object, Graph)

rdf_process_ntriples(+Input, :CallBack, +Options)

Call-back interface, compatible with the other triple readers. In addition to the options from rdf_read_ntriples/3, this processes the option graph(Graph).

CallBack

is called as call(CallBack, Triples, Graph), where Triples is a list holding a single rdf(S,P,O) triple. Graph is passed from the graph option and unbound if this option is omitted.

[semidet,multifile]rdf_db:rdf_load_stream(+Format, +Stream, :Options)

Plugin rule that supports loading the ntriples and nquads formats.

[multifile]rdf_db:rdf_file_type(+Extension, -Format)

Bind the ntriples reader to files with the extensions nt, ntriples and nquads.

7 library(semweb/rdfa): Extract RDF from an HTML or XML DOM

See also

- http://www.w3.org/TR/2013/REC-rdfa-core-20130822/
- http://www.w3.org/TR/html-rdfa/

This module implements extraction of RDFa triples from parsed XML or HTML documents. It has two interfaces: read_rdfa/3 to read triples from some input (stream, file, URL) and xml_rdfa/3 to extract triples from an HTML or XML document that is already parsed with load_html/3 or load_xml/3.

[det]read_rdfa(+Input, -Triples, +Options)

True when Triples is a list of rdf(S,P,O) triples extracted from Input. Input is either a stream, a file name, a URL referencing a file name or a URL that is valid for http_open/3. Options are passed to open/4, http_open/3 and xml_rdfa/3. If no base is provided in Options, a base is deduced from Input.

xml_rdfa(+DOM, -RDF, +Options)

True when RDF is a list of rdf(S,P,O) terms extracted from DOM according to the RDFa specification. Options processed:

base(+BaseURI)

URI to use for” . Normally set to the document URI.

anon_prefix(+AnnonPrefix)

Prefix for blank nodes.

lang(+Lang)

Default for lang

vocab(+Vocab)

Default for vocab

markup(+Markup)

Markup language processed (xhtml, xml, ...)

[multifile]rdf_db:rdf_load_stream(+Format, +Stream, :Options)

Register library(semweb/rdfa) as loader for HTML RDFa files.

To be done

Which options need to be forwarded to read_rdfa/3?

8 library(semweb/rdfs): RDFS related queries

The library(semweb/rdfs) library adds interpretation of the triple store in terms of concepts from RDF-Schema (RDFS). There are two ways to provide support for more high level languages in RDF. One is to view such languages as a set of entailment rules. In this model the rdfs library would provide a predicate rdfs/3 providing the same functionality as rdf/3 on union of the raw graph and triples that can be derived by applying the RDFS entailment rules.

Alternatively, RDFS provides a view on the RDF store in terms of individuals, classes, properties, etc., and we can provide predicates that query the database with this view in mind. This is the approach taken in the library(semweb/rdfs.p)l library, providing calls like rdfs_individual_of(?Resource, ?Class).^{5The SeRQL language is based on querying the deductive closure of the triple set. The SWI-Prolog SeRQL library provides entailment modules that take the approach outlined above.}

8.1 Hierarchy and class-individual relations

The predicates in this section explore the rdfs:subPropertyOf, rdfs:subClassOf and rdf:type relations. Note that the most fundamental of these, rdfs:subPropertyOf, is also used by rdf_has/[3,4].

rdfs_subproperty_of(?SubProperty, ?Property)

True if SubProperty is equal to Property or Property can be reached from SubProperty following the rdfs:subPropertyOf relation. It can be used to test as well as generate sub-properties or super-properties. Note that the commonly used semantics of this predicate is wired into rdf_has/[3,4].^{bugThe current implementation cannot deal with cycles}.^{bugThe current implementation cannot deal with predicates that are an rdfs:subPropertyOf of rdfs:subPropertyOf, such as owl:samePropertyAs.}

rdfs_subclass_of(?SubClass, ?Class)

True if SubClass is equal to Class or Class can be reached from SubClass following the rdfs:subClassOf relation. It can be used to test as well as generate sub-classes or super-classes.^{bugThe current implementation cannot deal with cycles}.

rdfs_class_property(+Class, ?Property)

True if the domain of Property includes Class. Used to generate all properties that apply to a class.

rdfs_individual_of(?Resource, ?Class)

True if Resource is an indivisual of Class. This implies Resource has an rdf:type property that refers to Class or a sub-class thereof. Can be used to test, generate classes Resource belongs to or generate individuals described by Class.

