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Semantics Developer's Guide (PDF)

Semantics Developer's Guide — Chapter 10

XQuery and JavaScript Semantics APIs

This chapter describes the XQuery and JavaScript Semantics APIs, which include an XQuery library module, built-in semantics functions, and support for SPARQL, SPARQL Update, and RDF. The Semantics API is designed for large-scale, production triple stores and applications. The complete list of semantic functions can be found at http://docs.marklogic.com/sem/semantics.

This chapter includes examples of using the Semantics API, which is an API designed to create, query, update, and delete triples and graphs in MarkLogic.

Additionally, the following APIs support the MarkLogic Semantics features; XQuery API, REST API, Node.js Client API, and Java Client API, using a variety of query styles, as described in the Loading Semantic Triples, Semantic Queries and Inserting, Deleting, and Modifying Triples with XQuery and Server-Side JavaScript chapters of this guide.

This chapter includes the following sections:

XQuery Library Module for Semantics

Some of the Semantics XQuery functions are built-in functions that do not require an import statement, while others are implemented in an XQuery library module that does require an import statement. To simplify things, MarkLogic recommends that you import the Semantics API library into every XQuery module or JavaScript module that uses the Semantics API.

Importing the Semantics Library Module with XQuery

You can use the Semantics API library module with XQuery by importing the module into your XQuery with the following prolog statement:

import module namespace sem = "http://marklogic.com/semantics" 
  at "/MarkLogic/semantics.xqy";

The prefix for all semantic functions in XQuery is http://marklogic.com/semantics. The Semantics API uses the prefixes sem: or rdf:, which are defined in the server. For details about the function signatures and descriptions, see the Semantics documentation under XQuery Library Modules in the XQuery and XSLT Reference Guide and the MarkLogic XQuery and XSLT Function Reference.

Importing the Semantics Library Module with JavaScript

For JavaScript you can use the Semantics API library module by importing the module into your JavaScript with this statement:

var sem = require("/marklogic/semantics.xqy");

The prefix for all semantic XQuery functions in JavaScript is /marklogic.com/semantics.xqy. With JavaScript, the Semantics API uses the prefix sem. , which is defined in the server. For details about the function signatures and descriptions, see the Semantics documentation under JavaScript Library Modules in the JavaScript Reference Guide and the MarkLogic XQuery and XSLT Function Reference.

Generating Triples

The XQuery sem:rdf-builder function is a powerful tool for dynamically generating triples in the Semantics API. (For JavaScript, the function is sem.rdfBuilder.)

The function builds triples from the CURIE and blank node syntaxes. Blank nodes specified with a leading underscore ( _ ) are assigned blank node identifiers, and maintain state across multiple invocations; for example, "_:person1" refers to the same node as a later invocation that also mentions "_:person1". For example:

xquery version "1.0-ml";
import module namespace sem = "http://marklogic.com/semantics" 
  at "/MarkLogic/semantics.xqy";
 
 let $builder := sem:rdf-builder((), sem:iri("my-named-graph"))
 let $t1 := $builder("_:person1", "a", "foaf:Person") 
 let $t2 := $builder("_:person2", "a", "foaf:Person") 
 let $t3 := $builder("_:person1", "foaf:knows", "_:person2") 
 return ($t1,$t2,$t3) 

=>

@prefix xs: <http://www.w3.org/2001/XMLSchema#> .

<http://marklogic.com/semantics/blank/4892021155019117627>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type/>
<http://xmlns.com/foaf/0.1/Person> .

<http://marklogic.com/semantics/blank/6695700652332466909>
        <http://xmlns.com/foaf/0.1/knows> 
        _:bnode4892021155019117627 ;
        <http://www.w3.org/1999/02/22-rdf-syntax-ns#type/>
        <http://xmlns.com/foaf/0.1/Person> .

In the example, there are three triples generated in Turtle serialization using sem:rdf-builder. The triples represent the following facts; that person1 and person2 are people, and that their relationship is that person1 knows person2.

