Assemblers are a general mechanism in Jena to describe objects to be built, often these objects are models and datasets. Assemblers are used heavily in Fuseki for dataset and model descriptions, for example.
SPARQL queries operate over an RDF dataset, which is a unnamed, default graph and zero or more named graphs.
Having the description in a file means that the data that the application is going to work on can be changed without changing the program code.
This is needed for use in Fuseki.
A dataset can be constructed in an assembler file:
@prefix tdb: <http://jena.hpl.hp.com/2008/tdb#> . @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> . @prefix ja: <http://jena.hpl.hp.com/2005/11/Assembler#> . [] ja:loadClass "com.hp.hpl.jena.tdb.TDB" . tdb:DatasetTDB rdfs:subClassOf ja:RDFDataset . tdb:GraphTDB rdfs:subClassOf ja:Model . <#dataset> rdf:type tdb:DatasetTDB ; tdb:location "DB" ; .
Only one dataset can be stored in a location (filing system directory).
The first section declares the prefixes used later:
@prefix tdb: <http://jena.hpl.hp.com/2008/tdb#> . @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> . @prefix ja: <http://jena.hpl.hp.com/2005/11/Assembler#> .
then there is a statement that causes TDB to be loaded. TDB initialization occurs automatically when loaded. The TDB jar must be on the Java classpath.
While order in this file does not matter to the machine, because in
this case the jena assembler system checks for any ja:loadClass
statements before any attempt to assemble an object is made, having
it early in the file is helpful to any person looking at the file.
[] ja:loadClass "com.hp.hpl.jena.tdb.TDB" .
And finally there is the description of the TDB dataset itself:
<#graph> rdf:type tdb:DatasetTDB ; tdb:location "DB" ;
The property tdb:location gives the file name as a string. It is
relative to the applications current working directory, not where
the assembler file is read from.
The dataset description is usually found by looking for the one
subject with type tdb:GraphDataset. If more than one graph is
given in a single file, the application will have to specify which
description it wishes to use.
An assembler can specify that the default graph for query is the union of the named graphs. This is done by adding tdb:unionDefaultGraph.
<#dataset> rdf:type tdb:DatasetTDB ; tdb:location "DB" ; tdb:unionDefaultGraph true ; .
TDB always stores data in an RDF dataset. It is possible to use just one of the graphs from the dataset. A common way of working with one graph is to use the default graph of the dataset.
A single graph from a TDB dataset can be described by:
@prefix tdb: <http://jena.hpl.hp.com/2008/tdb#> . @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . @prefix ja: <http://jena.hpl.hp.com/2005/11/Assembler#> . [] ja:loadClass "com.hp.hpl.jena.tdb.TDB" . <#dataset> rdf:type tdb:DatasetTDB ; tdb:location "DB" ; <#graph> rdf:type tdb:GraphTDB ; tdb:dataset <#dataset> .
A particular named graph in the dataset at a location can be assembled with:
<#graphNamed> rdf:type tdb:GraphTDB ; tdb:dataset <#dataset> . tdb:graphName <http://example/graph1> ; .
It is possible to create a dataset with graphs backed by different storage subsystems, although query is not necessarily as efficient.
To include as a named graph in a dataset use vocabulary as shown below:
@prefix tdb: <http://jena.hpl.hp.com/2008/tdb#> . @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> . @prefix ja: <http://jena.hpl.hp.com/2005/11/Assembler#> . [] ja:loadClass "com.hp.hpl.jena.tdb.TDB" . tdb:DatasetTDB rdfs:subClassOf ja:RDFDataset . tdb:GraphTDB rdfs:subClassOf ja:Model . # A dataset of one TDB-backed graph as the default graph and # an in-memory graph as a named graph. <#dataset> rdf:type ja:RDFDataset ; ja:defaultGraph <#graph> ; ja:namedGraph [ ja:graphName <http://example.org/name1> ; ja:graph <#graph2> ] ; . <#graph> rdf:type tdb:GraphTDB ; tdb:location "DB" ; . <#graph2> rdf:type ja:MemoryModel ; ja:content [ja:externalContent <file:Data/books.n3> ] ; .
Note here we added:
tdb:DatasetTDB rdfs:subClassOf ja:RDFDataset . tdb:GraphTDB rdfs:subClassOf ja:Model .
which provides for integration with complex model setups, such as reasoners.
@prefix tdb: <http://jena.hpl.hp.com/2008/tdb#> . @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> . @prefix ja: <http://jena.hpl.hp.com/2005/11/Assembler#> . tdb:Dataset a rdfs:Class . tdb:GraphTDB a rdfs:Class . tdb:DatasetTDB rdfs:subClassOf ja:RDFDataset . tdb:GraphTDB rdfs:subClassOf ja:Model . tdb:location a rdf:Property ; # domain is tdb:Dataset or tdb:GraphTDB # The range is simple literal . tdb:unionDefaultGraph a rdf:Property ; rdfs:domain tdb:Dataset ; # The range is xsd:boolean . tdb:graphName a rdf:Property ; rdfs:domain tdb:GraphTDB ; # range is a URI .