8.2 Collections and Containers

The RDF construct rdf:parseType=Collection constructs a list using the rdf:first and rdf:next relations.

rdfs_member(?Resource, +Set)

Test or generate the members of Set. Set is either an individual of rdf:List or rdfs:Container.

rdfs_list_to_prolog_list(+Set, -List)

Convert Set, which must be an individual of rdf:List into a Prolog list of objects.

rdfs_assert_list(+List, -Resource)

Equivalent to rdfs_assert_list/3 using DB = user.

rdfs_assert_list(+List, -Resource, +DB)

If List is a list of resources, create an RDF list Resource that reflects these resources. Resource and the sublist resources are generated with rdf_bnode/1. The new triples are associated with the database DB.

9 Managing RDF input files

Complex projects require RDF resources from many locations and typically wish to load these in different combinations. For example loading a small subset of the data for debugging purposes or load a different set of files for experimentation. The library library(semweb/rdf_library.pl) manages sets of RDF files spread over different locations, including file and network locations. The original version of this library supported metadata about collections of RDF sources in an RDF file called Manifest. The current version supports both the VoID format and the original format. VoID files (typically named void.ttl) can use elements from the RDF Manifest vocabulary to support features that are not supported by VoID.

9.1 The Manifest file

A manifest file is an RDF file, often in Turtle format, that provides meta-data about RDF resources. Often, a manifest will describe RDF files in the current directory, but it can also describe RDF resources at arbitrary URL locations. The RDF schema for RDF library meta-data can be found in rdf_library.ttl. The namespace for the RDF library format is defined as http://www.swi-prolog.org/rdf/library/ and abbreviated as lib.

The schema defines three root classes: lib:Namespace, lib:Ontology and lib:Virtual, which we describe below.

lib:Ontology

This is a subclass of owl:Ontology. It has two subclasses, lib:Schema and lib:Instances. These three classes are currently processed equally. The following properties are recognised on lib:Ontology:

dc:title

Title of the ontology. Displayed by rdf_list_library/0.

owl:versionInfo

Version of the ontology. Displayed by rdf_list_library/0.

owl:imports

Ontologies imported. If rdf_load_library/2 is used to load this ontology, the ontologies referenced here are loaded as well. There are two subProperties: lib:schema and lib:instances with the obvious meaning.

lib:source

Defines the named graph into which the resource is loaded. If this ends in a /, the basename of each loaded file is appended to the given source. Defaults to the URL the RDF is loaded from.

lib:baseURI

Defines the base for processing the RDF data. If not provided this defaults to the named graph, which in turn defaults to the URL the RDF is loaded from.

lib:Virtual

Virtual ontologies do not refer to an RDF resource themselves. They only import other resources. For example the W3C WordNet manifest defines wn-basic and wn-full as virtual resources. The lib:Virtual resource is used as a second rdf:type:

<wn-basic>
        a lib:Ontology ;
        a lib:Virtual ;
        ...

lib:CloudNode

Used by ClioPatria to combine this ontology and all data it imports into a node in the automatically generated datacloud.

lib:Namespace

Defines a URL to be a namespace. The definition provides the preferred mnemonic and can be referenced in the lib:providesNamespace and lib:usesNamespace properties. The rdf_load_library/2 predicates registers encountered namespace mnemonics with rdf-db using rdf_register_ns/2. Typically namespace declarations use @prefix declarations. E.g.

@prefix     lib: <http://www.swi-prolog.org/rdf/library/> .
@prefix    rdfs: <http://www.w3.org/2000/01/rdf-schema#> .

[ a lib:Namespace ;
  lib:mnemonic "rdfs" ;
  lib:namespace rdfs:
] .

9.1.1 Support for the VoID and VANN vocabularies

The VoID aims at resolving the same problem as the Manifest files described here. In addition, the VANN vocabulary provides the information about preferred namepaces prefixes. The RDF library manager can deal with VoID files. The following relations apply:

• VoID Dataset and Linkset are similar to lib:Ontology, but a VoID resource is always Virtual. I.e., the VoID URI itself never refers to an RDF document.

• The owl:imports and its lib specializations are replaced by void:subset (referring to another VoID dataset) and void:dataDump (referring to a concrete document).

• A description of the dataset is given using dcterms:description rather than rdfs:comment

• The RDF library recognises lib:source, lib:baseURI and lib:Cloudnode, which have no equivalent in VoID.