Note the following:

  • The first parameter accepts an optional set of prefix mappings, which in this example is an empty argument. Since 'empty' means default, the $common-prefixes are used for the first argument. The second argument is a named graph for the sem:rdf-builder output.
  • In the predicate position, the special value of "a" is expanded to the full IRI for rdf:type.
  • Human-readable CURIEs for common prefixes are used, such as foaf:knows instead of long IRIs. See Working with CURIEs.
  • The blank nodes produced in the third triple match the identity of those defined in the first and second.

Extracting Triples from Content

With the sem:rdf-builder function you can easily extract triples from existing content or the results of a SPARQL query and quickly construct RDF graphs for querying or inserting into your database.

Assume that you have a web page that lists cities and countries that are sorted and ranked by the cost of living (COL), which is based on a Consumer Priced Index (CPI) and CPI-based inflation rate. The inflation rate is defined as the annual percent change in consumer prices compared with the previous year's consumer prices. Using a reference point of Monterrey, Mexico with an assigned a value of 100, the Inflation value of every other city in the database is calculated by comparing their COL to that of Monterrey. For example, an Inflation value of 150, means that the COL is 50% more expensive than living in Monterrey.

These values are fictional and are not based on any official sources.

The underlying HTML code for the COL table may resemble the following:

<table class="city-index"
style="max-width:58%;float:left;margin-right:2em;">
    <thead>
        <tr>
            <th>Ranking</th>
            <th class="city-name">City</th>
            <th class="inflation">Inflation 
            <a href="#inflation-explanation">*</a></th>
        </tr>
    </thead>
          
           <tbody><tr>
              <td class="ranking">1</td>
              <td class="city-name">
              <a href="http://www.example.org/IncreasedCoL/london">
              London (United Kingdom)</a></td>
              <td class="inflation">270</td>
            </tr>
          
            <tr>
              <td class="ranking">2</td>
              <td class="city-name">
              <a href="http://www.example.org/IncreasedCoL/stockholm">
              Stockholm (Sweden)</a></td>
              <td class="inflation">266</td>
            </tr>
          
            <tr>
              <td class="ranking">3</td>
              <td class="city-name">
              <a href="http://www.example.org/IncreasedCoL/zurich">
              Zurich (Switzerland)</a></td>
              <td class="inflation">251</td>
            </tr>
          
            <tr>
              <td class="ranking">4</td>
              <td class="city-name">
              <a href="http://www.example.org/IncreasedCoL/geneva">
              Geneva (Switzerland)</a></td>
              <td class="inflation">247</td>
            </tr>
          
            <tr>
              <td class="ranking">5</td>
              <td class="city-name">
              <ahref="http://www.example.org/IncreasedCoL/new-york">
              New York City (United States)</a></td>
              <td class="inflation">225</td>
            </tr>

This example uses the sem:rdf-builder function to extract triples from the HTML content. The function takes advantage of the fact that the HTML code is already well-formed and has a useful classification node (@class):

import module namespace sem = "http://marklogic.com/semantics" 
  at "/MarkLogic/semantics.xqy";
declare namespace html="http://www.w3.org/1999/xhtml";

let $doc := xdmp:tidy(xdmp:document-get("C:\Temp\CoLIndex.html",
      <options xmlns="xdmp:document-get">
        <repair>none</repair>
        <format>text</format>
      </options>))[2]

let $rows := ($doc//html:tr)[html:td/@class eq 'ranking']
let $builder := sem:rdf-builder
                 (sem:prefixes("my: http://example.org/vocab/"))
for $row in $rows
let $bnode-name := "_:" || $row/html:td[@class eq 'ranking']
return (
        $builder($bnode-name, "my:rank", xs:decimal(
                 $row/html:td[@class eq 'ranking'] )),
        $builder($bnode-name, "rdfs:label", xs:string(
                 $row/html:td[@class eq 'city-name'] )),
        $builder($bnode-name, "my:coli", xs:decimal(
                 $row/html:td[@class eq 'inflation'] ))
        )

The results are returned as in-memory triples:

Parsing Triples

This example extends the previous example and inserts parsed triples into the database by using the sem:rdf-insert function:

import module namespace sem = "http://marklogic.com/semantics" 
  at "/MarkLogic/semantics.xqy";
declare namespace html="http://www.w3.org/1999/xhtml";

let $doc := xdmp:tidy(xdmp:document-get("C:\Temp\CoLIndex.html",
      <options xmlns="xdmp:document-get">
        <repair>none</repair>
        <format>text</format>
      </options>))[2]

let $rows := ($doc//html:tr)[html:td/@class eq 'ranking']
let $builder := sem:rdf-builder(
                  sem:prefixes("my: http://example.org/vocab/"))
for $row in $rows
let $bnode-name := "_:" || $row/html:td[@class eq 'ranking']
let $triples := $row
return (sem:rdf-insert(( 
      $builder($bnode-name, "my:rank", xs:decimal
            ( $row/html:td[@class eq 'ranking'] )),
      $builder($bnode-name, "rdfs:label", xs:string
            ( $row/html:td[@class eq 'city-name'] )),
      $builder($bnode-name, "my:coli", xs:decimal
            ( $row/html:td[@class eq 'inflation'] )))
))

The document IRIs are returned as strings:

For more information about inserting and parsing triples with XQuery, see Loading Triples with XQuery.

The parser ensures well-formed markup as the triples are inserted as schema-valid triples and indexed with the Triples index, provided it is enabled. See Triple Index: Understanding and Using.

Use fn:doc to view the contents of the documents and verify the triples.

fn:doc("/triplestore/2ca88f284b7dd96d.xml")

One document is created for each blank node identifier ($bnode-name).

During the generation process $builder maintains state eliminating the need to keep track of every blank node label and ensuring that they map to the same sem:blank value.

The Semantics API includes a repair option for the N-Quad and Turtle parsers. During a normal operation, the RDF parsers perform these tasks:

  • Turtle parsing uses the base IRI to resolve relative IRIs. If the result is relative, an error is raised.
  • N-Quad parsing does not resolve using the base IRI. If a IRI in the document is relative, an error is raised.

During a repair operation the RDF parsers perform this task:

  • Turtle parsing uses the base IRI to resolve relative IRIs. No error is raised for resultant relative IRIs.
  • N-Quad parsing also uses the base IRI to resolve relative IRIs. No error is raised for resultant relative IRIs.

Exploring Data

The Semantics API provides a number of functions to access RDF data in a database. This section focuses on the following topics:

sem:triple Functions

This table describes the sem:triple functions used to define or search for triple data:

FunctionDescription
sem:tripleCreates a triple object that represents an RDF triple containing atomic values representing the subject, predicate, object, and optionally a graph identifier (graph IRI)
sem:triple-subjectReturns the subject from a sem:triple value
sem:triple-predicateReturns the predicate from a sem:triple value
sem:triple-objectReturns the object from a sem:triple value
sem:triple-graphReturns the graph identifier (graph IRI) from a sem:triple value

In this example, the sem:triple function is used to create a triple that includes a CURIE for the predicate and an rdf:langString value as the object, with English (en) as the given language tag:

sem:triple(sem:iri("http://id.loc.gov/authorities/subjects/
sh85040989"), 
sem:curie-expand("skos:prefLabel"), 
rdf:langString("Education", "en"))

=>
@prefix xs: <http://www.w3.org/2001/XMLSchema#> .

<http://id.loc.gov/authorities/subjects/sh85040989>
<http://www.w3.org/2004/02/skos/core#prefLabel/> 
"Education"@en .

sem:transitive-closure

Use the sem:transitive-closure function to traverse RDF graphs in order to answer reachability questions and discover more information about your RDF data. (In JavaScript, you would use the sem.transitiveClosure function.) For example, you can find out which nodes are reachable from a given node.

You can use a vocabulary such the Simple Knowledge Organization System (SKOS) with the sem:transitive-closure function. SKOS is 'a common data model for knowledge organization systems such as thesauri, classification schemes, subject heading systems and taxonomies' as described by the W3C SKOS Simple Knowledge Organization System Reference:

http://www.w3.org/TR/skos-reference/

Assume that you have a file composed of triples for subject headings that relate to US Congress bills and that the triples are marked up with the SKOS vocabulary. The triples may look similar to this extract:

<http://id.loc.gov/authorities/subjects/sh85002310/>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type/>
<http://www.w3.org/2004/02/skos/core#Concept/> . 