• The RDF library recognises vann:preferredNamespacePrefix and vann:preferredNamespaceUri as alternatives to its proprietary way for defining prefixes. The domain of these predicates is unclear. The library recognises them regardless of the domain. Note that the range of vann:preferredNamespaceUri is a literal. A disadvantage of that is that the Turtle prefix declaration cannot be reused.

Currently, the RDF metadata is not stored in the RDF database. It is processed by low-level primitives that do not perform RDFS reasoning. In particular, this means that rdfs:supPropertyOf and rdfs:subClassOf cannot be used to specialise the RDF meta vocabulary.

9.1.2 Finding manifest files

The initial metadata file(s) are loaded into the system using rdf_attach_library/1.

rdf_attach_library(+FileOrDirectory)

Load meta-data on RDF repositories from FileOrDirectory. If the argument is a directory, this directory is processed recursively and each for each directory, a file named void.ttl, Manifest.ttl or Manifest.rdf is loaded (in this order of preference).

Declared namespaces are added to the rdf-db namespace list. Encountered ontologies are added to a private database of rdf_list_library.pl. Each ontology is given an identifier, derived from the basename of the URL without the extension. This, using the declaration below, the identifier of the declared ontology is wn-basic.

<wn-basic>
        a void:Dataset ;
        dcterms:title "Basic WordNet" ;
        ...

rdf_list_library

List the available resources in the library. Currently only lists resources that have a dcterms:title property. See section 9.2 for an example.

It is possible for the initial set of manifests to refer to RDF files that are not covered by a manifest. If such a reference is encountered while loading or listing a library, the library manager will look for a manifest file in the directory holding the referenced RDF file and load this manifest. If a manifest is found that covers the referenced file, the directives found in the manifest will be followed. Otherwise the RDF resource is simply loaded using the current defaults.

Further exploration of the library is achieved using rdf_list_library/1 or rdf_list_library/2:

rdf_list_library(+Id)

Same as rdf_list_library(Id,[]).

rdf_list_library(+Id, +Options)

Lists the resources that will be loaded if Id is handed to rdf_load_library/2. See rdf_attach_library/1 for how ontology identifiers are generated. In addition it checks the existence of each resource to help debugging library dependencies. Before doing its work, rdf_list_library/2 reloads manifests that have changed since they were loaded the last time. For HTTP resources it uses the HEAD method to verify existence and last modification time of resources.

rdf_load_library(+Id, +Options)

Load the given library. First rdf_load_library/2 will establish what resources need to be loaded and whether all resources exist. Than it will load the resources.

9.2 Usage scenarios

Typically, a project will use a single file using the same format as a manifest file that defines alternative configurations that can be loaded. This file is loaded at program startup using rdf_attach_library/1. Users can now list the available libraries using rdf_list_library/0 and rdf_list_library/1:

1 ?- rdf_list_library.
ec-core-vocabularies E-Culture core vocabularies
ec-all-vocabularies All E-Culture vocabularies
ec-hacks            Specific hacks
ec-mappings         E-Culture ontology mappings
ec-core-collections E-Culture core collections
ec-all-collections  E-Culture all collections
ec-medium           E-Culture medium sized data (artchive+aria)
ec-all              E-Culture all data

Now we can list a specific category using rdf_list_library/1. Note this loads two additional manifests referenced by resources encountered in ec-mappings. If a resource does not exist is is flagged using [NOT FOUND].

2 ?- rdf_list_library('ec-mappings').
% Loaded RDF manifest /home/jan/src/eculture/vocabularies/mappings/Manifest.ttl
% Loaded RDF manifest /home/jan/src/eculture/collections/aul/Manifest.ttl
<file:///home/jan/src/eculture/src/server/ec-mappings>
. <file:///home/jan/src/eculture/vocabularies/mappings/mappings>
. . <file:///home/jan/src/eculture/vocabularies/mappings/interface>
. . . file:///home/jan/src/eculture/vocabularies/mappings/interface_class_mapping.ttl
. . . file:///home/jan/src/eculture/vocabularies/mappings/interface_property_mapping.ttl
. . <file:///home/jan/src/eculture/vocabularies/mappings/properties>
. . . file:///home/jan/src/eculture/vocabularies/mappings/ethnographic_property_mapping.ttl
. . . file:///home/jan/src/eculture/vocabularies/mappings/eculture_properties.ttl
. . . file:///home/jan/src/eculture/vocabularies/mappings/eculture_property_semantics.ttl
. . <file:///home/jan/src/eculture/vocabularies/mappings/situations>
. . . file:///home/jan/src/eculture/vocabularies/mappings/eculture_situations.ttl
. <file:///home/jan/src/eculture/collections/aul/aul>
. . file:///home/jan/src/eculture/collections/aul/aul.rdfs
. . file:///home/jan/src/eculture/collections/aul/aul.rdf
. . file:///home/jan/src/eculture/collections/aul/aul9styles.rdf
. . file:///home/jan/src/eculture/collections/aul/extractedperiods.rdf
. . file:///home/jan/src/eculture/collections/aul/manual-periods.rdf

9.2.1 Referencing resources

Resources and manifests are located either on the local filesystem or on a network resource. The initial manifest can also be loaded from a file or a URL. This defines the initial base URL of the document. The base URL can be overruled using the Turtle @base directive. Other documents can be referenced relative to this base URL by exploiting Turtle's URI expansion rules. Turtle resources can be specified in three ways, as absolute URLs (e.g. <http://www.example.com/rdf/ontology.rdf>), as relative URL to the base (e.g. <../rdf/ontology.rdf>) or following a prefix (e.g. prefix:ontology).

The prefix notation is powerful as we can define multiple of them and define resources relative to them. Unfortunately, prefixes can only be defined as absolute URLs or URLs relative to the base URL. Notably, they cannot be defined relative to other prefixes. In addition, a prefix can only be followed by a Qname, which excludes . and /.

Easily relocatable manifests must define all resources relative to the base URL. Relocation is automatic if the manifest remains in the same hierarchy as the resources it references. If the manifest is copied elsewhere (i.e. for creating a local version) it can use @base to refer to the resource hierarchy. We can point to directories holding manifest files using @prefix declarations. There, we can reference Virtual resources using prefix:name. Here is an example, were we first give some line from the initial manifest followed by the definition of the virtual RDFS resource.

@base <http://gollem.science.uva.nl/e-culture/rdf/> .

@prefix base:           <base_ontologies/> .

<ec-core-vocabularies>
        a lib:Ontology ;
        a lib:Virtual ;
        dc:title "E-Culture core vocabularies" ;
        owl:imports
                base:rdfs ,
                base:owl ,
                base:dc ,
                base:vra ,
                ...

<rdfs>
        a lib:Schema ;
        a lib:Virtual ;
        rdfs:comment "RDF Schema" ;
        lib:source rdfs: ;
        lib:schema <rdfs.rdfs> .

9.3 Putting it all together

In this section we provide skeleton code for filling the RDF database from a password protected HTTP repository. The first line loads the application. Next we include modules that enable us to manage the RDF library, RDF database caching and HTTP connections. Then we setup the HTTP authentication, enable caching of processed RDF files and load the initial manifest. Finally load_data/0 loads all our RDF data.

:- use_module(server).

:- use_module(library(http/http_open)).
:- use_module(library(semweb/rdf_library)).
:- use_module(library(semweb/rdf_cache)).

:- http_set_authorization('http://www.example.org/rdf',
                          basic(john, secret)).

:- rdf_set_cache_options([ global_directory('RDF-Cache'),
                           create_global_directory(true)
                         ]).


:- rdf_attach_library('http://www.example.org/rdf/Manifest.ttl').

%%      load_data
%
%       Load our RDF data

load_data :-
        rdf_load_library('all').

9.4 Example: A metadata file for W3C WordNet

The VoID metadata below allows for loading WordNet in the two predefined versions using one of

?- rdf_load_library('wn-basic', []).
?- rdf_load_library('wn-full', []).

@prefix    void: <http://rdfs.org/ns/void#> .
@prefix    vann: <http://purl.org/vocab/vann/> .
@prefix     lib: <http://www.swi-prolog.org/rdf/library/> .
@prefix     owl: <http://www.w3.org/2002/07/owl#> .
@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      dc: <http://purl.org/dc/terms/> .
@prefix   wn20s: <http://www.w3.org/2006/03/wn/wn20/schema/> .
@prefix   wn20i: <http://www.w3.org/2006/03/wn/wn20/instances/> .

[ vann:preferredNamespacePrefix "wn20i" ;
  vann:preferredNamespaceUri "http://www.w3.org/2006/03/wn/wn20/instances/"
] .

[ vann:preferredNamespacePrefix "wn20s" ;
  vann:preferredNamespaceUri "http://www.w3.org/2006/03/wn/wn20/schema/"
] .