<http://id.loc.gov/authorities/subjects/sh85002310/>
<http://www.w3.org/2004/02/skos/core#prefLabel/> 
"Agricultural education"@en . 

<http://id.loc.gov/authorities/subjects/sh85002310/>
<http://www.w3.org/2008/05/skos-xl#altLabel/>
_:bnode7authoritiessubjectssh85002310 . 

_:bnode7authoritiessubjectssh85002310
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type/>
<http://www.w3.org/2008/05/skos-xl#Label/> . 

_:bnode7authoritiessubjectssh85002310
<http://www.w3.org/2008/05/skos-xl#literalForm/> 
"Education, Agricultural"@en . 

<http://id.loc.gov/authorities/subjects/sh85002310/>
<http://www.w3.org/2004/02/skos/core#broader/>
<http://id.loc.gov/authorities/subjects/sh85133121/> . 

<http://id.loc.gov/authorities/subjects/sh85002310/>
<http://www.w3.org/2004/02/skos/core#narrower/>
<http://id.loc.gov/authorities/subjects/sh85118332/> . 

<http://id.loc.gov/authorities/subjects/sh85133121/>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type/>
<http://www.w3.org/2004/02/skos/core#Concept/> . 
<http://id.loc.gov/authorities/subjects/sh85133121/>
<http://www.w3.org/2004/02/skos/core#prefLabel/> 
"Technical education"@en . 

In this dataset, 'Technical education' is a broader subject heading for 'Agricultural education' as defined by the skos:broader predicate:

<http://id.loc.gov/authorities/subjects/sh85002310/>
<http://www.w3.org/2004/02/skos/core#broader/>
<http://id.loc.gov/authorities/subjects/sh85133121/> . 

Although skos:broader is not stictly transitive, it is a sub-property of skos:broaderTransitive, which is a transitive.

If you load triples into MarkLogic with blank node identifiers, the original _:bnode identifiers are replaced with MarkLogic blank node identifiers. For example, _:bnode7authoritiessubjectssh85002310 in the original dataset becomes <http://marklogic.com/semantics/blank/17142585114552908287> on ingestion. This identity is maintained for all triples that are related to the original blank node.

This example uses cts:triples to find the subject IRI for a triple where the predicate is a CURIE for skos:prefLabel and the object is Agricultural education. The subject IRI found in the cts:triples query is subsequently used with skos:broader to determine broader subject terms to a depth of 3:

xquery version "1.0-ml";
import module namespace sem = "http://marklogic.com/semantics" 
  at "/MarkLogic/semantics.xqy";

let $triple-subject := sem:triple-subject(cts:triples((), sem:curie-expand("skos:prefLabel"), 
rdf:langString("Agricultural education", "en")))
return 
sem:transitive-closure($triple-subject, sem:curie-expand("skos:broader"), 3)

=>
<http://id.loc.gov/authorities/subjects/sh85133121/>
<http://id.loc.gov/authorities/subjects/sh85002310/>
<http://id.loc.gov/authorities/subjects/sh85026423/>
<http://id.loc.gov/authorities/subjects/sh85040989/>

Notice that in addition to the expected IRIs, for the following subjects:

  • <http://id.loc.gov/authorities/subjects/sh85002310/>
  • <http://id.loc.gov/authorities/subjects/sh85133121/>

IRIs were returned also in the results for the following subjects:

  • <http://id.loc.gov/authorities/subjects/sh85040989/>
  • <http://id.loc.gov/authorities/subjects/sh85026423/>

When we take a closer look at the dataset, the IRIs for 'Education' and 'Civilization' are also returned, since they are broader subjects still to 'Agricultural education' and 'Technical Education':

<http://id.loc.gov/authorities/subjects/sh85040989/>
<http://www.w3.org/2004/02/skos/core#prefLabel/> 
"Education"@en . 
<http://id.loc.gov/authorities/subjects/sh85040989/>
<http://www.w3.org/2004/02/skos/core#broader/>
<http://id.loc.gov/authorities/subjects/sh85026423/> . 
...
<http://id.loc.gov/authorities/subjects/sh85026423/>
<http://www.w3.org/2004/02/skos/core#prefLabel/> 
"Civilization"@en . 
...
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