<wn20-common>
        a void:Dataset ;
        dc:description "Common files between full and basic version" ;
        lib:source wn20i: ;
        void:dataDump
                <wordnet-attribute.rdf.gz> ,
                <wordnet-causes.rdf.gz> ,
                <wordnet-classifiedby.rdf.gz> ,
                <wordnet-entailment.rdf.gz> ,
                <wordnet-glossary.rdf.gz> ,
                <wordnet-hyponym.rdf.gz> ,
                <wordnet-membermeronym.rdf.gz> ,
                <wordnet-partmeronym.rdf.gz> ,
                <wordnet-sameverbgroupas.rdf.gz> ,
                <wordnet-similarity.rdf.gz> ,
                <wordnet-synset.rdf.gz> ,
                <wordnet-substancemeronym.rdf.gz> ,
                <wordnet-senselabels.rdf.gz> .

<wn20-skos>
        a void:Dataset ;
        void:subset <wnskosmap> ;
        void:dataDump <wnSkosInScheme.ttl.gz> .

<wnskosmap>
        a lib:Schema ;
        lib:source wn20s: ;
        void:dataDump
                <wnskosmap.rdfs> .

<wnbasic-schema>
        a void:Dataset ;
        lib:source wn20s: ;
        void:dataDump
                <wnbasic.rdfs> .

<wn20-basic>
        a void:Dataset ;
        a lib:CloudNode ;
        dc:title "Basic WordNet" ;
        dc:description "Light version of W3C WordNet" ;
        owl:versionInfo "2.0" ;
        lib:source wn20i: ;
        void:subset
                <wnbasic-schema> ,
                <wn20-skos> ,
                <wn20-common> .

<wnfull-schema>
        a void:Dataset ;
        lib:source wn20s: ;
        void:dataDump
                <wnfull.rdfs> .

<wn20-full>
        a void:Dataset ;
        a lib:CloudNode ;
        dc:title "Full WordNet" ;
        dc:description "Full version of W3C WordNet" ;
        owl:versionInfo "2.0" ;
        lib:source wn20i: ;
        void:subset
                <wnfull-schema> ,
                <wn20-skos> ,
                <wn20-common> ;
        void:dataDump
                <wordnet-antonym.rdf.gz> ,
                <wordnet-derivationallyrelated.rdf.gz> ,
                <wordnet-participleof.rdf.gz> ,
                <wordnet-pertainsto.rdf.gz> ,
                <wordnet-seealso.rdf.gz> ,
                <wordnet-wordsensesandwords.rdf.gz> ,
                <wordnet-frame.rdf.gz> .

10 library(semweb/sparql_client): SPARQL client library

This module provides a SPARQL client. For example:

?- sparql_query('select * where { ?x rdfs:label "Amsterdam" }', Row,
                [ host('dbpedia.org'), path('/sparql/')]).

Row = row('http://www.ontologyportal.org/WordNet#WN30-108949737') ;
false.

Or, querying a local server using an ASK query:

?- sparql_query('ask { owl:Class rdfs:label "Class" }', Row,
                [ host('localhost'), port(3020), path('/sparql/')]).
Row = true.

HTTPS servers are supported using the scheme(https) option:

?- sparql_query('select * where { ?x rdfs:label "Amsterdam"@nl }',
                Row,
                [ scheme(https),
                  host('query.wikidata.org'),
                  path('/sparql')
                ]).

[nondet]sparql_query(+Query, -Result, +Options)

Execute a SPARQL query on an HTTP SPARQL endpoint. Query is an atom that denotes the query. Result is unified to a term rdf(S,P,O) for CONSTRUCT and DESCRIBE queries, row(...) for SELECT queries and true or false for ASK queries. Options are

Variables that are unbound in SPARQL (e.g., due to SPARQL optional clauses), are bound in Prolog to the atom '$null$'.

endpoint(+URL)

May be used as alternative to Scheme, Host, Port and Path to specify the endpoint in a single option.

host(+Host)

port(+Port)

path(+Path)

scheme(+Scheme)

The above four options set the location of the server.

search(+ListOfParams)

Provide additional query parameters, such as the graph.

variable_names(-ListOfNames)

Unifies ListOfNames with a list of atoms that describe the names of the variables in a SELECT query.

Remaining options are passed to http_open/3. The defaults for Host, Port and Path can be set using sparql_set_server/1. The initial default for port is 80 and path is‘/sparql/`.

For example, the ClioPatria server understands the parameter entailment. The code below queries for all triples using _rdfs_entailment.

?- sparql_query('select * where { ?s ?p ?o }',
                Row,
                [ search([entailment=rdfs])
                ]).

Another useful option is the request_header which, for example, may be used to trick force a server to reply using a particular document format:

?- sparql_query(
       'select * where { ?s ?p ?o }',
        Row,
        [ host('integbio.jp'),
          path('/rdf/sparql'),
          request_header('Accept' =
                         'application/sparql-results+xml')
        ]).

sparql_set_server(+OptionOrList)

Set sparql server default options. Provided defaults are: host, port and repository. For example:

    sparql_set_server([ host(localhost),
                        port(8080)
                        path(world)
                      ])

The default for port is 80 and path is /sparql/.

sparql_read_xml_result(+Input, -Result)

Specs from http://www.w3.org/TR/rdf-sparql-XMLres/. The returned Result term is of the format:

select(VarNames, Rows)

Where VarNames is a term v(Name, ...) and Rows is a list of row(....) containing the column values in the same order as the variable names.

ask(Bool)

Where Bool is either true or false

[det]sparql_read_json_result(+Input, -Result)

The returned Result term is of the format:

select(VarNames, Rows)

Where VarNames is a term v(Name, ...) and Rows is a list of row(....) containing the column values in the same order as the variable names.

ask(Bool)

Where Bool is either true or false

See also

http://www.w3.org/TR/rdf-sparql-json-res/

11 library(semweb/rdf_compare): Compare RDF graphs

This library provides predicates that compare RDF graphs. The current version only provides one predicate: rdf_equal_graphs/3 verifies that two graphs are identical after proper labeling of the blank nodes.

Future versions of this library may contain more advanced operations, such as diffing two graphs.

[semidet]rdf_equal_graphs(+GraphA, +GraphB, -Substition)

True if GraphA and GraphB are the same under Substition. Substition is a list of BNodeA = BNodeB, where BNodeA is a blank node that appears in GraphA and BNodeB is a blank node that appears in GraphB.

GraphA	is a list of rdf(S,P,O) terms
GraphB	is a list of rdf(S,P,O) terms
Substition	is a list if NodeA = NodeB terms.

To be done

The current implementation is rather naive. After dealing with the subgraphs that contain no bnodes, it performs a fully non-deterministic substitution.

12 library(semweb/rdf_portray): Portray RDF resources

To be done

- Define alternate predicate to use for providing a comment
- Use rdf:type if there is no meaningful label?
- Smarter guess whether or not the local identifier might be meaningful to the user without a comment. I.e. does it look‘word-like’?

This module defines rules for user:portray/1 to help tracing and debugging RDF resources by printing them in a more concise representation and optionally adding comment from the label field to help the user interpreting the URL. The main predicates are:

• rdf_portray_as/1 defines the overall style
• rdf_portray_lang/1 selects languages for extracting label comments

[det]rdf_portray_as(+Style)

Set the style used to portray resources. Style is one of:

prefix:id

Write as NS:ID, compatible with what can be handed to the rdf predicates. This is the default.

writeq

Use quoted write of the full resource.

prefix:label

Write namespace followed by the label. This format cannot be handed to rdf/3 and friends, but can be useful if resource-names are meaningless identifiers.

prefix:id=label

This combines prefix:id with prefix:label, providing both human readable output and output that can be pasted into the commandline.

[det]rdf_portray_lang(+Lang)

If Lang is a list, set the list or preferred languages. If it is a single atom, push this language as the most preferred language.

13 Related packages

The core infrastructure for storing and querying RDF is provided by this package, which is distributed as a core package with SWI-Prolog. ClioPatria provides a comprehensive server infrastructure on top of the semweb and http packages. ClioPatria provides a SPARQL 1.1 endpoint, linked open data (LOD) support, user management, a web interface and an extension infrastructure for programming (semantic) web applications.

Thea provides access to OWL ontologies at the level of the abstract syntax. Can interact with external DL reasoner using DIG.

14 Version 3 release notes

RDF-DB version 3 is a major redesign of the SWI-Prolog RDF infrastructure. Nevertheles, version 3 is almost perfectly upward compatible with version 2. Below are some issues to take into consideration when upgrading.

Version 2 did not allow for modifications while read operations were in progress, for example due to an open choice point. As a consequence, operations that both queried and modified the database had to be wrapped in a transaction or the modifications had to be buffered as Prolog data structures. In both cases, the RDF store was not modified during the query phase. In version 3, modifications are allowed while read operations are in progress and follow the Prolog logical update view semantics. This is different from using a transaction in version 2, where the view for all read operations was frozen at the start of the transaction. In version 3, every read operation sees the store frozen at the moment that the operation was started.

We illustrate the difference by writing a forwards entailment rule that adds a sibling relation. In version 2, we could perform this operation using one of the following:

add_siblings_1 :-
        findall(S-O,
                ( rdf(S, f:parent, P),
                  rdf(O, f:parent, P),
                  S \== O
                ),
                Pairs),
        forall(member(S-O, Pairs), rdf_assert(S,f:sibling,O)).

add_siblings_2 :-
        rdf_transaction(
            forall(( rdf(S, f:parent, P),
                     rdf(O, f:parent, P),
                     S \== O
                   ),
                   rdf_assert(S, f:sibling, O))).

In version 3, we can write this in the natural Prolog style below. In itself, this may not seem a big advantage because wrapping such operations in a transaction is often a good style anyway. The story changes with more complicated constrol structures that combine iterations with steps that depend on triples asserted in previous steps. Such scenarios can be programmed naturally in the current version.

add_siblings_3 :-
        forall(( rdf(S, f:parent, P),
                 rdf(O, f:parent, P),
                 S \== O
               ),
               rdf_assert(S, f:sibling, O)).

In version 3, code that combines queries with modification has the same semantics whether executed inside or outside a transaction. This property makes reusing such predicates predictable.

rdf_statistics/2

Various statistics have been renamed or changed:

• sources is renamed into graphs
• triples_by_file is renamed into triples_by_graph
• gc has additional arguments
• core is removed.

rdf_generation/1

Generations inside a transaction are represented as BaseGeneration+TransactionGeneration, where BaseGeneration is the global generation where the transaction started and TransactionGeneration expresses the generation within the transaction. Counting generation has changed as well. In particular, comitting a transaction steps the generation only by one.

rdf_current_ns/1, rdf_register_ns/2, rdf_register_ns/3

These predicates are renamed into rdf_current_prefix/1, rdf_register_prefix/2, rdf_register_prefix/3. The old predicates are still available as deprecated predicates.

rdf_unload/1

now only accepts a source location and deletes the associated graph using rdf_unload_graph/1.

Acknowledgements

This research was supported by the following projects: MIA and MultimediaN project (www.multimedian.nl) funded through the BSIK programme of the Dutch Government, the FP-6 project HOPS of the European Commission, the COMBINE project supported by the ONR Global NICOP grant N62909-11-1-7060 and the Dutch national program COMMIT.

Index

?

compressed data

4.2

concurrent/3

4.6

dc:title

gz, format

4.2

gzip

4.2

lang_equal/2

lang_matches/2

lib:source

load_data/0

9.3

owl:imports

rdf/3

1 8

rdf/4

4.6

rdf_active_transaction/1

rdf_alt/3

rdf_assert/3

rdf_assert/4

rdf_assert_alt/3

rdf_assert_alt/4

rdf_assert_bag/2

rdf_assert_bag/3

rdf_assert_list/2

rdf_assert_list/3

rdf_assert_seq/2

rdf_assert_seq/3

rdf_attach_db/2

4.6

rdf_attach_library/1

9.1.2 9.1.2 9.2

rdf_bag/2

rdf_bnode/1

8.2

rdf_cache_file/3

rdf_canonical_literal/2

rdf_compare/3

rdf_create_bnode/1

rdf_create_graph/1

rdf_current_db/1

4.6.1

rdf_current_literal/1

rdf_current_ns/1

14

rdf_current_ns/2

rdf_current_predicate/1

rdf_current_prefix/1

14

rdf_current_prefix/2

rdf_current_snapshot/1

rdf_db:rdf_file_type/2

rdf_db:rdf_input_info/3

rdf_db:rdf_load_stream/3

rdf_db:rdf_open_hook/3

rdf_db:url_protocol/1

rdf_db_to_file/2

rdf_default_graph/1

rdf_default_graph/2

rdf_delete_literal_map/2

rdf_delete_snapshot/1

rdf_destroy_literal_map/1

rdf_detach_db/0

rdf_equal/2

rdf_equal_graphs/3

rdf_estimate_complexity/4

rdf_file_type/2

4.1 4.1

rdf_find_literal_map/3

4.5.1

rdf_find_literals/2

4.5 4.5

rdf_flush_journals/1

rdf_gc/0

rdf_generation/1

14

rdf_global_id/2

rdf_global_object/2

rdf_global_term/2

rdf_graph/1

rdf_graph_property/2

rdf_has/3

rdf_has/4

rdf_has/[3,4]

8.1 8.1

rdf_input_info/3

4.1

rdf_insert_literal_map/3

4.5.1

rdf_insert_literal_map/4

rdf_iri/1

rdf_is_bnode/1

rdf_is_iri/1

rdf_is_literal/1

rdf_is_name/1

rdf_is_object/1

rdf_is_predicate/1

rdf_is_resource/1

rdf_is_subject/1

rdf_is_term/1

rdf_journal_file/2

4.6.1

rdf_keys_in_literal_map/3

rdf_last/2

rdf_length/2

rdf_lexical_form/2

rdf_list/1

rdf_list/2

rdf_list_library/0

9.1 9.1 9.2

rdf_list_library/1

9.1.2 9.2 9.2

rdf_list_library/2

9.1.2 9.1.2

rdf_literal/1

rdf_load/1

rdf_load/2

3.4 3.4 4 4.1

rdf_load_db/1

3.4 3.4 3.4

rdf_load_library/2

9.1 9.1 9.1.2 9.1.2

rdf_load_stream/3

4.1 4.1

rdf_load_turtle/3

rdf_make/0

rdf_match_label/3

rdf_member/2

rdf_monitor/2

1 3.4 4.5 4.5.1

rdf_name/1

rdf_new_literal_map/1

rdf_node/1

rdf_nth0/3

rdf_nth1/3

rdf_object/1

rdf_persistency/2

rdf_portray_as/1

rdf_portray_lang/1

rdf_predicate/1

rdf_predicate_property/2

rdf_process_ntriples/3

rdf_process_turtle/3

rdf_reachable/3

rdf_reachable/5

rdf_read_nquads/3

rdf_read_ntriples/3

rdf_read_turtle/3

rdf_register_ns/2

9.1 14

rdf_register_ns/3

14

rdf_register_prefix/2

14

rdf_register_prefix/3

14

rdf_reset_db/0

3.4

rdf_reset_literal_map/1

4.5.1

rdf_resource/1

rdf_retract_list/1

rdf_retractall/3

4.6.1

rdf_retractall/4

rdf_save/1

rdf_save/2

rdf_save_canonical_trig/2

rdf_save_canonical_turtle/2

rdf_save_db/1

rdf_save_db/2

4.6

rdf_save_footer/1

rdf_save_header/2

rdf_save_ntriples/2

rdf_save_subject/3

rdf_save_trig/2

rdf_save_turtle/2

rdf_seq/2

rdf_set/1

rdf_set_cache_options/1

rdf_set_graph/2

rdf_set_predicate/2

rdf_snapshot/1

rdf_source/1

4.6 4.6.1

rdf_source_location/2

rdf_statistics/1

rdf_statistics/2

14

rdf_statistics_literal_map/2

rdf_subject/1

rdf_term/1

rdf_token_expansions/2

rdf_tokenize_literal/2

rdf_transaction/1

rdf_transaction/2

3.4 3.4

rdf_transaction/3

rdf_unload/1

3.4 14

rdf_unload_graph/1

14

rdf_update/4

4.6.1

rdf_update/5

rdf_update_duplicates/0

rdf_version/1

rdf_where/1

rdfs_assert_list/2

rdfs_assert_list/3

8.2

rdfs_class_property/2

rdfs_container/2

rdfs_container_membership_property/1

rdfs_container_membership_property/2

rdfs_individual_of/2

rdfs_list_to_prolog_list/2

rdfs_member/2

rdfs_nth0/3

rdfs_subclass_of/2

rdfs_subproperty_of/2

read_nquad/2

read_ntriple/2

read_ntuple/2

read_rdfa/3

sparql_query/3

sparql_read_json_result/2

sparql_read_xml_result/2

sparql_set_server/1

time_file/2

4.1

tokenize_atom/2

4.5 4.5

turtle_pn_local/1

turtle_write_quoted_string/2

turtle_write_uri/2

xhtml

4.3

xml_rdfa/3

lib:CloudNode

ClioPatria

13

Collection,parseType

8.2

I

owl:versionInfo

N

lib:Namespace

O

lib:Ontology

OWL2

13

P

Persistent store

4.6

R

RDF-Schema

8

S

SPARQL

13

T

Thea

13

parseType,Collection

8.2

U

lib:baseURI

V

lib:Virtual

{

/1